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Overall gearing change formula?
carcentric - 24/3/06 at 11:12 PM

If you increase tire diameter from 25" to 26.25" (~5% difference), and
increase the ring(crown)/pinion ratio from 3.09:1 to 3.545:1 (~15% difference the other direction) . . .

. . . has the OVERALL GEARING been changed ~10% (TD percentage difference minus RP percentage difference)?

If not, what is the correct formula to determine overall gearing change?

Thanks.

[Edited on 24/3/06 by carcentric]

Mark Allanson - 24/3/06 at 11:52 PM

you make my head hurt

cossey - 25/3/06 at 12:15 AM

speed will now be 91.5% of orginal speed if ive got my maths right ie (26.25/25)*(3.09/3.545)

carcentric - 25/3/06 at 12:23 AM

quote:
Originally posted by cossey
speed will now be 91.5% of orginal speed if ive got my maths right ie (26.25/25)*(3.09/3.545)


That was what I thought at first, too, until I considered an example where tire diameter went UP 10% and r/p ratio went DOWN 10%. Using multiplication, there would be a 1% overall change - but logic would tell me there'd be no change because one change would offset the other . . . that's why I'm asking.

cossey - 25/3/06 at 08:44 AM

they dont offset each other and you would get a 1% change.

eg a 25" diameter wheel at 100rpm travels 25*pi*100 inches per minute =7854"/min
a 10% shorte diff means that the wheels for the same propshaft revs now only turn at 90rpm. 10% bigger wheels have a diameter of 27.5" so 27.5*pi*90=7775"/min

7775/7854=0.99 or 99%