FWIW..... My old 1990 slushbox (chipped and custom exhaust) got its best mileage above 75 mph....by my calculations there was a sharp increase in mileage somewhere between 75 and 80. My theory is the lil 3.6 was really getting deep into its torque band (best effiecency range) around there somewhere. I observed mileage as high as 28mpg, hand calculated.