that's a huge variance... more likely that driving style has changed it, but there could be something wrong with the car i suppose.

my 2.0 slow does better on ultimate than on bp premium. i worked that out manually over several months. ultimate was always more efficient. someone else on another forum with the same car as me did the same test (without me knowing) and when i saw his results the figures were exactly the same to a tenth of a litre. so for teh 2.0 slow at least i think ultimate is better.