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My 83 will still get ~30MPUSG on the highway if held at about 60-65MPH. I could get 32-33 in the old 55MPH days! Here's what I do, and you could try....
NGK BPR6ES spark plugs gapped to Volvo's spec at .028, and new Bougicord wires.
Mann air filter replaced every 20k mi, sooner if it looks dirty.
Throttle body cleaned up.
92C thermostat.
Base ignition timing at 12deg Volvo spec.
Valve clearances checked and within spec.
Mobil-1 synthetic 5W30 in the engine.
Redline MTL in the M46 gearbox. (side benefit: reduced OD slipping when hot)
Redline 75W90 rear axle lube (notably less "stiff" on cold winter startups).
Tires are slightly oversized 195/70-14 at 32psi. (I've never detected any difference in MPG by running higher pressure, even if theory says it should - but then I've only varied from 28-36psi)
Airbox preheater checked and working OK (you can disconnect/bypass this in AZ).
Given everything in decent shape, the biggest factor on fuel economy is directly under your right foot.
--
Bob (son's 81-244GL B21F-M46, dtr's 94-940 B230FD, my 83-244DL B23F-M46, 89-745(LT1 V8), 98-S90, 77MGB and four old motorcycles)
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