Final Drive Ratio changes and resulting RPM\'s...

[OB_shane]

If your tire is 190/50/17 (width in mm/height % of width/rim size in inches)...
then your circumference is (190*(50/100)/25.4+17)*pi [pi is 3.14 or so]...
this gives:
Tire Circumference: 76.91 (this assumes no tire wear!)

The formula is...
MPH=RPM/(gear*primary*secondary)*circumference/(5280/12)*60

primary 83/52
secondary 40/17 (rear/front sprocket)
gears and MPH @ 4000 rpm (actual - not what you see on your speedo)
1 - 34/13 - 30
2 - 31/16 - 40
3 - 29/19 - 51
4 - 27/21 - 60
5 - 25/22 - 68
6 - 24/23 - 74

with 42/17 (42 rear sprocket)@ 4000 rpm now gives...
6 - 24/23 - 71

 

[OB_Warp13Busa]

I'm considering going up 1 or 2 teeth on the rear sprocket and would like to hear from anyone who's done this. What was the difference in RPM in 6th gear near 80mph? Currently, my tach's showing approximately 4000-4100 rpm at 80. I don't have any of the internal gear ratio or tire circumference info to do this for myself. Thanks. Jim

 

[OB_highpwr]

If you do the math, adding 2 teeth to the rear causes a 5% increase in engine RPM for any given speed. So if you normally cuised at 80mph/4100rpm you will now be turning 4305rpm at that same speed.
Keep in mind that your speedo readings will also be affected by this gearing change.

Larry S.

see "the pocket sized rear wheel stand" at
http://quickstand.home.netcom.com