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OK, I know if you change your tire size, it will mess up your speedometer reading. AND if you put SE gears on a 3.5/3.2 without the 2.7 tcm it will mess it up too, RIGHT?

OK, with that being said, could you buy different size tires INSTEAD of the TCM to counter the gear difference? and IF so, what size?
 

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7% larger.......probably too big for your wheel wells........PLUS upping the size of your tires will offset the effect of the gears, and your car will accelerate exactly the same as it currently does.
 

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jfsram said:
A different tire size will only change the speedometer reading if the overall diameter is different.
Yes, if you changed the tire size and changed the rim size you can end up with exactly or close enough total diameter as stock. For instance, stock SE size is 225/60/16. If you went up to a 17" rim but changed the tire profile from 60 to 50, then you'll end up very close to stock and have no affect on the speedo.

As far as I know, changing the gearing shouldn't affect the speedo because I believe the pcm reads the output shaft speed to come up with road speed. Since the output shaft speed will be affected by changing the gears, so is the signal being read. This is different than a rear wheel drive car, where the speed sensor is part of the output shaft and the differential is in the rear (separate from the tranny), so when you change the gearing in the differential it will change the speedo. Any tranny guys on here familiar with ours know what exactly the pcm monitors to determine vehicle speed?
 

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The tranny output sensor is where the car gets the signal to calculate speed. Since that's before the gears it will change the speedo - the car has no way of knowing the tire size or the gear ratio.

To offset the gear ratio you would need an 18" tire size of 255/50/18 or 245/55/18 - 19" would be 245/50/19 - but those are only kinda' close, there's still some variance. I'd be a little worried about rubbing the wheel well at that point ...
 

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D76G12 said:
The tranny output sensor is where the car gets the signal to calculate speed. Since that's before the gears it will change the speedo - the car has no way of knowing the tire size or the gear ratio.
Ok that was the part I wasn't sure about. Then it is very similar to a rear wheel drive, where the speedo signal is before the differential, so any differentiakl changes will affect the speedo. I knew which sensor but wasn't sure where it was located as far as before or after the diff. Thanks for the clarification and good to know.

D76G12 said:
To offset the gear ratio you would need an 18" tire size of 255/50/18 or 245/55/18 - 19" would be 245/50/19 - but those are only kinda' close, there's still some variance. I'd be a little worried about rubbing the wheel well at that point ...
 

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froggy81500 said:
Ok that was the part I wasn't sure about. Then it is very similar to a rear wheel drive, where the speedo signal is before the differential, so any differential changes will affect the speedo.
Yep - except the old RWDs had a way to keep the speedo right when you changed differential gears - a simple change in the speedo cable gear would do it. Treps are a whole different ballgame as some people are finding out ...
 

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D76G12 said:
Yep - except the old RWDs had a way to keep the speedo right when you changed differential gears - a simple change in the speedo cable gear would do it. Treps are a whole different ballgame as some people are finding out ...
Yeah I know. Simple change of the gear on the speedo cable or the gear in the tailshaft and problem solved. Nowadays, its all computerized.
 
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