Track Tac

tire change ratio

J

Jim46

Member
Im going from a 33.75 Vega with a 66 gear to a 37.25 Hoosier . what gear will I need . does any one have a ratio chart for these changes
 
The goal I believe is to keep engine rpm and ground speed relatively close after tire change.
To calculate, need to know peak engine rpm, and both gears on original, along with the information provided in original post.
The formula would look like this,
Basic setup.
6000 rpm
15/60 gear ratio equals 4 to 1 ratio
33.75 tire rollout original
37.25 final tire size

Engine rpm/gear ratio = axle rpm.
6000/4 = 1500

Axle rpm times original tire rollout equals inches per minute distance covered.

1500 × 33.75 equals 50625 inches per minute

Distance covered divided by new tire size equals axle rpm.
50625 /37.25 = 1359.06 rpm

Engine rpm divided by axle rpm equals new gear ratio.

6000/1359 = 4.415
15 × 4.415 = 66

So 15/66 or equivalent would be starting point using the numbers I substituted.
 
Simplify this… going bigger circumference on tires is equal to taking teeth off the rear gear..
Going smaller in circumference is equal to adding teeth to rear gear..
So with 1/2 change I would think only 1 tooth or 2 at the most..
Now if that’s only RR change and the left does not change then this changes everything
 
Jim46 said:
Im going from a 33.75 Vega with a 66 gear to a 37.25 Hoosier . what gear will I need . does any one have a ratio chart for these changes
Click to expand...
There's a calculator somewhere on our old website that you can download and do this. KartCalc I believe it was called...
or do the math longhand.


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Brian Carlson
Carlson Racing Engines
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www.CarlsonMotorsports.com
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