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Redex fuel additive trial: the verdict

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I’ve just finished a three-month trial of Redex fuel system additives in my Mazda MX-5 to see if I could feel any increase in my car’s performance or measure any significant difference in its fuel economy.

Since May, I’ve been recording my fuel economy every time I fill up the tank – manually noting down the litres added and mileage covered, rather than relying on the car’s trip computer – and adding a dose of Redex to the tank.

As I outlined in previous reports, the type of fuel additive that I’ve been using is a fuel system cleaner rather than an octane booster. The goal isn’t to turn regular unleaded into premium unleaded by increasing the car’s octane number, but simply to give all the tiny pipes and injectors a thorough clean so they can do their thing without being clogged up.

Given that your engine creates dozens of explosions every single second, there’s a lot of combustion going on under your bonnet. With any burning of fuel, there will be deposits and debris that forms. Sometimes it’s obvious, like the ash that forms when you burn a log in a fireplace. Other times, like in your car’s engine, these deposits are microscopic, but the principle is the same. Over time, these deposits build up and affect your ability to keep burning the fuel properly.

The purpose of the Redex fuel system cleaners is to remove this build-up of deposits and keep the fuel injection system clean. By doing so, the fuel will burn more cleanly and therefore give you better performance and fuel economy as the engine is working more efficiently.

As I’ve made clear previously, this isn’t a laboratory test in controlled conditions but rather a real-world trial that gives a decent indication of what any driver can expect in everyday life.

My car is a 2013 Mazda MX-5 2.0-litre manual. Unless it’s raining, the roof is always down (and thankfully, we’ve enjoyed a cracking summer) and the vast majority of my driving is a 25-mile round trip to and from the office each day. My car only has about 30,000 miles on the clock with a full Mazda service history, so it’s in pretty good nick.

Fuel economy results

Below is a table of my last recording before starting with the Redex products, followed by one tank of the heavy-duty Redex Advanced 5-in-1 system cleaner and then six tanks using the regular Redex petrol system cleaner.

 MPG readingFuelling notesDrivingTraffic
130.2Initial baseline – no Redex productsCommutingModerate
233.9Advanced 5-in-1 system cleanerMixed motorway and commutingVaried
329.690ml petrol system cleanerCommutingModerate
427.490ml petrol system cleanerCommutingHeavy
529.990ml petrol system cleanerCommutingHeavy
631.590ml petrol system cleanerCommutingVaried
733.290ml petrol system cleanerMixed motorway and commutingLight
832.790ml petrol system cleanerCommutingLight

The initial baseline figure of 30mpg is pretty indicative of my usual fuel economy result, although usually that’s relying on the car’s trip computer reading rather than direct measurement.

Over about 2,000 miles of mainly commuting in three months, my average fuel consumption using the Redex fuel system cleaners was 31.1mpg. My driving circumstances in that time have been a pretty representative example of my usual activities, so superficially it looks like a 1mpg improvement since using Redex.

In reality, the most obvious conclusion from the fuel economy results is that they largely followed the driving conditions more than anything else. With lighter traffic conditions during August, my fuel economy improved by more than 10% compared to the same journeys in heavier traffic. The best results were, unsurprisingly, achieved at times when my regular commute was combined with longer A-road and motorway driving.

Redex petrol system cleaner and Advanced 5-in-1 concentrated system cleaner

Driving observations

Given that the additives I’ve been using are not performance boosters as such, I wasn’t expecting to feel any noticeable differences to my car’s accelerative qualities. If that was the point, Redex has products like its Petrol Power Booster. Alternatively, I could just fill my tank with premium unleaded fuel, which has a higher octane level than regular unleaded and provides extra power.

The principle with the fuel system cleaner additives is that a clean fuel system will perform more efficiently than a clogged fuel system, much like your heart being able to pump blood more freely through clear arteries than clogged ones. Clogged arteries also mean your heart has to work harder and is at greater risk of heart damage, and your engine is no different. Over time, clogged fuel injectors can potentially lead to expensive engine damage.

There has been no noticeable difference in performance, and I suspect you’d need a laboratory to really show any improvement. In the variable nature of the real world, and in a well-maintained modern car, I couldn’t detect any difference.

I mentioned in my last report that I’d previously noticed a slight cough or misfire from the Mazda on a couple of occasions prior to using the Redex products, but hadn’t noticed anything since using the fuel system cleaners. I can report that the engine has still been running cleanly over the last six weeks, so the stuff appears to be doing its job in that respect.


My fuel economy figures haven’t shown any changes that could really be attributed to the Redex products. The variation of 3-4mpg above or below the average based on driving conditions and traffic outweighs any potential gain from the products. Again, if you were running a laboratory experiment that directly measured fuel consumption and removed all other variables then you may be able to show an improvement, but it’s nothing really noticeable.

So why would you consider using fuel additive products like these ones I’ve been testing?

Firstly, if you have an older or higher-mileage car, you are far more likely to see noticeable improvements in performance and fuel economy compared to a newer, lower-mileage vehicle. Given that fuel deposits and debris build up in your engine over time, it makes sense that you’ll notice a greater improvement in an older engine that has seen a few million more fuel explosions than a newer engine. If I’d been running the same test on an older car I am sure I would have seen some more noticeable results, but unfortunately I don’t have an older car to call upon for such a purpose (and buying an old banger to drive for three months didn’t really appeal!)

Secondly, I felt a definite sense of confidence that I was taking positive steps to keep my engine clean and operating at peak efficiency. Every litre of petrol has to make a long and tortuous journey to get from a fuel refinery into a tanker truck, then out of the truck and into a giant storage tank under a service station, then into a fuel bowser and finally into your car’s petrol tank. There are plenty of opportunities for the fuel to collect debris and impurities anywhere along that journey that can clog up your fuel system, so adding a system cleaner every now and again seems like a worthwhile investment in preventative maintenance to protect my engine against future damage.

After the initial 500mL dose of Redex  Advanced 5-in-1 system cleaner, I have been adding a small 90mL bottle of Redex petrol system cleaner every time I have filled up my fuel tank for three months. In my situation, I don’t think I really need to keep using the additives at that rate in my car. I am likely to add a 90mL bottle every month or so and probably use the concentrated cleaner every six months to keep the Mazda’s fuel system sparkling clean. If I had an older car (or when this one starts getting older and higher-mileage), I’d keep using it every tank or at least every other tank.

Everyone’s circumstances are different, so your mileage may vary (both literally and figuratively!). The single-shot 90mL bottles are about £3 each, or you can buy a larger reusable bottle that will save a bit of money and reduce plastic waste, so it’s certainly worth giving it a go to see for yourself. The big bottle of concentrated cleaner is about £17, but if you use that every six months then it’s still less than £3 per month, which seems like a sensible measure to keep your engine operating at its best.

If you have any questions or comments about this trial, fire away in the comments section below and I’ll answer them as quickly as I can.

This review is sponsored by Redex.

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Stuart Masson
Stuart Massonhttps://www.thecarexpert.co.uk/
Stuart is the Editorial Director of our suite of sites: The Car Expert, The Van Expert and The Truck Expert. Originally from Australia, Stuart has had a passion for cars and the automotive 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.