I managed my best and read that your on about putting weight Ballast on some of the cars
I just did some testing, well time trialing in each of the GT Cars, with soft tyres, and 4 laps of fuel. I also monitored the amount of fuel used per lap seen as though this will be a long race at 99 laps and looking at 2 pit stops for some cars.
GT1
F550: 1:10:8 - Using 0.77 Gallon of fuel per lap
F575: 1:11:5 - 0.78 Gallon of fuel per lap
GT2
C5R - 1:12:2 - 0.72 Gallon of fuel per lap
Viper (Force One - 655HP) - 1:12:5 - 0.76 Gallon of fuel per lap
Viper (636HP) - 1:12:9 - 0.75 Gallon of fuel per lap
GT3
Z4 GT3 - 1:16:3 - 0.58 Gallon of fuel per lap
SLS GT3 - 1:16:1 - 0.61 Gallon of fuel per lap
It seems in some cases you pay for speed but for not as good MPG. The F550 and 575 are no way competative for each other, i found the F575 to be really bad on tyres especially compared to the F550. Perhaps use a different car for the GT1 Class?
Hope i've been of some help