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JCummings 01-13-2013 05:45 PM

Help with Tire size
I am looking for a knowledgable person who can explain to me how to figure out how changing tire size affects final final gear ratio. I understand the concept but can not figure out exactly how to do the math. I am looking to pick up a set of smaller drag tires for the truck, but I am not sure what size I am after.

chefred112 01-15-2013 08:55 AM

the sweet spot for drag tires is about 28"...I run 28" Hoosiers or 325/45-17 M&H drag radials..

chefred112 01-15-2013 08:57 AM

..and I always forget to change tire size at track, dang ..I don't think it affects gear ratio as much as it affects shift points..

chefred112 01-15-2013 09:02 AM

here is a simple calculator.... plug in stock tire diameter and current gear ratio and new diameter to compute new ratio..

chefred112 01-15-2013 09:06 AM

but, plug stock tire dia in 'new' tire dia and proposed drag tire dia in 'old' tire dia field ... the guy mislabeled the fields...

JCummings 01-15-2013 06:16 PM

Thx guys. Are you running 1/4 mile or 1/8th? What rpm and gear are you crossing the traps at?

chefred112 01-15-2013 07:19 PM

run both the 1/8th (7.8x) and 1/4 (12.1x), I know I'm in 3rd when i cross in the 1/4 and I think I do in the 1/8th as well as i don't think I can be in 2nd at 90+ mph. And I want to stay out of 4th at the 1/4...

JCummings 01-15-2013 09:24 PM

Fred, I only have a 1/8th mile track near me. I think that I would like to be topping out 2nd prime when I am crossing the finish. What are your thoughts? I know that you have much more experience than I do. Also, I plan on installing a super charger within a year. I am planning aloof my mods around that setup. Thanks again, joe

JCummings 01-21-2013 07:47 AM

Anyone have any knowledge on this matter?

chefred112 01-22-2013 07:08 PM

I hope 2nd prime is more stable in your 4th gens then it was on our 3rd gens...we would at all cost try to avoid 2nd prime because it eats transmissions (I should know)..we would only be able to access on downshifts anyways, not on upshifts..

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