PhilipJ

I want to come up with five numbers: I want to be able to multiply the speed in mph by each of these five numbers to get the corresponding engine rpm in 1st, 2nd, ... 5th gears ... or vice versa (engine rpm to mph in each gear). I'm refering to a stock Rebel.

I know how to do the math to get from the rear wheel sprocket rpm to mph, BUT I'm puzzled by the data in my manual. It says that "primary reduction" is 3.631, then a "final reduction" of 2.357, then the gear ratios varying from 2.846 (1st) to 0.913 (5th). From the gear ratios it's obvious what to do with the numbers (make sure the engine is running faster in 1st than 5th at a given rpm), EXCEPT I want to be sure that those numbers refer to rpm changes just in the gearbox (input to/output from), and are NOT including speed changes from engine sprocket to rear wheel sprocket. I also do not know what to do with "primary reduction" and "final reduction". I assume that it means that if the engine crankshaft is running 3.631 X 2.357 rpm the output shaft (to the gearbox) is running at 1 rpm.

If anyone can clarify these queries, I'll compute the five numbers, so we can easily compute, say, what your rpms are when you're doing 45 mph in 5th ... or has someone already done this?

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Blue 2009 ... a beauty!

Guba

Red line for engine rpm's is 8,650. If you look at the speedometer, the red marks numbered one through five are the engine red line for that gear. If you change sprockets or tire sizes the red line marks on the speedo won't apply. They aren't super accurate anyway though. When I glance down at my speedo and see the gauge halfway between one of the five red numbers and zero mph, I assume the engine rpm to be around 4,300.

So in a sense, Honda already did it.

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Some days beer tastes good...other days beer tastes better!

'07 Rebel 250 26,000 miles

Soul Searcher

Greetings PhilipJ,

You have two inputs you need to compute the number. One is the error in the Rebel Speedometer. Another is a Tachometer, so you know what the rpms are at a certain MPH.

A simple, inexpensive solution for finding the error in your speedometer. Use a lead person in a cage, to set the speed in 5 mph increments. If the cage is a GM product, 1996 or later, the error will be less than .025 percent in the cage speedometer.

A GPS, either owned or borrowed will also give you the error in the Rebel's Speedometer.

Then use the chart that describes you Rebel in this Thread.

Calculate Engine RPM's without a Tachometer.

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PhilipJ

... oops, I forgot to search on this topic, which would have found me your useful post!

... thanks, Soul Searcher!

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Blue 2009 ... a beauty!

Soul Searcher

Most welcome PhilipJ. Hope it was helpful in answering your questions.

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