Speedo calculation

[FrkyMnky]

Alright, I'm trying to buy a speedo for the bike and I know 2:1 is the std ratio for 19" wheels. Im trying to see how close the speedo will be with my particular tire on the front. Im pretty good with math, but I cannot figure out what I am missing.
27.59 Dia
86.67 Circ
2240RPM@60mph

So its made simpler because we are covering a mile a minute so I don't have to worry about conversions for the rpms. However, when I figure out distance covered with the circumference it should be ~731 RPMs for 60mph. WTH am I missing or screwing up?

 

[TwoManyXS1Bs]

I've seen that '2240RPM@60mph' thing numerous times, and like you, thought

I, too, would like to know where that comes from...

 

[Paulrxs650]

I believe it refers to the RPM of the speedometer cable and not the engine / wheels etc.

 

[pamcopete]

The standard Japanese motorcycle speedometer cable turns 2240 RPM at 60 MPH. If your wheel is rotating at 731 RPM at 60 MPH, then the speedometer adapter will have to have a ratio of 2240/731 = 3.06

However, you cannot just use the circumference of the tire to obtain RPM of the wheel at 60 MPH. Due to its semicircular cross section, a motorcycle tire has an infinite number of circumferences all of which are affected by the tire design, rubber compound, inflation pressure, weight of the motorcycle etc. The only accurate number to use in place of a calculated circumference is rolling distance. Rolling distance is the distance that the motorcycle rolls forward for one revolution of the tire. Simply mark the sidewall of the tire and the floor and push the bike forward until the mark on the sidewall is once again on the bottom. Measure the distance between the two marks on the floor. I think that you will come up with a smaller number than 86.67 and a bigger number than 731 and the ratio will be less than 3 but not exactly 2. That is where the urban myth of 2:1 comes from.

 

[TwoManyXS1Bs]

Here's a good thread: http://www.xs650.com/forum/showthread.php?t=18013