If you have different gears, but no PCM prog.

[DominusEtDeusNatus]

Lets say you put in 3.42s with a 3.08pcm. Now you also put a taller tire. This could make the gears into a equivalent of 3.23s on stock height tires. Taller tires help to correct it a bit. Higher gears make car think it's going faster, taller tires do the opposite.

[12secondv6]

To explain this I will use my car:
Computer programmed for 308 gears
323 gears in the rear
245 50 16 tires in the back.
Automatic tranny

A taller tire will travel a farther distance with once revolution compared to a shorter tire.
It will 'technically' make the car think it has a lower gear ratio in the rear axle.
Hmmmm, that really didn't sound right.....
OK, you have certain preset gears in your tranny (either manual or auto). You then have a certain gear in your rear. One way you can modify the 'gearing to the ground' is taller or shorter tires.
Here is are two examples:
1) Monster trucks, they must have lots of 'torque' to the wheels. They have lots of power but they also run monster gearing. To compensate they run REALLY tall tires. If they ran much shorter tires their top speed would be drastically cut.
2) nitro funny cars (john force) You know how they run really wide and tall tires in the back right? They also have monster gears in the rears. They use much taller tires (also they grow at higher top speeds) to obtain those insane top speeds.
Computer programmed for 308's, 323's in the rear and I have 245 50 16 tires in the back.
First let us figure out the tire height of the stock tire.
In inches a 245 50 16 tire has a height of 25.65 inches.
I plan on going to a 26 inch tire.
Not much of a difference but it should in theory it should help me compensate for a computer programmed for 308's but 323's in the rear.
It may not be exact but it should make it much closer than being off by 5%
Hell, maybe it might be around 1-2% off so:
If it is off by 1% then 58 rpm's
If it is off by 2% then 116 rpm's
If it is off by 3% then 174 rpm's
If it is off by 4% then 232 rpm's
If it is off by 5% then 290 rpm's
So lets stay with 2% for the following statement:
I presently shift the follwoing rpm's
1 to 2 upshift at 5800 rpm's
2 to 3 upshift at 5500 rpm's
3 to torque converter lock up 5200 rpm's
Each shift I lose about 300 rpm's
2% difference put in
1 to 2 upshift at 5800 rpm's
2 to 3 upshift at 5684 rpm's
3 to torque converter lock up 5568 rpm's
Each shift I lose about 116 rpm's
I would (with a taller tire) use more rpm because the taller tire would slightly compensate for the msimatched computer programming of 308's to 323's in the rear.
Now let me throw you for a loop :D
Taller tires may slightly grow at high rpm's due to very fast rotation and that tire height might slightly grow a bit higher that 26 inchs but that I think is a bit above me.

I think what I posted makes sense in my head :D and I hope it makes sense to you.

[AMinor65]

I like this calculator. Has some of the stuff you guys been talkin' about.

http://www.venommotorsports.com/howt...ldiameters.htm

[ March 31, 2002: Message edited by: AMinor65 ]</p>