I suspect that fuel consumption with a 650 will be similar to with a 500. If you drove at an equivalent speed then your 650 won't be thrashed as much as a 500 would be. This would negate the slightly higher consumption of a bigger engine. If you thrash the 650 you will go faster than with a 500 for about the same consumption because the car is lighter than a 126. I'll let you know for certain in a few weeks.?