My 940 (non-turbo, automatic) and the 83 240 will both do about 30MPG (US) at a steady 70mph. The average turbo is unlikely to match that, but should still be in the upper 20's somewhere. I suspect the PO got his "30" going down I-5 with a tailwind!

Cleaning the throttle body is a good thing to do for smooth idle and clean running at low throttle openings, but is unlikely to affect highway fuel economy. With all due respect to those who like fuel injector cleaners-in-a-bottle, I've kept careful track of before and after results and found zero benefit to any of them. Others here have shared that experience.

Do put in a new air filter as suggested, and a fresh set of spark plugs, and perhaps wires. If driving at a reasonable clip still doesn't bring the fuel economy up, have the O2 sensor function checked.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD and 89 745 (LT-1 V8); hobbycar 77 MGB, and a few old motorcycles)