Calculate Engine RPM's without a Tachometer. - Honda Rebel Forum

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Old 07-09-2010, 12:12 AM   #1
 
Join Date: Jan 2010
Location: Summerfield,FL
Posts: 662
Cool Calculate Engine RPM's without a Tachometer.

How to figure your estimated Engine RPM's without a Tachometer for 250 CMX.

Using this Calculator at http://www.gearingcommander.com/

I've assembled some charts for different Sprocket Sets.

With the known error in our Rebel's Speedometer we can use the 10% Rule for Safety.
The easy way to do this is subtract 1 mph for each 10 mph Indicated on Speedometer.

An Example : Your Rebel is equipped with stock gearing, 14/33.
The Speedometer is indicating 20 mph in 2nd gear.

We subtract 2 mph from indicated speed, so we use 18 mph to find our engine rpm's.

Our Selected gear is 2nd. In the first Chart find 2nd gear.
Using the last column "RPM's for each 1 MPH in Selected gear" we find the number "212".
Just multiply 212 times 18 and you have 3,816 rpm.

For our Rebel Riders who love speed, let's use the Speedometer tick mark for first gear.
20 mph minus 2 mph is 18 mph. In the last column of the first chart we find 344. 344 times 18 equals 6,192 rpm's.

Remember the Safety Factor. Being the MAX speed for 1st gear is 24 mph at 8,250 rpm's.

If you have a gps on board and you use it's speed, your rpm's will be with very accurate.

My Garmin 255W and Tiny Tach has verified the last chart as being very accurate, within 50 rpm's.

. . . . . . . . . . Now the Charts. . . . . . . . . . .

All numbers are rounded down (0-4), or rounded up (5-9).
Max MPH for Selected Gear is at 8,250 engine rpm's for each Sprocket Set.
Max MPH is corrected, not indicated on Speedometer.

Sprocket Set 14/33._____Final Drive Ratio 2.357.
Selected_____Max MPH for_____RPM's for each 1 MPH
_Gear_______Selected Gear_______in Selected gear
_ 1st___________24__________________344
_2nd___________39__________________212
_3rd___________52__________________159
_4th___________64__________________129
_5th___________76__________________109

Sprocket Set 15/33._____Final Drive Ratio 2.200.
Selected_____Max MPH for_____RPM's for each 1 MPH
_Gear_______Selected Gear_______in Selected gear
_ 1st___________26__________________317
_2nd___________42__________________196
_3rd___________56__________________147
_4th___________69__________________120
_5th___________81__________________102

Sprocket Set 14/30._____Final Drive Ratio 2.142.
Selected_____Max MPH for_____RPM's for each 1 MPH
_Gear_______Selected Gear_______in Selected gear
_ 1st___________27__________________306
_2nd___________43__________________192
_3rd___________57__________________145
_4th___________71__________________116
_5th___________84___________________98

Sprocket Set 15/30._____Final Drive Ratio 2.000.
Selected_____Max MPH for_____RPM's for each 1 MPH
_Gear_______Selected Gear_______in Selected gear
_ 1st___________29__________________284
_2nd___________46__________________179
_3rd___________61__________________135
_4th___________76__________________109
_5th___________90___________________92

Sprocket Set 14/27._____Final Drive Ratio 1.928
Selected_____Max MPH for_____RPM's for each 1 MPH
_Gear_______Selected Gear_______in Selected gear
_ 1st___________30__________________275
_2nd___________48__________________172
_3rd___________64__________________129
_4th___________78__________________106
_5th___________93___________________89
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