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post #1 of 6 (permalink) Old 06-21-2011, 01:15 PM Thread Starter
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This question has probably been asked several times before, but I did not find it so I ask again.

I have 3.08 in my Dif. I'm going to change it and I do not go Drag Racing, but I still would like it to be quicker. My normal driving is often between some places like 50 miles or more, but the time when I go crusing in town it would be fun to not always be left behind. The RPM today at 90 KMH (~55MPH) is ~1600.

The wheels are 235/55x17 and that will give the original diameter.

How much would the RPM be if I change to 3.55 or even 3.73?

The 3.27 does not seems to be worth to change to as it is relatively close to what I have today.

Have a nice day

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post #2 of 6 (permalink) Old 06-21-2011, 01:29 PM
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You won't want to go as high as a 3.73 without a tune. The rule of thumb is that the PCM can compensate in the shift schedule no more than two steps in gearing in either direction. After that and you might start to bounce off the rev limiter when shifting hard.

If you were to go a 3.55, that would increase your RPM by a factor of about 15%, or about 1840 RPM at the speed where you were running 1600 RPM.

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post #3 of 6 (permalink) Old 06-21-2011, 01:55 PM
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Good link that will let you plug in different combinations:


All the way to the bottom has the speed/rpm/tire size/etc. calculator.

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post #4 of 6 (permalink) Old 06-21-2011, 02:01 PM
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You will get left behind still if you choose 3:55.

At a min 3:73 will help with TL.

You have no way around it you need a tune to get anything out of these cars!

From the factory they are as lazy as most teenagers. LOL
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post #5 of 6 (permalink) Old 06-21-2011, 03:52 PM Thread Starter
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Great to have this forum. Thanks for your help.

I know that the T-Bird is lazy, therfore I'm going to put in PI heads plus things around it and then tune it all. The plan is that it happens this winter. Then the tune in spring tie.

In order to save money and a lot of time in the garage, I ask before I buy a 3.55 and a 3.73 and then try which one I like. It has to be set correct and that one I need some help with, which I have.

So far I have Vogtland springs. Koni adjust Shocks, thicker sway bar in rear, the new front sway is in the garage and will be changed when the engine is out. Poly bushings in rear, The rear axle frame totally welded, Half shafts in tool steel. Probably a lot of things forgotten that I have changed, but I do a little rebuild each winter and run on the summer.

Summer is great here now. Sun comes up 03.40 and go down 22.33

Again, thanks for your help. Drive safe

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post #6 of 6 (permalink) Old 06-21-2011, 03:53 PM Thread Starter
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Sorry, missed an M, tune in Spring time
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