Well, on my trip to pa last weekend I got 26 mpgs on the way down and 24.5 mpgs on the way back. This is with my 5.0, it's pretty much stock save for the full exhaust and the stock "sport" 3.08 gears, lol. I had my car on cruise at 75. But, when I used to drive back and forth from college I got about 20-21 mpgs, I never was able to do any better or worse, it included a little bit of city driving and mostly on two lane roads at 60. As long as your '81 had a tune up recently I would say it would be better than the dodge.