Tesco's all the way for my Tubby. Every test i have seen Tesco has consistently out performed all its come against. However at the moment its not much cheaper than optimax, however i use Tesco's out of preference. It used to be Tescos was a godd 5-6p a litre cheaper, but only tends to be 1-2p difference now. As for the mpg, unless you were on a rolling road, running in perfectly identical conditions on each fuel, how can you honestly tell the difference? 20-30 mile differences could be anything from getting stuck in a bit more traffic, colder weather, heavier use of throttle, anything.