Tach Trouble - Help please

Okay, 650L, 15/45 Sprockets. What am I doing or what is wrong here? I realize the speedo is likely not exact and neither is the tire O.D., but I am not even getting close.

Indicated 4360RPM at 65MPH in 5th gear.

(4360RPM/2.209 primary reduction) x 0.84 final gear ratio x (15T/45T)

x 2.2667' tire O.D. x 3.14159 = 3935.43 feet per minute

BUT, 65MPH = 5720 feet per minute

Is my math okay, or is the Tach messed up, or both, or what? Isn't the primary reduction the fixed ratio between the crankshaft and the transmission input? I have a new TTO Tach, and I can't get it to change functions, but from where I am at I don't know what to tell them if and when I call for help. I thought the tach was reading high, but not two times actual, so I went to check and - duh.... :smirk:

Thanks for any input.

Hmmm... got noodle on that one abit. I'm getting 6196.7 feet per minute.:smirk:

65 miles/hr x 5280 ft/mile / 60 minutes/hr = 5720 ft/min .............. I hope.

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