Swap engine - tire size = what gear?

Old October 14th, 2018, 07:05 PM
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Swap engine - tire size = what gear?

If I swap an Olds engine from a late 60's early 70's car with stock 14" wheels, 300hp+ motor & 3.08 stocking gearing... and I am putting it into a late 70's car with stock 15" wheels & 2.73 gear - should I account for the tire size difference to match the original gearing of the motor?

In other words, switch to a 3.23 gear with 15" wheels that is equivalent to a 3.08 gear with 14" tires.
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Old October 15th, 2018, 05:38 AM
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Wheel size is irrelevant to rear end gearing, tire diameter is what gearing is based on.
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Old October 15th, 2018, 07:25 AM
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Apologies your correct. the 14" stock was 26.4"

My TA tires are stock size... BFG's are 27.4" so 1" difference.
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Old October 15th, 2018, 08:14 AM
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Originally Posted by Via3d View Post
Apologies your correct. the 14" stock was 26.4"

My TA tires are stock size... BFG's are 27.4" so 1" difference.
Math is your friend...

Simply ratio the diameters. 27.4/26.4 = 1.04 (ie, a 4% difference). 1.04 x 3.08 = 3.20 gearing for the same effective ratio at the tires. 3.23 is the closest available ratio.
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Old October 15th, 2018, 09:15 AM
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I like the term you used Joe "effective ratio".

That is what my main question is, should I aim to achieve the same effective ratio going to a 3.23 ? (that was setup with the motor originally with 3.08 gear and 26.4" tires)

Assumption being this was the performance setup from the Olds engineers that may be the the best overall street cruiser/highway setup ???
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