Gear Set ratio calculations?
Turbo Mini Forums

Right so i have a bunch of boxes and was wondring what ther ratios are in each gear so counted up the teath and still coundent work out how its divided or what ever to give something that looks correct..

so i counted on clubman set witch med says to be 1st 2.583, 2nd 1.711, 3rd 1.250, 4th 1
input gear 19
3rd 22
2nd 25
1st 29
laygear 25 23 19 15

if the gears that mesh are devided up you get 0.5175, 0.760, 1.045, 1.315 haha so i then thought about how power is transfered from input gear to main shaft via sync hub to fd in 4th gear
3rd gear whould be input gear to lay gear to 3rd/main shaft and so on

my question is what is the math to get from drop gear to wheels? id like to find out my true road speed (ignoring wind and things)

do i take engine rpm x drop gears in my case 1-1 so can ignore that x input teeth x laygear teeth (19/25) that gets us to lay gear rpm right? so then x first ratio (15/29) then x FD to get wheel speed to get distance traveled per rev that can then be changed to kph

i know there is guess works but it only goes by like 165 50 13 you cant actualy mesure the rolling distance per rev and tell it to work from that as im sure wheels differ from brand to brand amount of tread ect

reason for all this is i want to know my new GPS based speedo is corect and also can then know at what rpm im doing x speed to back it up

sure i could get a modern car with its crappy speedo and follow then doing x but not sure id get a proper reading?

Use a satnav to double check your cars speedo, they're pretty accurate these days.

Your maths is essentially correct but if you want to do the comparison, just do it in 4th gear because then (as your drop gears are 1:1) you only need to know the FD ratio.

And, yes, the actual revolutions per mile (or kilometer) do vary slightly between makes of tyre and with treadwear, some manufacturers actually quote them, but the only way to be really accurate is to push it down the road a set distance with a mark on the tyre, you can't calculate it accurately from the physical dimensions as the "rolling" diameter isn't the same as the actual diameter because of distortion in the contact area.

Also, to do this, you need a very accurate rev counter (once you have the MS3 in you get a very accurate one by default...)

Schrödinger's cat - so which one am I ???

might give that a go rod i know a nice road with a slight decline saves me pushing

might need to run after the car tho...