I have a diff ratio in my Mk2 of 3.1, how many RPM do I need to do in order to travel at 60KPH in 4th gear?
My tyres are 145 x 80 Nankangs. Is there a formula?

Use this website - with:
145 tyres
3.1 diff
60kph = 37.5mph
37mph is at 2000rpm, so 37.5mph is 2027rpm

http://www.guess-works.com/Tech/ratio.htm

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Gearcalc is good informative calculator. Free download and you can use it for other cars. Can calc in imperial or metric. I used it when building my westfield 7 clone. Very handy !

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1970 Mk2 Cooper S under restoration

of course there is!!

distance (mm) times distance per minute (km) over distance per revolution (mm) times diff ratio times gear ratio = engine revolutions per minute

(1000000*60/60)/(((145*80/100)*2 + (10*25.4)) * pi)*(59/19*1) = 2033.81rpm

What's more interesting though is that I put a 3.1 diff under my new motor, and my standard mk2 S speedo is as near to perfectly accurate as you could hope for. I did 100km or so on Saturday afternoon doing my ignition timing and the GPS on my phone was spot on with what the speedo indicated. Dad has a 3.1 under his MK1 S, and it's the same, absolutely accurate, no fiddling with speedo drive ratios (6/17) or modifying speedo gearing/springs/whatever

Previously I had a 3.44 under mine, which was a fair way off - the speedo said I was doing 100mph, but the GPS speed it was only about 90 - any other speed had similar % errors

I reckon the 1312 TPM on the MK2 speedo face is wrong - I think it's meant to be 1280 like a MK1

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did I tell you that I won a trophy?

Does anyone know the TPM of an Australian clubman GT speedo ?

Thanks

it ought to be the same as a MK2 S - 3.44 diff, 6/17 drive gear, 120mph indicated - all the same, but how correct it actually is... pfft

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did I tell you that I won a trophy?

The "standard" ratios are 16.4 mph/thousand revs for 3.44 diff; 18.1 for 3.1 ..and I think 14.6 for 3.76

These were the factory specs with standard wheels/tyres... And don't forget to calibrate you nacho as well as the speedo.

Speedo is calibrated by altering the distance between the drive and driven wheels in the speedo itself (but i leave it the pros to actually do it.. . It (used) to be job the didn't take long or cost much..

My speedo was calibrated years (decades?:) ago for a 3.44 diff. Since then I've swapped to a 3.1 ..and installed a good tacho which I trust to keep me legal... The figures in Timmy's table above pretty well match my "settings" (although I have 165 tyres).

Cheers, Ian