How Rare? 1976 Dart Lite

Not just the weight but the timing and lean burn tune on the carb. I had a 71 Sebring with 318 auto and highway gears that could get well over 20mpg on a trip from Washington to California at interstate speeds. A 6 a body with overdrive and the right tune at the 55 mph they used to do the mileage test on I believe could easily be in the 30s.