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Trying to achieve 3.8-1 diff ratio
norm007 - 23/8/11 at 08:32 AM

Hi.

As the title suggests, I am try to achieve a 3.8ish-1 ratio.

I have a 3.92 diff in my car at the moment and I am considering going back to my 3.64.

But I am now thinking about changing from 50 profile tyres to 55 profile instead to try and get somewhere in between the two ratios.

I would be grateful if somebody could enlighten me as to wether I am thinking straight or barking up the wrong tree??????



Cheers in advance

mcerd1 - 23/8/11 at 09:00 AM

what size are your tyres at the moment ?

norm007 - 23/8/11 at 09:03 AM

quote:
Originally posted by mcerd1
what size are your tyres at the moment ?



195/50x15

mcerd1 - 23/8/11 at 09:22 AM

3.64 diff would be ~ 7% higher geared than a 3.92
3.8 would give +3%


195/55 R15 gives about a 3% increase compared to the 195/50's so about the same as a 3.8 diff

(based on a 1870mm circumference for the 195/55 and 1809mm for the 195/50 - you can get a better calc than that if you get the revs/mile figures for the specific tyres your look at)


(205/50 R15 would give +1.7% and 205/55 R15 would give +5%)

[Edited on 23/8/2011 by mcerd1]

[Edited on 23/8/2011 by mcerd1]

norm007 - 23/8/11 at 09:27 AM

3.64 diff would be ~ 7% higher geared than a 3.92
3.8 would give +3%


195/55 R15 gives about a 3% increase compared to the 195/50's so about the same as a 3.8 diff


(205/50 R15 would give +1.7% and 205/55 R15 would give +5%)

Thank you very much for the info.

How did you work out the equation, or is there software out there to do it???

mcerd1 - 23/8/11 at 09:40 AM

just a ratio of the old diff (or tyre) by the new one or vice versa (depending on which why round your looking at it)

so 3.92 / 3.8 = 1.0316 or 103.16% of the 3.92 (aka +3.16% on top of the 3.92)

with the tyres I did it from calculated circumferences, but revs pre mile would be better (most manufactures quote these on there websites)

or there are a few free gear cal programs out there if thats easier - personaly I like this one:
http://locost7.info/gearcalc.php

[Edited on 23/8/2011 by mcerd1]

[Edited on 24/8/2011 by mcerd1]

norm007 - 24/8/11 at 09:10 AM

Thank you again for your help.

I have downloaded the calculator program for future reference

Mopple - 26/8/11 at 06:10 AM

Another possibility would be 3.77 ratio Sierra diff.

mcerd1 - 26/8/11 at 07:15 AM

quote:
Originally posted by Mopple
Another possibility would be 3.77 ratio Sierra diff.

isn't that the 6.5" diff ?
they are as rare as hen's teeth over here