Formula required: gear ratios / final drive / rpm / mph

[Matt UK]

Morning all, I really hope someone can steer me on this one!

I'm looking to create / understand / find a formula that will allow me to understand how gear ratios / final drive / rpm / mph all fit together.

I am considering changing my final drive and want to see how it would change the mph at a set rpm in each gear ratio - if I get it all into excel I can them play around with the variables.

Any help would be very much appreciated!

[MX7]

I'd have thought that a Google on "gear calculator" would give some ideas.

[300bhp/ton]

Yeah, there are some online calculators and even some that might specialise in your car.

I've got some of these on spreadsheets for various purposes, although I'd have to dig them out.

You'll need to know all of your gear ratio's and current final drive as well as tyre diameter. From this you can derive mph.

What car is it?

[Sticklebrick]

Here you go: It is pretty simple to work out if you think about it in steps.

Engine RPM/Gear Ratio = Propshaft RPM (or equivalent).

Prop RPM/Final Drive = Road Wheel RPM

Road Wheel RPM * Pi * wheel diameter(metres) = Road speed (in metres/minute)

Now just a case of converting to mph:

(Road Speed (mpm) * 60)/1609.344 = Road Speed (mph)

Hope this helps.

[Matt UK]

300bhp/ton said:

You'll need to know all of your gear ratio's and current final drive as well as tyre diameter. From this you can derive mph.

What car is it?

Yup got all of that - the car is a Caterham.

I've got a feeling though that when it had a 6spd box put in, it retained the 3.9 diff as it's very frantic. Previous owner seems to think it was the 3.6 so just doing a few sums to work it all out.

[Matt UK]

Sticklebrick said:

Here you go: It is pretty simple to work out if you think about it in steps.

Engine RPM/Gear Ratio = Propshaft RPM (or equivalent).

Prop RPM/Final Drive = Road Wheel RPM

Road Wheel RPM * Pi * wheel diameter(metres) = Road speed (in metres/minute)

Now just a case of converting to mph:

(Road Speed (mpm) * 60)/1609.344 = Road Speed (mph)

Hope this helps.

That is great, thanks.

I've built up the formula below but yet to pressure-test it with some figures - I'll use your maths also as it looks easier!

(mph X 12 (inches per foot) X 5280 (feet per mile) X axle gear ratio X transmission gear ratio) OVER
(pi (3.14159265) X tire diameter X 60 (minutes per hour))

[Matt UK]

doogz said:

I'll send you it if you want it. You can have a look at how it works and make your own or whatever.

Thanks chap

[D900SP]

Just go to www.teammfactory.com/gear-calculator

Does everything you want to check and calculate.

[FlatPack]

Or just download GearCalc which even has the ratios for your Caterham 6 speed box built in.

Do you know if your speedo is right? I know of some Caterhams that had the 6 speed box retrofitted, kept the original diff and the speedo ended up incredibly optimistic!

[Matt UK]

D900SP said:

Just go to www.teammfactory.com/gear-calculator

Does everything you want to check and calculate.

FlatPack said:

Or just download GearCalc which even has the ratios for your Caterham 6 speed box built in.

Do you know if your speedo is right? I know of some Caterhams that had the 6 speed box retrofitted, kept the original diff and the speedo ended up incredibly optimistic!

Thanks for all the help chaps - great that there are some online downloads (that google didn't find!) to check my work - but like some of you out there, when I get it into my head to build a spreadsheet, I like to build a spreadsheet!

I've not checked the speedo vs the GPS, but will do later this week.

But yes, my Caterham has a 6 speed Superlight retrofit.
And I suspect the Roadsport 3.92 diff.
And 185/60 R13's.

Couple all of that together and I have one busy little beaver at cruising speed.

So just doing some homework to start mapping out some changes. 205's on the rears will help and potential getting to a 3.62 final drive. A spreadsheet to see what this would do is going to be a helpful steer.

[Noger]

Yes, Peter Odgen's GearCalc is all you should need.

http://locost7.info/gearcalc.php

If you jack the 7 up, and turn the wheel you can work out what diff you have.

[Matt UK]

Just finished my maths!

I'll use the websites posted to check I've done it right.

Thanks all.

[tikendra yadav]

thanks to all of you to share your knowledge , but these all are information not enough for my question please any one can tell me the exact relation b/w wheels rpm and speed of a car .