calculating speed based on RPM, diff ratio and tyre size. bit of a guide.

**love ke70** · 11-02-2007, 04:02 PM

hey guys, well a little earlier i was playing around with numbers trying to work out the best ways of dropping my RPM at highway speed.
im running a 1ggte in a ke70 with W57 and 4.3 final drive alfa romeo lsd.
at 110 she will sit at around 3500. which is just shit.
so i was thinking, maybe i could drop it using tyres.
wanting to drop it to around 2800 RPM.-
so i set about playing with sums, and thought this may come in handy, so thought i would post it up. i will try to explain as best i can. without taking all day, or making it too messy.

so, let me begin:

ok, i gave up begining and did it in word to make it neat. heres a link to download the file
http://www.esnips.com/doc/83fe059a-5...1/speed-vs-rpm

i have linked to any references i used. and have tried to explain things as best i can.

theres more i could add. but was getting a bit over it and its all just rearranging numbers really.

thought it may come in handy tho, for people wanting to see the result of changing tyres, gear ratios or diff ratios.

so feel free to add any comments or anything you would like to see added.
i could add one about changing tyre size. as i did some playing around with that but couldnt be bothered to type all that up.

let me know what you think

cheers, andrew

**Norbie** · 11-02-2007, 04:48 PM

Nice work, some good info in there.

For those who are lazy, here is an online calculator that figures it all out for you:
http://akb.norbie.net/Public/Gearing.aspx

**love ke70** · 10-04-2007, 09:19 PM

just realised, the key problem with that calculator is you have to trial and error your rpm to get a rpm value at a given speed....
that said, i still did trail and error rather than math just then when checking something :embarassed:

**joecoolmk2** · 10-04-2007, 09:42 PM

that's an awesome calculator norbie, good work

+ rep

**Norbie** · 10-04-2007, 10:45 PM

Originally Posted by love ke70

just realised, the key problem with that calculator is you have to trial and error your rpm to get a rpm value at a given speed....

Hmmm, so you want to be able to enter a road speed and get the RPM in each gear? Shouldn't be too hard, I'll have a crack at it later in the week.

**Mos** · 17-04-2007, 06:25 PM

Do any of these calcs take into account tyre rolling diameter? (always less than actual diameter).
Mos.

**love ke70** · 17-04-2007, 06:39 PM

shhhh!

the difference is likely negligible, and it will cause the speedo to read less than road speed, so its likely disregarded as part of that acceptable error.
one must also look into the amount of tread on tyres...
so many variables its only ever a close guess

**Norbie** · 17-04-2007, 10:11 PM

Originally Posted by Mos

Do any of these calcs take into account tyre rolling diameter? (always less than actual diameter).

I could add that variable if you can tell me how to calculate it! I have a feeling it's all about tyre pressure and vehicle mass though, which makes things way too complicated for a quick and easy calculator.

**Mos** · 17-04-2007, 10:15 PM

The way I saw it expressed on a tyre manufacturer's spec sheet was in percentage - ie 97% or whatever of the nominal diameter. I was trying to find a reference but couldn't.
But yes, many variables that will affect that percentage.

Mos.