How accurate is your car speedometer?

Car Technology
How accurate is a car speedometer compared to satnav?

“If my satnav shows I’m going four to five miles per hour slower than my car speedometer says, which do I believe?” Asked by Tabitha, September 2012

Having recently bought a used car, Tabitha began to notice that there was a consistent discrepancy between the speed showing on her speedometer and that on her portable satellite navigation unit. At most speeds, the speedo reading was several mph faster than her satnav unit was indicating.

Many other drivers have reported the same thing over the years, and this situation is actually common to nearly every car on the market. But why is this the case? Surely car manufacturers can make their speedos accurate to the precise mph or km/h you are travelling at?

How a car speedometer works

The Car Expert explains why your car speedometer may not be accurateSpeed is the measurement of distance over time. But a car speedometer doesn’t actually measure how fast you travel from Point A to Point B. Car speedos usually work by measuring rotation of the car’s driveshaft, axle or wheel. They then use some basic maths to extrapolate that rotation and determine how fast you are travelling. It’s a very similar concept to a bicycle speedometer.

However, if the diameter of the wheel/tyre alters, the extrapolation calculation will be incorrect. For example, the diameter will increase if you put new tyres on the car (more tread, which wears down over thousands of miles) or increase the tyre pressure.  This means that, for each revolution of the wheel, the car is travelling further, meaning your speed is greater.

If the diameter decreases (eg – worn tyres, less air in the tyres, a different brand of tyre with slightly different dimensions, more load in the car weighing it down and compressing the tyres), then the car will be travelling a shorter distance for each revolution of the wheel, therefore you will be going slower.

Margin of error in a car speedometer

The differences in wheel diameter resulting from the above circumstances could be tiny (maybe a few millimetres), but at 30mph your car wheels are rotating 6-7 times every second, so it can quickly make a difference of a few miles per hour. This margin for error is taken into account in how the law is applied, and how manufacturers calibrate their car speedos.

How a satnav speedometer works

Is the speed on your satnav more accurate than your car speedometer?Satellite navigation units (either portable or integrated into the car) calculate your car’s speed by measuring actual distance travelled over time using GPS satellite tracking.  They repeatedly locate your exact position on earth via satellite and calculate how far you have travelled, then divide by the time it took for you to travel that distance. Satnav accuracy is determined by satellite signal quality and is unaffected by your car’s tyres. Many satnavs are unable to account for changes in vertical direction, so may be less accurate if you are travelling up or down a steep hill. They are also inherently more accurate at higher speeds, as a larger distance over time reduces rounding errors, but a satnav will usually be much closer to a car’s true speed than the speedometer.

Some factory satnav systems will also use data from the car to integrate with the GPS signal to improve overall accuracy.

The law for car speedometers in the UK

The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.

To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring.

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You should also read: The Car Expert satnav shopping guide
How does your satnav speed compare to your car speedometer?

Stuart Masson

Stuart is the Editor of The Car Expert, which he founded in 2011, and our new sister site The Van Expert. Originally from Australia, Stuart has had a passion for cars and the car industry for over thirty years. He spent a decade in automotive retail, and now works tirelessly to help car buyers by providing independent and impartial advice.

47 Comments

  1. Would a digital speedo be more accurate than an analogue dial-type speedo? Or do they have the same fudge factor?

    Reply
    • Stuart Masson

      Hi Connor, it shouldn’t make any difference, as the calculation to display the speed is the same for both analogue and digital speedometers. These days, they often use a digital signal from the wheel sensors anyway and convert it to an analogue dial display. So yes, they both have the same ‘fudge factor’.

    • I took a driving test today, the speedometer was showing a different speed to the sat nav, as the sat nav was digital I followed that, but after my test I had failed as I had been over the limit several times. So was I wrong? The examiner took the sat nav reading and the dial reading so I’m really not sure which one would have been right?

    • Stuart Masson

      Hi Rachel. Difficult to say without having been there, but it is entirely possible you managed to exceed the limit while you were looking out the windscreen, in the mirrors or over your shoulder (ie – anywhere but the speedo or satnav).

      Normally, the speedo should read higher than the satnav. I’m not sure which reading they use to determine that you are speeding.

  2. The speedo on my Ford seems to be perfectly accurate. Am I just special?

    Reply
    • Yes, but not in a good way.

  3. How do we know that the satnav is accurate? I thought that the satellite signals were deliberately designed to be less accurate than possible for “security reasons”?

    Reply
    • Stuart Masson

      As far as I know, you are correct in the level of accuracy for civilian GPS units (like satnavs) being lower than military applications. But if you are moving at 30mph, you are covering more than 13 metres per second, which makes it possible for a satnav to measure your speed accurately. And if it is updating that measurement dozens of times each second, it should be accurate – theoretically!

    • The system known as ‘selective availability’ which introduced a timing error for civilian GPS use was turned off a number of years ago – mainly because it had become quite easy to circumvent the introduced error using differential GPS (a fixed installation in a known location on the earth can be used to calculate the timing error and pass it on to any other GPS capable of receiving that extra info).
      As far as I’m aware, there’s no longer any difference in the accuracy of civilian vs military GPS.

  4. I’m surprised that manufacturers haven’t come up with a better way of measuring actual speed than measuring rotation. Surely it couldn’t be that difficult to bounce a tiny radar or sonar off the ground to measure real speed?

    Reply
  5. Air pressure in the tyre makes no difference to the measured speed of the vehicle. The length of the outer perimeter of the squashed circle is the same as the circumference of the circle as would be on a properly inflated tyre. One rotation of the whole wheel requires the entirety of the outside surface to travel along the road and since that hasn’t changed, neither has the number of revolutions of the wheel, gearbox or measurement system.

    Fitting different size tyres will affect it though as you have stated because then you’re actually changing the length of the rubber on the outside of the tyre.

    Additionally, some sat-nav speedometers don’t account for the fact you’re travelling up or downhill, so their speed indicates only the horizontal displacement of the vehicle, and Pythagoras tells us that the hypotenuse (road) of a triangle must be a greater distance than the horizontal (where vertical is height), so the speed on your sat-nav may be slightly lower than your actual speed in this case. For example: If you drive up a 30 degrees hill and your sat-nav says 40mph you may be going closer to 50mph.

    Reply
    • Hi Matt, The air pressure will affect the speedometer, as the car is calculating the tyres circumference based on the tyres radius. So if you have less air in the tyre the radius will decrease as the weight of the car squashes the tyre fooling the car into thinking the tyre has a smaller diameter

  6. UK-driven trucks have calibrated speedos which is fine but causes problems in certain circumstances. Roadworks usually have 50mph so us HGV drivers drive close to the speed limit if we can, but hey that car in front using the second lane is also travelling at 50mph, so us HGV drivers in lane one catchup to car we also start using lane two, the car will not move faster or move into lane one, so us HGV drivers might give car driver a quick flash which might get the car driver to move into lane one. Then you might get lucky and get backup to 50mph. But car driver thinks you are speeding and then does an illegal move which is apply their brakes for NO reason. ALL manufacturers should stick to the truth and issue calibrated speedos.

    Reply
  7. The ECU of my car reports a speed that is within 0.01 mph 0f the speed shown by my sat-nav. But the speedo shows 70mph when the satnav reports a speed of 65mph.

    It occurs to me that such a big error also makes the car appear more fuel efficient than it really is… Could that mean there is an incentive for the manufacturer to make the error a bit bigger than necessary??

    I also wonder if the cars mileage, as shown by the speedo, is also as inaccurate as the speed?

    Reply
  8. Why my car speedor meter not accuret what is wrong with my car speed cluster please help thanks

    Reply
  9. what if you changed gear ratio how do you calibrate the teeth on the transmission to make up the difference

    Reply
    • stuart

      Hi Joseph. If a modern car is using the wheel sensors to measure rotation speed, the gear ratios shouldn’t matter – the wheels are still turning at the same speed even if the engine revs are higher or lower. To illustrate, if you are driving your car at 30mph in 2nd gear, 3rd gear or 4th gear, your ratios are very different but the speedo is still accurate.

  10. How come when a car publication (I can't remember which one) tested several second hand cars, they found the Alfa 156 was reading 30 mph and the car was actually doing 33 mph. It was a completely standard car. As the car went faster the speedo started to read faster than actual speed.

    Reply
    • Stuart Masson

      Hi Lyndon. Always difficult to assess the validity of used car results, as you don't know the history of the vehicle in question. Unless you tested a large number of the same model and found a consistent problem, it's impossible to blame an inherent problem with the car. I used to own an Alfa Romeo 156 and never had this issue.

  11. So I take this screws it up for owners of classic cars who have worn out 40-50 year old speedometers and cant get replacement ones. Mine is 1998 and (Calibrated) Digital so I'm ok just another money making scheme.. now people are buying more Hybrid / Electric cars and not spending as much on fuel they need to get the loss back some how..

    Reply
    • Stuart Masson

      I'm not sure what you mean, Chevymanuk. Are you suggesting that old cars should be exempt from speeding laws because their speedometers may no longer be accurate?

  12. If the circumference of the tyre is smaller then the wheel will have to rotate more times to cover the same distance so speed will appear higher. Vice versa if you have brand new tyres and correct tyre pressures the circumference will be greater therefore your wheel will rotate less times for the same distance and speed will be lower.
    You stated it the other way round.

    Reply
    • Stuart Masson

      Hi Steve. Your maths is absolutely correct, but you are calculating backwards. Distance travelled is an unknown for the car, not a known.

      Wheel circumfererence (known, but with fudge factor as described above) x wheel rotation speed (known, measured from axle or wheel sensors) = vehicle speed. So a larger circumference for a given rotation speed will yield a higher vehicle speed.

  13. The indicated speed on my 2007 (8th Gen) Civic reads 4.5mph over what I'm actually travelling at tested against trucks from our own fleet !
    I understand perfectly what Les was saying above re: travelling through roadworks. The muppets double up and are actually travelling around 43mph in a 50 zone and point blank refuse to return to Lane 1. Have a guess what the hypocrites then do as soon as they get out of the AVERAGE speed camera zone ?
    The mind boggles.

    Reply
  14. How does this inaccuracy affect the recorded mileage on the vehicle? If the speedo is over reading by 10% does this resulting a10% increase in miles and hence more services and money spent at a garage?

    Reply
    • Stuart Masson

      Hi Brian. In a word, yes.

  15. Stuart Masson

    Hi everybody. This article has been getting huge numbers of referrals from Facebook over the last few weeks, which is absolutely fantastic, but Facebook doesn't allow website owners to track where exactly on Facebook the link has come from. If the link is on a public page, can somebody please point me in the direction so I can find it?

    Many thanks, stuart.

    Reply
  16. I am aware that EU law requires that speedometers never under-read but they can over-read by up to 10 per cent + 6.25mph.

    I have a part-time delivery job and the van's speedo regularly and consistently over-reads by 20 per cent as calibrated against my satnav

    Is this legal? What implications does it have on the vehicle's MoT check assuming the examiner was to find it (and as yet the last three tests that I have been aware of haven't). Would he check for that and if so, how?)

    Reply
    • Stuart Masson

      Hi Trevor. It's not part of the MOT test, as far as I know. If it's over-reading it's not really a problem as you're not likely to be going over the speed limit. You could probably send the van to the dealership to have it fixed, but unless it's still under warranty it will cost you.

  17. I recently bought a 2012 Ford Focus from a local Ford dealer. I soon noticed I was getting tailgated a lot when going at exactly the speed limit, also the engine was labouring in the gear I would expect to be most appropriate for the speed (eg 4th for 30mph) so I went out with the sat nav and sure enough, the speedo reads higher than true speed, by as much as 5mph. While this wouldn’t usually be a problem, it means I would need to take the car to higher than the speedo reading to suit the gear (and risk breaking the speed limit unknowingly), or else put up with the vibration of a labouring gear, or drop down to a less economic gear for higher engine revs. Would the dealer be able to tune the speedo so that the error is reduced? Or will I have to learn to drive at “35” and “45” and so on?

    Reply
    • Stuart Masson

      The most appropriate gear for your speed will depend on whether the engine is working in its optimal rev range. As more cars get six-speed manual gearboxes and seven- or eight-speed automoatic gearboxes, they tend to prefer being in a higher gear than you may be used to.

      It’s unlikely the dealer will be able to do much to adjust the speedo, so you’ll have to do what everybody else does and either drive slightly under the limit or allow for the speedo inaccuracy. Can’t guarantee that it will stop the tailgating, as that is happening more and more on our roads anyway.

  18. We have just had a speeding notice come through our door. Saying doing 36 in a 30 zone. However I know where the camera in question is. I also know where I was going. And I know for certain I was doing what my speedo said was 30mph. I know this as I was behind a relative and another car was in front of him, as I go through cameras I always check my speed, also the two other drivers in front have had nothing sent to them. Is it possible the speedo on my car could be out that much? If so how can I prove it. Thanks. Neil

    Reply
    • Stuart Masson

      Hi Neil. If your speedo said you were doing 30mph, then you should have been doing no more (and possibly a bit less) than 30mph.

      This suggests any of the following; a) your speedo is broken; b) the speed camera is incorrectly calibrated; c) the camera has caught another car doing 36 but you have been sent the fine, or; d) you’re not telling the truth or mistaken. The first step is to request the photo of the incident, as that will determine if it’s actually your car in the photo.

      For more information, have a read on our article about being caught by a speed camera.

  19. Hi the actual Stuart, thanks for ur time. I am in UAE, Sharjah, driving Ford Figo 2012, original tyre size written in the car is 175/65 R14 82, on 15 oct 2016 tyre shop put 185/70 R14 88 tyre when I changed them. In the next three days I got 3 speed fines, I was driving under the speed limit. I checked in internet that tyre size affects to the actual speed and also I compare to the speed application and realized that car speedo showing 4_5 km/hr less than application. I went to ford dealer centre and they said its recommended to put original tyre size, but they are saying the tyre size does not effect the speed, they made test with laptop connected to the car program I think, then the result was both were the same. I think they did not do actual speed test which is depends on tyre size.
    I am so confused, because I need proof or statement to show that tyre size affects actual speed and cause speedo error to make tyre shop pay the fines I got. I am sure that this is from tyte size. I can not prove by logic, orally or speed application totake action against tyre shop for his mistake.
    Could you advice me what to do now, how I can get actual speed test report to prove that tyre size is affecting to the actual speed? How is it done actual speed test to check speedo error, how it is called? I am tyring to find here but still no result. Do you have any office or contact here to solve this problem. Appreciate your advice and information. Thx.

    Reply
    • Stuart Masson

      Changing a 175/65 R14 tyre to a 185/70 R14 tyre will increase your speed by about 5% for every rotation of the wheel, so it MAY mean you are now travelling 5% faster than before for the same speedo reading. However, there are other factors which will also affect this, such as tyre pressure, rolling resistance, increased drag and so on, so there’s no guarantee you are going any faster on the new tyres than you were with the old ones.

      It’s also unlikely that that extra 5% in diameter would be enough to suddenly be responsible for three speeding fines in three days – unless you were already speeding but just under the threshold used for speeding tolerances in the UAE and this has tipped you over the edge. If you were under the speed limit before, the tyres on their own should not have been enough to put you over the threshold.

      Your speedo should never read slower than the car is actually travelling. It is possible that when the tyre fitters changed your tyres, they have knocked the speed sensor somehow and it is now giving false data.

      It is possible that the new tyres are significantly quieter than the old tyres, meaning your subconscious audio cues equated the noise levels to a lower speed and you were driving faster than you thought. A lot of things are possible.

  20. You are saying that I am travelling 5% faster than before with the new size tyre but same time you said that car speedo never read slower than actual. It might be because of tyre size, now tyre bigger than original and speed sensor connected with wheel size rotation not tyre size rotation, so error might happen. I also checked some GPS speed applications that car speedo shows less than GPS. I checked this in other cars and result is opposite, GPS showing less than car speedo. By logic also bigger diameter tyre makes more distance than smaller tyre in particular time, speed equal to distance/time. In this case, car speedo reading based on the original tyre size. What could you advise?

    Reply
    • Stuart Masson

      I said you MAY be travelling 5% faster, not that you ARE travelling 5% faster. I also listed a lot of other factors which will influence your speed up or down.

  21. As car expert could you give me some solution for the current problem, instructions how to identify the exact problem, to whom contact? Could you provide your contact details to discuss this matter in details? Here I could not find any expert to check this and solve it. thx.

    Reply
  22. great article Sir…
    If i am driving in uphill/downhill, will this satnav speedometer give me accurate reading…

    Reply
    • Stuart Masson

      Hi Rahul. If you are going up or downhill, there will be an inaccuracy in your satnav speedo reading. How much it will affect you depends on the angle of the hill.

  23. What system do police speed cameras (hand-held or in-car) use to determine a vehicle’s speed ?

    Reply
  24. Makes sense to me. well explained. Suppose it will never explain why I was caught doing 51 in a 30

    Reply
  25. Whatever the truth is about relative speeds per rotation of tyre or whatever the problem caused is that some drivers still adopt rightly or so the ida that they will stay safe and do the max speed limit as shown on their speedometers. That being 30 mph even though they may only be doing 27/8 mph they are quite happy with this progress. Unfortunately there are those that will push the envelope and understanding or believe that no action would be taken against them for doing 30 mph plus 10% plus 2 mph and so drive with the speedometer reading 35 mph and whose actual speed may be 30 mph or thereabouts.

    The problem is that in a town environment one can now apparently have two lawful speeds. One of 30 mph and one of 35 mph. Several years ago The Chief of Police Officers, then known as ACPO. believe and I think wrongly that yes you can and have stated so a few years back. This leads to the problem that some driver now have. One driver near the limit is getting caught up by other drivers who drive over the limit. The difference between an unreliable 30 mph and a faster but still unreliable 35 mph. So we have the phenomena called Tailgating.

    What we and drivers need to know is what is right as both of them cant so its got to be one or the other and a new direction should be given by the Police as to exactly where we stand. Then we should abide by it without question. Do we do 30 or 35 mph? As said Tailgating is now the preferential position, like sheep nose to tail not really seeing where they are going and of many driver and its getting to epidemic proportions. Tailgating is not just about being too close to the vehicle in front so that one can not guarantee stopping in the distance that can be seen to be clear ahead and that is dangerous enough there are other consequences. Its also about the inability to see whats happening ahead and to be seen. In close proximity to the vehicle in front there is less of a chance that you are seen by other drivers turning or pedestrians crossing and therefore putting oneself and other road users in a dangerous position. Of being so close a proximity that one has visual fixation on the rear of that vehicle and being transfixed on their rear brake lights.

    People are being killed or becoming seriously injured due to this circumstance and so its a matter that should be sorted out in order to save lives.

    Reply
    • Stuart Masson

      The speed limit is the speed limit. Different police forces may adopt different tolerances for how they enforce it, but there is no such thing as two speed limits.

      Tailgating is an issue at any speed limit and is a separate problem. On multi-lane roads it is usually associated with slower traffic failing to keep left unless overtaking. Unfortunately there is generally very little policing of either problem.

  26. I stand by what I said in the second paragraph. Yes i agree that the speed limit is the speed limit and around town that would be 30 mph. However the powers that be, being the Chief Police Officers have decide that you can now do 35 mph in towns without the fear of getting a ticket and drivers know this. This is adopted throughout the police services nationwide. It further means that at increased speeds one can, with impunity exceed the legal limit without being brought to book.

    Yes Tailgating is a becoming a common driving practise and one which can end up in multiple pile ups as do happen particularly on our motorways and part of this is due to the large discrepancy between vehicles speedometers and the tolerances of the authorities.

    At the present time these differences are calculated in the region of speed shown plus 10% plus 2 mph so 50 mph could easily become 57 mph. However they have found in America that the accuracy of speedometers of many vehicles tested was found to be that some registered correct at 100% and some were slightly less accurate down to 97% correct. Only a 3% discrepancy. So some vehicles will have been travelling only 3% slower than the speed shown. at 50 mph that equates to only 1.5 mph. ie 48.5 mph

    So what discrepancy should we adopt a difference of 7 mph too fast or one of 1.5 mph slower. I know which one I would choose.

    Reply
  27. Hi Stuart, congratulations on a great (and very popular) article!

    When I purchased my new Ford from the dealer, there were 4 different wheel size options,
    215/60R16 Diameter 26.2″ Width 8.5″ Sidewall 5.1″ Circumference 82.1″ Revs/Mile 795.
    225/50ZR17 Diameter 25.9″ Width 8.9″ Sidewall 4.4″ Circumference 81.2″ Revs/Mile 804.
    235/45ZR18 Diameter 26.3″ Width 9.3″ Sidewall 4.2″ Circumference 82.7″ Revs/Mile 789
    235/45ZR19 Diameter 27.3″ Width 9.3″ Sidewall 4.2″ Circumference 85.8″ Revs/Mile 761

    Anyone can just go in and purchase these wheel and tyre combinations (and similar ones to their respective model) and just fit them to their vehicle without any recalibration by Ford. There is a difference of 43 Revs/Mile between the 17″ and 19″ wheel and tyre combination. So I was wondering, how does this affect the speedo and odometer readings?
    So I did a few calculations and it surprised me how little it did affect the readings.
    I used the sizes from the 225/50R17 and 235/45ZR19 as thats where the largest size difference in circumference/revs per mile:

    Speedometer error between 225/50R17 and 235/45ZR19
    Speedo showing 20mph 30mph 40mph 50mph 60mph 70mph 80mph 90mph
    Actual mph 21.1 31.6 42.2 52.7 62.2 72.8 84.3 94.9

    The difference to the odometer is also very small. There is a 4.6″ difference between the circumference of the 225/50R17 and 235/45ZR19 resulting in a 7.2 mile difference over 100,000 miles.

    I hope this helps someone understand a little easier as it did me.
    Daijudo

    Reply
    • Stuart Masson

      Hi Daijudo. All of those wheel/tyre combinations are approved by Ford because they don’t change the overall rolling diameter significantly. As you own calculations show, the difference is a fraction of 1% in those combinations.

      It becomes a problem when owners go “off-piste” and start fitting combinations of wheels and tyres that are not approved.

What are your thoughts? Let us know below.

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