As for your comment on the vacuum advance. Yes it is only supposed to have vacuum upon acceleration. Not all the time. Is the trans shifting OK and when its supposed to. If its not down shifting at the right time it could drag you milage down by lugging the motor. Or if its shifting to early.
High gears are good for milage if you do mostly highway driving. If you are just around town in my past experience lower gears actually give you the best milage because the motor doesn't have to work all the time to accelerate all the time Were as on the highway high gears = the motor doesn't have to work all the time to maintain a constant speed.
My 83 K 10 305, SM 456, Np 208, with 2:73 got 19 plus MPG on the highway. but if I ran around town allot it was only about 13 MPG or so.
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