Join Date: Apr 2012
Location: 9000 ft. ASL in Southwest Colorado, USA
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Your tachometer reads crankshaft revolutions, whereas the speedometer gets its info from the speed sensor, located in the transmission, which reads revolutions of the transmission output shaft (aka countershaft). So the speedo is factory calibrated for a 14T drive sprocket, and a 38T driven sprocket. Any changes in sprocket sizes that result in a final drive ratio which differs from the stock ratio will change your true road speed relative to what the speedo indicates. The Speedo DRD from 12 o'clock labs allows you to correct the speedometer. Basically it takes the signal from the speed sensor in the transmission and factors it before the signal is received by the speedometer. If you go to the 12 o'clock labs website, they have a calculator in which you plug in your sprocket sizes, rear tire size, and factory speedo error (typically 4 - 5%). When you change to a smaller front sprocket, you will get a negative correction factor (changing to a larger front sprocket, or taller ratio, would give you a positive correction factor), which you then program into the Speedo DRD.
Once you have corrected the speedo, your tach will display higher rpm's relative to your road speed in 6th gear. On my CBR (with a 13T front sprocket & stock rear sprocket, and SpeedoDRD) running in 6th gear at a steady 60 mph, the tach displays 7000 rpm.
Last edited by MotoMike; 12-02-2012 at 07:55 PM.