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Question for Mr Eaton and others


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I've read that the final drive on the 91 Reatta is different (lower ratio) than the previous models. It has been said this was done to enhance performance. It is also true that the 91 models have larger wheels. Wouldn't this combination result in the same final drive ratio at the road? Has anyone done the math? Just curious.

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Although the 1991 used 16" wheel, compared to the earlier 15", the overall diameter is insignifient.

1988-1990 215/65-15 did 776 revolutions per mile.

1991 215/60-16 did 771 revolutions per mile. Roughly 1/2 of 1% differenct. So the wheel tire combination did not affect straight line performance.

Final drive.... this is a little more complex and I don't know all the answers. There are at least two ways of changing the final drive ratio (engine RPM in relation to the wheel RPM) on cars with front wheel drive. Since the transmission is under and not behind the engine, they are connected by a chain. Each end has a sprocket. By changing the number of teeth on the sprocket you can change the final drive ration. The parts book show a 35 tooth and a 37 tooth gear. I suspect you don't want to change this too much as this also screws with shift points etc since the change is on the front of the transmission.

Next is the final drive pinion. The parts book list 4 different ratio differentials, 2.84, 2.97, 3.06, and 3.33

Of note here is if you ever have your transmission rebuilt, and they actually exchange the transmission, somehow confirm you get the same final drive you started with, otherwise you might get one from a LeSabre with 2.84 or a Riviera 3.06

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<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">The parts book show a 35 tooth and a 37 tooth gear. I suspect you don't want to change this too much as this also screws with shift points etc since the change is on the front of the transmission.

Next is the final drive pinion. The parts book list 4 different ratio differentials, 2.84, 2.97, 3.06, and 3.33 </div></div>

I have an 88 model. Was there a standard ratio for that model year? How can we tell what sprocket tooth count and final drive ratio we have? Maybe a tag on the transmission or a code somewhere?

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