If you mean the equivalent of BP's "Ultimate" Diesel.....
It depends very much on your car and its usage. They say it will increase performance, promote a cleaner engine and create less pollution.
I am assuming this is if you use it all the time, it would be pointless putting the odd bit in now and then.
I put £50-00 worth of Shell V-power diesel in yesterday for my drive back from London. It was 10p/litre more expensive than the bog standard fuel. I found the car ran much better on this diesel. However, do I get a better MPG to cover the extra cost?
Honest John, in the Telegraph, seems to favour the "super" diesels, both BP and Shell, for regular use. Assuming "bog" diesel was £1.40 per litre, and your normal fuel produced 40 mpg, at £1.50 per litre you would need to get 43 mpg+ to break even. There could, of course, be other advantages, in terms of wear and tear, servicing costs, etc.
"Super Diesel" probably isn't a rip off. It contains a higher proportion of synthetic diesel and has a higher Cetane Rating ( simply put a measure of how well the droplets burn under compression). The fuel produces less soot which should result in fewer carbon deposits in the engine thus improving "breathing".
The fuel is less dense than normal diesel and has less energy per unit volume.
Modern diesel engines have particulate filters which can prove troublesome if the engine is never worked hard. These fuels may help.
Better mpg ? This is difficult to compare as there are so many variables but if it is significantly better it should be obvious.
My Dad has been using ultimate or super unleaded for years based on the priciple of efficiency and cleaner engine. His mpg is slightly better on the whole so he feels he has broken even.
Unsure how much cleaning and other side affects it does, but I do know when I tried it for a while my mpg remained similar to before. Tend not to bother now unless i pull up to the side of the "wrong" pump.
Providing you Vehicle's filters are changed on a regular basis, your Injectors are clean & not depositing black smoke your motor should be running well, add RedX to a full tank of derv every couple of months when you fill up.
A few things can increase the fuel Consumption Blue, the height of your van, Mirror size, Under inflated tyres, Roof Bars / Rack, badly serviced motor.
TWR, the car is between services, tyre pressures checked weekly, driven in a similar manner at all times, and the roof bars are a permanent fixture to the vehicle, oh, and I haven't changed the mirrors either.
pdq - that can bugger up the chemistry of the fuel that you have been running on to clean the engine. Use a Shell or BP, claims in the States indicate >5% mileage improvement...and having seen early results, this is very much the low end. IMO, I would work tfrom 7.5 to 10 per cent depending on age and set up of the vehicle.
I have done a long running experiment with super diesel against normal and have found (that driving in exactly the same way) that the super gives 5 - 6 miles extra per gallon which represents 15.60% -18.75% increase in mileage. However, using super and cruising under 65mph on motorways produced an increase in mileage per gallon of nearly 25%. I do not know what the super does to the engine but if you want to save on fuel bills you can achieve 20% - 25% more mileage just by driving under 65mph.
I use Shell regular fuel save diesel and find it gives me better mpg than supermarket fuels - VW Passat Estate 1.6 Blue Motion 70+ mpg on a run and 65 over 20k miles and Isuzu Trooper 3.0 diesel Auto around 32mpg, down to 28 when towing. I have not been able to detect any significant benefit when I have tried premium fuels but I do appreciate that independent testing shows some marginal improvement in MPG and but only Shell appears to burn cleaner - I therefore try to run around 500 miles on Shell V Power prior to taking a car for an MOT