I occasionally put in 10 litres of Shell V-nitro diesel, and always notice the engine running more smoothly soon afterwards. Can't suggest that consumption improves though. I guess it cleans injectors and the EGR valve etc. So spending an extra quid or so that way instead of buying a bottle of injector cleaner for about £5 seems reasonable to me.

A bottle of injector cleaner can last a lot longer than one tank of super diesel - you don't have to use it all at once!

<< A bottle of injector cleaner can last a lot longer than one tank of super diesel - you don't have to use it all at once! >>

I did say I only put in 10 litres at a time. And the effect takes some time to wear off. So I don't think the overall cost is that different. The label on the injector cleaner tells you to lob it all in, which I never do.

If you're careful, you can buy a decent injector cleaner like Redex in a 4-dose bottle when it's on offer in Halfords for about £5 - £6. For the most part, the difference between a full tank of regular fuel and super fuel is about the same, £5 or so, thus the injector cleaner is 4x the value.

As we've said on many occasions, using superfuels is really only worth it if either:

• You own a car that is tuned to fully take full advantage of the performance benefits (as well as the mpg increase under normal loads) of that fuel, and even then most only get about a 5% increase in one or the other (not both at the same time). The fuel costs about 7-10% more (not to be confused with supermarket superfuels, which don't have anywhere near as much cleaning additives, hence why they are only about 3-5% more expensive than their own regular fuels);
• You do a lot of slower-speed urban driving and thus its beneficial to the engine generally, especially diesel car owners who can reduce issues with the EGR and DPF as a result of the extra detergents in the fuel;
• The car is old and/or hasn't been maintained well or driven sympathetically. It (similarly for injector cleaners) can help to reduce acceleration flat spots and reduce carbon deposits, which would make the engine run more smoothly and efficiently.

I seriously doubt that putting in one fifth of a tankful of superfuel would do very much - if I recall correctly, HJ himself said to see any meaningful effects, especially on the types of car I mentioned above, it normally takes 3-4 tankfulls (the same goes for the injector cleaners) of the branded superfuels.

Once either has been burnt during combustion, the effects will immediately disappear, but the benefits will slowly wear off if the car is used/maintained in an unsympathetic way as above and the deposits return at the rate they did before the treatment/superfuel use. Big difference.

<< I seriously doubt that putting in one fifth of a tankful of superfuel would do very much - >>

I usually add fuel to a fairly empty tank, so my 10 litres of superdiesel is not diluted too much by the rest. I don't think I can quantify the effect it has, but within a few miles, when it will have reached the engine, there is a noticeable softening of the engine note and a better throttle response. I therefore consider it something worth doing occasionally. I don't know whether 'injector cleaner' does any more than just that, so if the EGR valve also benefits, perhaps superdiesel is a better remedy?

I suspect a lot very much depends upon how badly gummed up all these components are - the injector cleaner probably has some positive effect on the EGR, as does the superfuel, but bear in mind both will do better before they are burnt in the combustion chamber.

Far better, if you can easily remove and refit the EGR, throttle body or MAF sensor that they be cleaned using specialist cleaning sprays, which aren't that expensive themselves and can provide a huge difference. Some cars, like my specific model, is more difficult to get to those parts compared to ones with other engines.

Using higher grade branded fuels may overe a long time give a decent effect, but my point is that you'll pay a considerable amount more for doing so than using the off-the-shelf additives and specialist cleaners. If you've got the money, aren't mechanically inclined or don't have the time, especially if you're annual mileage isn't that high, then using superfuels aren't so much of an issue. I also suspect they are more beneficial if you use them from when the car is new, not if you buy the car second hand and its had a hard life up 'till then.