Best Rear Gear for T-5 Swap

66ford289

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Apr 10, 2005
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Right now I just have the stock c-4 2.80 setup, I have the T-5 sitting in the garage right now. If anyone is running this setup what are you using? Don't plan on doing too much highway driving so I am open to taller gears. Will stepping up to something like 3.55's make that big of a difference?
 
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Stock 5.0s with T-5s came with 2.73s or optional 3.08s and automatics had the option of 3.27s. Believe me, with a 25.5 to 26" tire, 2.73s are not that bad. I would say that in a classic application, 3.50s will be a good middle ground. If you truly do very little highway driving, 3.70-somethings will be better.
 
This formula might help.

MPH=(rpm x tire diameter) / (gear ratio x 336)

The tire diameter is in inches, and the gear ratio is overall (multiply the trans gear by the dif gear).

A little algebra will rearrange the formula to solve for rpm, gear ratio, etc.

Hope that helps
 
I would say about 3.25 or 3.5 to one,unless you are using slicks or something. I put in 3.89 to one in my car when I thought I was going to put in an automatic.I put in a five speed instead, and with the 3.27 to one first gear it is hard to leave hard without blowing away the tires.If your car is stock then first gear times rear is probably something like six to one.With a five speed and the same rear gears it would already be more like nine to one. Thats about fifty percent more without changing the rear. If you try that first and decide that its not quick enough off the line, then add more rear gear(3.5 or 3.25). Don't do what I did by changing both at once, all you'll do is make first gear useless.
 
MitchGT said:
I would say about 3.25 or 3.5 to one,unless you are using slicks or something. I put in 3.89 to one in my car when I thought I was going to put in an automatic.I put in a five speed instead, and with the 3.27 to one first gear it is hard to leave hard without blowing away the tires.If your car is stock then first gear times rear is probably something like six to one.With a five speed and the same rear gears it would already be more like nine to one. Thats about fifty percent more without changing the rear. If you try that first and decide that its not quick enough off the line, then add more rear gear(3.5 or 3.25). Don't do what I did by changing both at once, all you'll do is make first gear useless.
I agree with Mitch.

I have an '85 GT with a mostly stock 5.0 (tired too, with well over 100,000 miles on it) and 2.73 rear posi. It's super easy to fry the rear tires at will in first. In second they chirp if I wind first a little, and they break loose for more than a chirp if I bring first up to 4k or so.

If you have a really weak motor, or you're going to run slicks, a higher numerical ratio makes sense. 2.73s are great for mileage and showing off with street tires, though. Not that I advocate spinning the tires on public roads. :nono: :D :banana:
 
What is the your 5th gear ratio? This will make a big difference in your rear gear selection.

T-5's come with 5th gear ratios of .90, .80. .70 and .63. If your 5th gear is .9 or .8, a 3.25 or 3.55 rear will provide a nice final drive ratio, but if your 5th gear is .63 you may want to run 3.73 or even taller.

Tim
 
65 fastback said:
What is the your 5th gear ratio? This will make a big difference in your rear gear selection.

T-5's come with 5th gear ratios of .90, .80. .70 and .63. If your 5th gear is .9 or .8, a 3.25 or 3.55 rear will provide a nice final drive ratio, but if your 5th gear is .63 you may want to run 3.73 or even taller.

Tim
5th gear ratio only matters if you want to run slicks at the dragstrip IMO.
 
Hack said:
5th gear ratio only matters if you want to run slicks at the dragstrip IMO.

I wouldn't think you would even make it into an overdrive gear in a 1/4 mile pull. The 5th gear shouldn't matter at all at the dragstrip.

But the rear gears and 5th gear will make up your final drive ratio in highway driving.

Tim
 
65 fastback said:
I wouldn't think you would even make it into an overdrive gear in a 1/4 mile pull. The 5th gear shouldn't matter at all at the dragstrip.

But the rear gears and 5th gear will make up your final drive ratio in highway driving.

Tim
My point is that with 2.73s your final drive ratio will be plenty slow, so the final drive ratio doesn't matter. I run about 1500 rpm at 65 mph, which is plenty slow. I usually want to shift down to accelerate on the highway.

You might as well use 2.73s unless you're going to the drag strip or you're running on 6 cylinders. With a V8, stick and 2.73s you should be easily able to spin street tires, hence higher numerical gears are not useful.
 
My 2000 GT has a T5 tranny. I have 410's in the rear and it's fine. It didn't even barely effect the gas milage. I think because it's easier now for the engine to get the car moving or something like that.
As far as rpm's and speed. It turns 3k at 90mph in 5th. So as you can see I kept my freeway speed but gained a lot off the line. It's great! I love'em and would never go back!
I hope that helps ya with your decision I know it was a tough one for me too but I'm so glad I went with the 410's.
 
The overdrive ratio is so tall you can run 5.14's without harming it's highway manners. 3.73 is almost ideal to me, it turns 2500rpm at 85 mph in 5th and yet 5th is still usable down to 50mph. If you went with 3.00's you couldn't use 5th very often with a carb'd motor and any sort of a cam.