Just for reference, you might be able to get a couple more mpgs, but I can't suggest how. I have an 84DL 244 auto with 320K- an early 84 with the unheated sensor, running Avid T4s at 32lbs 90% highway 100 miles a day, no load heavier than a full size spare, occasionally wound out in first gear. Been using synth oil before beginning accurate mpg spreadsheet a couple years ago. The mileage runs 24.5 in the summer even with A/C but drops to 23+ in RFG time.

The 89 auto tops 25 as a grocery getter. The 79 kjet auto barely crests 21 and its odo runs about 5% to its benefit.
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Art Benstein near Baltimore