Hope this puts the debate to rest

No chance. In spite of any empirical evidence, all the naysayers (with the scientific knowledge of a duck) will continue to deny it, and rant on about how you, and I (and others) are either being conned by the marketing, and how HJ (who also recommends 'premium' fuels) is undoubtedly taking a backhander off Shell.

All the naysayers please note : only reply to this thread once you've put your tinfoil hat on. Don't want 'them' reading your thoughts, do you !

A couple of years back, I decided to experiment and see if premium diesel made any difference to economy in my 2011 Volvo V60 D3 (163bhp) with manual box.

I ran 10 consecutive tankfuls of Shell V-Power diesel (i.e. about 6000 miles over a 6 month period using NO other fuel than V-Power), and compared it with the 10 previous tankfuls of Shell's ordinary Fuelsave diesel (this is easy for me to do, as Shell is my nearest filling station and the same price as the local supermarkets).

So I'd done 6,000 miles in around 6 months of ordinary Shell diesel, followed by 6,000 miles in 6 months of nothing but V-Power.

I got a brim-to-brim average of 46.29 on V-Power compared with 45.12 on Fuelsave, with the same type of driving.

Or, to express it in percentages, I got a 2.6% improvement in economy for spending the extra 5.7% cost of V-Power (it was 8p per litre more at the time). I reckon that the 10 tanks / 6000 miles on each fuel was sufficient to reduce the influence of variables like ambient temperatures, type of journey etc.

There was no detectable difference in performance at all. I concluded that while V-Power made a small difference to economy, it wasn't worth paying the extra for, compared with the ordinary, cheaper Fuelsave diesel.

I hate to be yet another "naysayer", but I ran two six-week trials back-to-back in my car on Shell 95 and Shell 97, and the difference was less than 1mpg. Too close to give a definite answer as far as I'm concerned. And an awful lot of my driving is on the cruise control, so it's not really influenced by me. I convinced myself it ran smoother on the higher octane and responded better in higher gears, but that might have just been placebo! May be my car is just too old to produce a significant improvement. Round my way, Shell Fuelsave is only a penny more than supermarkets (and Esso is often the same price) but V-Power is a 10p more; it would need to improve by 3mpg to break even. But to be fair to HJ, I think he reommends V-Power as much for the extra additives over Fuelsave as much for any marginal mpg gains. I tend to stick some snakeoil in every 6 months or just before the MoT.

With Top Gear shortly to reappear on BBC and 'Top Gear' shortly to appear on Amazon TV, this sounds like a suitable article and test to be carried out scientifically on one of these programmes....

What perhaps some people are missing is that some cars are designed to take advantage (as some higher performance cars and many VAG-engined cars in the recent past [at least]) of higher grade fuels (e.g. by having a knock sensor that detects the fuel octane [or equivalent for diesel] rating and adjusts the engine to suit) which will provide better performance and/or fuel economy than lower grade 'standard' fuels.

Most cars do not have this feature, and will only benefit from the additional cleaning agents the likes of Shell and others use, which (according to HJ) helps keep the engine in better knick than standard fuels. No performance gain at all. I am mainly speaking from both personal experience with my petrol Mazda 3 (not the MPS), and more generally about petrol-fueled cars - I admittedly have less knowledge about which diesel-fueled cars may have sensors than can detect and take advantage of the higher grade diesel fuels on offer.

I personally noticed no discernable difference in either performance or mpg in my car when I filled up for (and throughout) my annual holiday to Cornwall (same place both times), including on the 300 mile plus runs there and back.

I personally find that an occasional 'Italian tune-up' (once the car is fully warmed up of course) does wonders for the engine to rid itself of any cr4p built up through town driving. Far cheaper (for me at least) than paying 7-10p a litre more for fuel all the time.