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My '87 745T with the M46 manual tranny gets 27 - 30 mpg on the highway during the summer. The mileage drops to 25 - 27 during the winter. My wife's '88 745T with the AW71 automatic tranny gets 25 - 28 highway during the summer, and 22 - 24 in the winter. City driving is a different story. The '87 does about 25 mpg city during the summer, and about 20 in the winter. The '88 does about 21 mpg city during the summer, and maybe 18 during the winter.
I suspect the mileage difference is mostly due to the tranny differences. Except for the fact the two cars have different turbo charges, (Garrett T3 for the '87 vs. a Mitsu. for the '88), the engines are virtually identical. Maybe the roof rack on the '88 is also hurting me a little. The arguement I hear for not putting a lockup tranny on the automatic turbo cars is to allow the engine to rev a little more to get the turbo spinning when the gas is pushed in. Thing is, a manual tranny is the ultimate lockup tranny, and it works just fine with a turbo. I think the lockup torque converters in the automatic trannies can't take the torque, so the issue is more of durability.
One thing I noticed in your post is you are using 89 octane gas. The owners manual I believe calls for 91 octane. With lower octane, the knock sensor may be kicking in and calling for less ignition advance. This could result in lower efficiency, hence lower gas mileage. I always run 91 octane, pay the difference in gas, and live with it. Cars seem happier. And they run great, and give reasonable mileage for turbo cars.
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