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Old 10-30-2012, 06:33 PM   #4 (permalink)
EcoModding Lurker
Join Date: Oct 2012
Location: Iowa
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Putting this in here so I can remember later on:
1984 EPA rating:City-13 Comb-15 HWY-20
Now the new motor is a 1987 5.0 H.O with a 1988 harness and computer
1988 EPA rating:City-14 Comb-17 HWY-23
My car has a 3.08 rear gear and the 1988 manual should come with either 2.73 or 3.08. I am just writing them both off as 3.08
My car currently has 225/70r14 and the stock tire on a 88 is 225/60-15
The 14" tire is 26.40" and the 1988 is 25.62"'
So if I stay with that size tire the operating RPM would naturally be lower. I can even go bigger if need be. (14" tires are hard to come by in this size)
Another thing I have going for me is the stock gear ratio of the t-5. I got this off of the interwebs so might not be 100% correct.
T-5s by year
84 1st 2.95 2nd 1.94 3rd 1.34 4th 1.00 5th .625

85-89 1st 3.35 2nd 1.93 3rd 1.29 4th 1.00 5th .675
5th gear has a lower ratio in the 84 versus the 88. Thus improving over a stock 88.
To recap the rear ends are the same, I am running close to 1" taller tire on the rear slowing rpms at speed, and my transmission is .050 lower than a stock 1988 Mustang GT. So given the EPA "rated" the 1988 at 23 HWY I believe with those 2 things I should be better than 23. Whether thats 25 MPG we will see.
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