OK, assuming the Dart has a 16 gal tank and it can pull all the gas out of it, and it got 16 mpg, that means he went 256 miles. Divide that by the 3 hour drive, and it averages 85.33 mph. I believe 16 mpg isn't bad mileage for a slant 6 at 85 mph for 3 hours. I doubt anything in the 1973 era would do much better.

One also needs to consider the road conditions of that 3 hour drive. I would be concerned about how much gas would remain after a 260 mile trip in anything I own, especially at 85 mph. Gene