I drove an 80 Diplomat every day in 96-98. It was a stock 318 with lockup converter and high gears-2.45 I think. Only mods were electronic ignition and dual exhaust with glass packs. It got 27mpg Canadian (22 1/2 US) on the highway in 90 degree heat with the A/C blasting. Great car. Shoulda kept it.
Then I got a 86 5th Ave with same driveline but lean burn still functioning. It would get 25mpg Canadian (20 1/2 US) on the highway. The low first gear gave them enough power to pull out without feeling sluggish. Great cars. Both were super reliable.