SILLY QUESTION FROM A DAFTY about Tyre Size

[colinwoodburn]

Rather than going down the route of changing a rear axle from 4.08 TO 3.27, would swapping the wheel and tyre combination from 14s to 15s with 70 series tyres not help to bring down the revs while travelling at 60/70 MPH? Just an idea. Your thoughts would be appreciated.

COLYWOLY

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[Shvegel]

Measure the distance around a present tire(circumference) then measure the distance around the proposed new tire. Divide the old circumference by the new and you will get a number around .90 or so. Take that number and multiply it with the number of revs your engine turns at highway speeds and you will get your new revs at highway speed.

According to this web site:

http://www.1010tires.com/tiresizecalculator.asp

A 195/70/14 circumference is 77.72 inches while a 195/70/15 tire is 80.86 inches in circumference. Divide 77.72 by 80.86 and you get .96 or a 4 percent reduction in revs across the board so if you were turning 5000 revs at a given speed you would turn 4800 revs with the bigger tires.



For reference if you run the numbers on your  rear end swap(3.27 divided by 4.08) you would get .80 which would give you a 20 percent reduction in revs or in the previous example 5000 revs becomes 4000 revs.

[waqas]

Watch out for the tyres rubbing on chassis parts if you increase the tyre size too much.