Since you are planning on keeping your motor pretty much stock, go with a minimum of a 3.73! Go with at least 3.73 Especially if you if have an AOD and seriously consider 4.10's.
If you have a manual and are going to keep it relatively stock go with 3.73.
My car with the AOD and 3.73's gets 25.5 MPG on the highway at 75 MPH and about 27 MPG on the highway at about 60 MPH.
Since the 5-speed has a 5th gear(overdrive) ratio of 0.68 and the AOD's overdrive ratio is 0.67 I would expect you to get very similar MPG at constant highway speeds. Around town it would be a little different, but it shouldn't hurt you that bad.
Bottom line, it's WORTH IT to get 3.73's!
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1991 GT, AOD, Moroso Cold Air, 3.73s, Pulleys, 3-chamber(the left one fell off-NO time+No Money=College), FMS c-springs, and KYBs
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