Overdrive ratio for highway economy

Discussion in 'Drivetrain' started by SIIaCanuck, Oct 21, 2015.

  1. SIIaCanuck

    SIIaCanuck New Member

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    I'm going to rebuild a T-5 for my SN95 GT with a 2.95 gear set. It's got 3.90 gears and I want to retain the rear gears but still get decent highway economy.

    The engine will have ported GT40p heads, ported Cobra lower, XE266 cam and Bassani mid-length headers & o/r x-pipe and Magnaflow cat-back.

    The 0.68 OD currently in it is too low so I'm considering the 0.63 or, if I retain the 0.68 fifth gear set from a 3.35 tranny, I'll have 0.59 with the 2.95 setup.

    Thoughts?
     
  2. B.mad

    B.mad Well-Known Member Preferred Vendor

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    I had a similar mindset you did, until i talked to modern driveline and said that you cant exactly pick and chose what 5th gear you want. The only time i've seen anything lower than .63 is with a 6-speed tranny. Unless you find someone who is selling to rebuilt a tranny with a different combo in the set, but i dont think that is possible. The .63 will give you better as mileage than the .68, but it is a very minimal difference. And even from the .63 to the .59 is not a big difference either. The RPM thing is a combo of 3 things: tire height, rear end gear, and OD gear. If you dont want to change the rear end gear, change the OD and tire height. If you dont want to change the OD, change the tire height and rear end. If you dont want to change the tire height, change the OD and rear end. I use this http://www.tremec.com/calculadora.php to figure out combos. Ideally you want to be at 1500-2000 at 65. that will give you the best gas mileage. More and you will see it decrease.
     
  3. duh09

    duh09 Legend

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    On highway, my 4.10 geared 95 GT did not get that much worse than my factory geared 94 GT, around 2-3mpg. If I sped up to 75ish it would begin to drop but doing the speed limit was not a huge difference. I was actually pretty disappointed with the 94 after being so used to the 95. Don't fear the gear!