That car was designed to run on 100+ octane fuel, and today is only available as racing fuel and very expensive...anything else will blow the engine. So your not going to drive it enough to worry about gas mileage..
Since most of it's driving is down down the 1/4 mile at full throttle, expect about 2 miles to the gallon or 4 runs max..
If you have the cash to own a true GT500 and can afford the risk of driving it daily. You should not be concerned about the fuel costs. The insurance alone will would be a ton. But for arguing I bet you could sneak 12 or so on the highway. I doubt you will get double digits in town. How about this I will pay for all the gas if I get to drive????????