I took my gtx on a 750mile round trip a few months ago and averaged 10 miles to the gallon driving 65-70mph. Engine is a 528 bored .060 so 542 cubic inches with a hyd cam 242/242@50 with stagev intake with 2 650 carbs and 354 gears. Years ago had a stock 426 in it with factory 1970 hyd cam and took the same trip and averaged 14mpg.