I have been using a veggie mix in my 21/4 diesel now for about 6/9 months and it serves me well, but for the last 2 months I've been adding acetone to the mixture. Not just has it upped the MPG, I now don't have to change down gear as much on hills.
I have friends who are using this in their petrols and it's even better in there's
It's a cheep enough way of getting a bit more power up hills and if it helps with the MPG, well you can't lose.