Speedometer Error with non-stock rear sprocket

I changed my rear sprocket from the stock 42 tooth to a 50 tooth.  The speedometer is off by as much as ten miles per hour at higher speeds.  Other motorcycles I have owned had the speedometer keyed off the front wheel.  My 2015 WR must read the RPM's from the motor to present the speeds.  Also, because the speeds are incorrect, I assume my odometer reading must be off as well. Comments?

yes, changing the gearing affects your speedo and odo. In stock form the odo is pretty close to reality, the speedo is off by about 7-10%. With your changed gearing now both the odo and speedo will be off. You can get a speedo healer / speedoDRD to fix this. But you can either set to correct odo reading or correct speedo reading. can't do both.

I suggest you use the search function. This topic has been discussed many many times on the forum. You will find all the info you need!

This will answer a lot of questions.

Yep, the speedometer sensor is on the output shaft of the transmission. Pulses are sent to the dash display for calculation before displaying speed and distance in either metric or imperial units. From the factory speed is 7.7% conservative (supposedly intentionally) and odometer is accurate. Changes to gearing and/or tire circumference affect readings further.

You can correct either speed or odometer with a device like 12 O'Clock Lab's SpeedoDRD, which is basically a programmable multiplier/divider that goes inline between the sensor and the display. But the 7.7% difference between speed and distance is hard-coded into the display, so both readings will never be accurate at the same time.

I have a SpeedoDRD on my WRR with 13/51 gearing and Dunlop D606 rear tire and my speedometer reads dead-on accurate at all speeds. It plugs inline into the factory wiring harness. 12 O'Clock Labs' website has a calculator to help you program it easily and quickly.

Compared to GPS my speedo and odo are both off my 25%. That means if I'm doing 40 MPH the display says 50. If I ride 40 miles it says I rode 50. I started building a GPS based speedo/odo that runs off an Arduino (well, technically a friend of mine did the building since he had the parts laying around) but decided it's easier to just mentally multiply the display speed or distance by 0.8 to get the actual. Even easier is that at normal driving speeds I can just subtract 10 from what the display says to get close enough.