I thought the speedo was reading high, today got my first quick readings with a GPS;
these may not be exact but should be close.
Speedo GPS
reading reading
30 20.7
40 29.6
50 39.7

I have no history on the tranny gearing, but these readings should only be affected by the R & P gears and i didn't think there were any options there. Rear tire diameter is 23.75. Not sure what the OEM size would have been.

What's going on here ??

Check your tires vs the stock 165/80R15's - it's usually mostly in tire size induced error, & not as much gearing - but then that's a huge speedo error.

https://www.tacomaworld.com/tirecalc


Tom
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Wow that are very large difference....even with wrong tyre diameter you would only be out by roughly 2 to 5% ,thinking your speedo might be shot or maybe you cable is on the way out !!!.
Good luck .

Well the numbers are not linear, they are close but not quite right.

The speedometers work by Turing a magnet inside the gauge with a mother magnet on the shaft that turns in sync, around and around it goes, but if it does something odd it is not directly connected. Odd in this case would be going in reverse.

? is your odd accurate with a measured mile ? It is gear driven, the speedo is driven by a magnetic copasetic function. If the odo is slipping or binding it can cause the speedo to reduce its readings. Not hard to fix, but a pain in the booty to do it right.

You have to open it up, remove the drive gear on the Odo shaft it is the first plastic gear you will find, take it off the shaft, measure it. Order one that is bigger from ODOgears, and then sand it to make it fit. Get everything back one, hint tape all the odo gears, and press the gear back on, lube everything up, and reassemble.

Again not hard, but not fun.