Gear Calculator

[silverlock]

Been toying with re-gearing 3rd and 4th on my 915/61 to reduce the gap between 2nd and 3rd / 3rd and 4th.

Dropping 3.2k RPMs when shifting up to 2nd tends to pull the car "off cam" — still in the powerband but not on the hottest part of the cam. Just a big RPM drop because the stock 23:29 3rd is just... long. A 21:31 or 21:29 reduces that drop to ~1740RPM or ~2320RPM and a 29:22 4th to bring it a bit closer to 3rd. I run a longer 29:22 5th already so I'm covered for those highway "commutes" to rallies.

I've been playing with some other tools + looking at a lot of Bill V posts/charts, but wanted something more dynamic + that I could plug in available options from Guards and visualize RPM drops and gaps.


So I built something: https://ratio.questionable.services/ — plug in your tooth counts, R&P, tire size, max RPM & shift point, and generate a table for each gear with RPM drops and a chart to show the gaps.

[Bill Verburg]

Originally Posted by silverlock

Been toying with re-gearing 3rd and 4th on my 915/61 to reduce the gap between 2nd and 3rd / 3rd and 4th.

Dropping 3.2k RPMs when shifting up to 2nd tends to pull the car "off cam" — still in the powerband but not on the hottest part of the cam. Just a big RPM drop because the stock 23:29 3rd is just... long. A 21:31 or 21:29 reduces that drop to ~1740RPM or ~2320RPM and a 29:22 4th to bring it a bit closer to 3rd. I run a longer 29:22 5th already so I'm covered for those highway "commutes" to rallies.

I've been playing with some other tools + looking at a lot of Bill V posts/charts, but wanted something more dynamic + that I could plug in available options from Guards and visualize RPM drops and gaps.


So I built something: https://ratio.questionable.services/ — plug in your tooth counts, R&P, tire size, max RPM & shift point, and generate a table for each gear with RPM drops and a chart to show the gaps.

It looks like you did a really nice job!