I just got 26-27 on my last tank in a 65 mustang. The only modifications I have done are a Holley 1940 with a larger throttle plate (the carb may be from a 140 or 300), and a distributor from a 68. I have spent considerable time tweaking the distributor curve over he past few months, and it paid off. Before I got 18-20 on the highway at 70-75. The last tank I got 26-27 driving the same route. Even if the tank did not get quite as full the second fill up, and even if my odometer is off by 1%, that's still over 25 mpg. Oh, and the net elevation gain over this trip was about 2500 feet.