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Old 02-11-2012, 07:07 PM
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rear ratio formula?

I got 273s in my lincoln 9 3/8 rear. (this is a low budget build so getting the gears changed is not an option) I know going to a smaller diameter tire will have the same effect as slightly lowering the rear axle ratio but my question is ,Is there a formula that figures out what dia rear tire to use to get specific final drive ratios?
ex. Original tires on my Lincoln before cutting it up were L78/15's so going to an N50/15 will have the same effect as keeping the original tires and lowering the gear ratio by how much?..
I guess i am looking for something like" for every inch you drop the dia of the rear tire is the same as raising the ratio by .75..." or take the ratio and divide it by the dia times...blah blah blah... any thoughts or anybody know?

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Old 02-11-2012, 07:41 PM
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Google "tire ratio calculator" There's a bunch of them. Here's just one.

Play around with them and you will get a idea. There is NO after market support for that 9 3/8 so your idea of playing with the tire Dia is a good idea.

RPM = (MPH x rear end gear ratio x 336) divided by the tire diameter
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Old 02-12-2012, 12:01 AM

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What you propose is somewhat fun, but your gearing is what it is, your car is what it is, so pick tires that look acceptable and are load-rated high enough for the car, let the effective gearing fall where it will. If it was built with L78-15s, then a 295/50R15 is going to look really short.
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