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1965 GT Fastback, original 9", t5 trans
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Discussion Starter #1
I have a 1965 Fastback A code true GT with a 91 T5 and an original 9" rear end
Thanks for any help. As best as I can determine, the 1991 Mustang T5 has the following ratios:

1st. 3.35

2nd 1.99

3rd. 1.33

4th 1.00

5th. 0.68

The tire size is 205/60R15. I measured the circumference at 77 1/2 inches.

The original 9" in this car had a 3.89 ring and pinion. The current rear end has a Richmond Gear 4.11 ring and pinion. The car is a rocket out of the hole but I run out of gears pretty fast. The recommendations I received from RG is:

If you have a 4.11 ratio now you are running 2654 rpm at 70 mph when in od.
Going to a 3.89 ratio you would be running 2518 at 70 and with a 3.50 ratio you would be running 2266 at 70.

It makes me wonder if I have the 91 T5 ratios correct as it seems to have a higher rpm at 70mph.

Any thoughts or recommendations?
 

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Replace your 3.35 transmission gear set with a 2.95 gear set. Replace the .68 OD with a .80 OD. Change your rear end to 3.70, 3.89 or 3.91. That's easy enough!
 

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What rpm are you running at 70? Those are the correct ratios for a 91 GT. The 90s fox body 5.0 mustangs came with 2.73 and 3.08 gears so the 3.35 first was not too short for them. I had a 90GT with 2.73 gears, T5 and it ran 2000rpm at 82mph.
 

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1965 GT Fastback, original 9", t5 trans
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Discussion Starter #5
Thanks for your replies and sharing your experiences. It's giving me a lot to think about. I'm leaning to a 3.50 rear end. I don't want to lose to much of the bottom end and don't do much freeway driving so a little more on the top end would be fine.
 

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With stock 5.0L/T5/3.25’s in my coupe, it will still spin the tires. I will eventually change to 3.40’s in near future. 3.50 would be more than plenty.
 

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Any plans to change to a larger wheel and tire? If you go to, say, a 225/45/17 or similar wheel/tire set-up you will pick up some tire height (about half an inch), which will do the same as moving to a numerically lower (taller) rear end. Just factor that into your axle ratio decision if a bigger tire is in your plan. I am running a 3.89 in my 9" with a T5 and it's a rocket in the lower gears, but still reasonable on the freeway and I am running a 17" wheel. Good luck!
 

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I created a gear ratio calculator in XL. I would post it for everyone, but the file extension is not allowed. PM me your address and I'll email it to you. Here's what it looks like. You enter your tire diameter and axle ratio and it will tell you the RPM at a given speed. It's easy to plug in ratios for different transmissions too.


757012
 
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