The 1971 Ford GTHO Phase 3 got 8 miles to the gallon at 100 mph:
"The fuel bill will also cause a few headaches - hard-pushed, the car does a neat 8 mpg. With slightly quieter driving it- will give 10.4 mpg - don't expect much more. As a fine, ultra-high performance car that will be beaten by nothing under $20,000"
https://www.whichcar.com.au/features...phase-3-review
So 10 mpg seems a bit thirsty.