Picked up Motorsport 7 a week ago and first impressions on the fuel system seems like it's reasonably close to irl.
Just did a test to see what the economy was with the G8 GXP with a fully upgraded engine and positive displacement supercharger and race transmission geared for top speed in the higher gears w/o an overdrive gear and smoother acceleration in the low gears and drove 39 laps going 60 mph, coasting until hitting 55 mph before speeding up to 60 again, not going below 50 on turns and speeding back up to 60 on straights on the 1.5 mile oval track(RPMs never exceeded 2000). 12.5 percent of the gas tank was used by that point and the conversation to the IRL cars 19 US gallons gas tank came out to 2.375 gallons to 58.5 miles driven which came to be 24.63 US MPG. The IRL GXP has a rating for 20 US MPG on the highway stock but the same LS3 in a C6 corvette is rated for 24-26 US MPG highway with many claims that say they've gotten over 31 US MPG with their C6s and I've seen a few people on the G8 forums claim being able to get 23-26 US MPG highway in their GXPs. I was surprised it was as close as it was.