Revs are reduced at 3 points between crankshaft and rear axle:
- primary drive, a pair of gears that drives the clutch from the crank
- transmission, 6 pairs of gears, which you select from with your left foot
- final drive, the front and rear sprockets
First, calculate rear wheel RPM. Divide engine RPM by primary and final ratios and by the selected transmission ratio.
rear tire circumference from its nominal dimensions. Note that this isn't very accurate, because the nominal dimensions will vary from reality. Circumference of a 190/50 17 rear tire is:
(17 inches + (190 mm × .50 ÷ 25.4 mm/inch) × 2) × 3.14159 = 76.91 inches
Then multiply estimated circumference by rear axle RPM to get speed in inches per minute. And finally, divide speed in inches per minute by 1056 to get miles per hour.
Here are ratios I found at Honda Japan
. I couldn't find them at American Honda
. Verify in your owner's or service manual if possible, because I can't guarantee them.
final: 2.562 (41/16)
As an example, here's 2nd gear at 11,500 RPM:
axle RPM = 11500 / (1.52 * 1.933 * 2.562) = 1528
(I've substituted "*" for "×" and "/" for "÷" in the expression to facilitate use of Microsoft's Windows calculator. Highlight from "11500" through the final equal sign, copy, and paste into the calculator.)
Now calculate speed by multiplying by tire circumference and converting to MPH:
MPH = 1528 * 76.91 / 1056 = 111