Owen_1987

All the talk on here about changing final drive gearing is getting me quite interested about the differences it will make. I'm wanting to make track car that is still usable for every day driving so I don't want a too extreme ratio.

The question is, what is the formula for finding engine RPM if the MPH, final drive ratio and transmission gear is known? I know it's never going to be 100% accurate because of all the different factors involved but I was just looking for an approx figure. I just want to make sure that I wouldn't be cruising at 70 MPH in 6th gear doing 4000RPM!

or if anyone has a decent website calculator that's trustworthy because I'm not sure about the answers on some i've tried.

lovegroova

You can play here http://www.turnzero.com/technical_resource...gear_calculator (with thanks to eSeM)

And have a read of this thread https://www.s2ki.com/forums/index.php?showtopic=760779 lots of good links in it.

Owen_1987

Thanks, must of missed that thread on here

chrisr111

As a rule of thumb a 4.77 is like using one gear lower, so sixth is like fifth, fifth is like fourth etc.

A 4.44 is roughly half way between the OEM 4.1 and a 4.77.

Chris.

Owen_1987

thanks, good info.

Still would like a formula if anyone knows it to work out what motorway cruising RPM would be at 70 MPH with 4.44 and 4.77

lovegroova

Just do the maths per that site:

4.10 = theoretical max speed of 165.4 mph @ 8900 rpm
Therefore that's 18.6mph/1,000 rpm
70mph = 3,766 rpm

4.44 = theoretical max speed of 152.7 mph @ 8900 rpm
Therefore that's 17.2mph/1,000 rpm
70mph = 4,079 rpm

4.77 = theoretical max speed of 142.2 mph @ 8900 rpm
Therefore that's 16mph/1,000rpm
70mph = 4,381 rpm

Owen_1987

didn't think about doing that, thanks for your help

lovegroova

You can get a pretty good rough idea by looking at the graphs produced as well.