2002 Toyota Tacoma Speedometer Accuracy

I know how tire sizes and speedometers work, but mine is puzzling me. I have a (used) stock Tacoma with it’s standard R17 tires on it. It’s just a regular truck, no crazy off-road equipment (though it is a 4x4). When my speedometer is steady at 70mph, my gps is steady at 68.5mph. If I’m figuring things out right, my speedometer is actually calibrated for a larger tire size, right? This, even though the R17 are the largest standard size available. I guess my questions are 1) am I right? 2) any way to fix this, and 3) what size tire could I put on to fix it?

Speedometers can have a reading error of as much as 5%. That difference between your GPS and the speedometer of 1.5 MPH isn’t something you want to put money into to try to get the numbers to match up.


Yeah, I probably won’t spend any money on it. I knew about the 5-10% error, but it’s just seems strange that it’s consistently off by the same amount.