Inaccurate Speedometer

I thought my speedo was inaccurate and tested it on a  speed detector down the road from me. The speedo showed 35 but the detector showed 40 (12% off).

I assume this means my odometer will be slightly off as well.

Is this something that can be fixed?


 

First thing would be to pull the driven gear out of the transmission or transfer case and see how many teeth are on it. A lower tooth count will read higher. Example - current tooth count is 21, a 20 tooth gear will show a higher speed a 22 tooth gear will show a lower speed.

One other item, tire size, if you went up to a larger diameter tire, it will cause your speedometer to read low.

We have a page dedicated to speedometers, including a calculator on how to correct them here: https://www.garysgaragemahal.com/speedometer-gears-cables--sensors.html

we deal with this every time we install larger tires on a truck. it takes a little math and sometimes a willingness to compromise. I generally get the mileage as close as possible with the gear swap on the cable drive but there are times when you really wish there was a 1/2 tooth, haha. and sometimes it's just do your best. fine tuning is doable but a little fiddly.

Thanks, I will check the link. This a stock truck so no big tires.

has anything been repaired, replaced, altered along the way? often things get out of sync this way. before going too far down any "rabbit hole". understand that not all means of measuring are reliably correct. I travel a lot and every time I get the chance to speed check, whether approaching a construction zone or even a police reminder box, I go by right above the posted limit to get a reading and therefore a confirmation. there was one near Knoxville two years ago that I passed multiple times and as often as it was 3 mph high it was also 3 mph low. now at 71 mph that is a considerable range when I'm driving the same truck each time. occasionally when my wife is navigating with her gps I will settle on a speed limit and have her check. its more accurate than the boxes are. but do this on long level stretches.

The tables Gary mentioned are very useful.
With the help of this tool, I was able to calculate that original 18 tooth driven gear + 7 tooth drive gear (difficult to find on web) could be replaced by the team drive 6t + driven 16t, giving an accurate speedometer.

In my case, the calculation table was very helpful.