Diff ratio - help an idiot.....

[Ranger 6]

Pardon the apparent idiot question....

I have 235/55/17 tyres driven through a 3.46 final drive.

Which final drive would I have to fit to be able to run 235/65/17 tyres and not need to change the speedo etc?

A well known tuner has built an off-road version with a 3.91 final drive which is actually a straight swap of parts from the petrol version - the other OEM parts swap alternative is a 3.73 diff

I'm useless with maths and these sort of things - is there an easy way of working things out?

[Ranger 6]

Anyone?

[Benbay001]

http://www.planet4x4.net/forums/calculator.php

[Ranger 6]

Thanks!!

now why didn't that come up when I googled.....??

[AyBee]

Surely easier just to get the speedo re-calibrated? I don't suppose you'll get an exact diff that matches and the difference will be very small anyway.

[Ranger 6]

No good just re-calibrating the speedo, the engine revs/speed characteristics will be wrong. With a bigger tyre the engine would need to work harder off the line and it will certainly slow the rate of acceleration but cruising would be easier.

By just putting the tyres on the speed would be about 75 at an indicated 70.

Found one - http://www.roversd1.nl/sd1web/wheelcalc.html

According to that calculator the right diff is the 3.71