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Author Topic:   Gear ratio change w/tires
flatfoot27
Dirt Maniac

Total posts: 103
posted March 27, 2005 07:29 AM  
Can someone tell me how to figure the change in gear ratio when I use diferent size tires. I cureently run 273 gears with 215-70-15 tire. I want to run 308 gears with 235-70-15 tires. Is their a way to figure my final ratio? Thanks

racer17j
Dirt Forum Champ
Total posts: 5390
posted March 27, 2005 08:21 AM  
I have a formula for finding how rpm and gear ratio relate to wheel speed
Using this formula you can tailor your gear to the needs of your cam

rpm div by final drive ratio=wheel rpm
to increase rpm while keeping the same wheel rpm
----------
final drive ratiox1 div (percent increase) = new ratio
new ratio x wheel rpm = new rpm

example
7000 div 5.67=1235
5.67x1.20=6.80
6.80x1235=8,400

flatfoot27
Dirt Maniac

Total posts: 103
posted March 27, 2005 08:26 PM  
I want to run the bigger tire to put more tire on the ground. Right now I'm running around a 688 final and I would like to get that down to a 700 or 713. But with stock chevy gears my choice is either 273 or 308. The 308 with my tires I'm running now would be a 783. So I was wondering if I go to the 235-70-15 tires and the 308 if I would be in the 700 range. I can go down as far as a 750 and be all right. But thats turning the 406 a little hard.

Speedway65
Dirt Freak

Total posts: 344
posted March 28, 2005 08:28 AM  
I have a formula i will post it on Tuesday morning!

Speedway65
Dirt Freak

Total posts: 344
posted March 29, 2005 07:14 AM  
Old roll out divided by new roll out times old rpm =new rpm
Example
old roll out-82
new roll out-86
old rpm-6100
82divided by 86=.953x6100=5816

so the new RPM will be 5816

This is just one way to help figure it out,there is another formula but i forgot it at home and i always post from work.

Hope this helps
Jeff

[This message has been edited by Speedway65 (edited March 29, 2005).]

    

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