There was a thread earlier this week that addressed speedometer accuracy. I put in my 2 cents worth of comment about tire wear changing the accuracy of the speedometer and was told that tire wear was negligable for speedo acccuracy. A little research this AM shows that tire wear in the worst case can effect the speedo by as much as 3%.

A couple of assumptions;
Tread depth of 10mm.
Complete wear (10mm gone).
Overall diameter of a stock 285/35/19 is 681.99mm.
Overall diameter of a worn 285/35/19 is 661.99mm.

Without getting into complicated math by using 1010tire.com I found that a 255/35/19 has a stock diameter of 660.90mm. Again using 1010tires.com that makes a fully worn 285/35/19 THREE PERCENT+ off. At 60MPH actual the speedometer would be reading 61.9MPH.

Seems my original statement of the speedometer being accurate at only one point on the tires wear was correct. If the speedometer is accurate at new unworn tread it would be over 3% high at fully worn.

Whether the speedometer is correct at new or worn isn't the problem, the problem is that the speedometer can't be accurate at both. It's only accurate at one point of the tires wear cycle, probably at mid point of wear, around 4mm before the wear bars show.

A good GPS is the best speed accuracy measuring device, your speedometer in the car can be 3% off stock.

 

Every vehicle I own reads 3 MPH higher than actual speed. Seems to be some kind of industry standard error.

 

I noticed this 3 mph error also. Now I drive seven mph over the speed limit to keep within the grace speed.

 

My 2009 Coupe with original tires with 12,000 miles is dead on accurate as near I can tell with the few radar speed signs that I drive by on a regular basis. So I don't think they all read high. My wife's CTS and our old minivan that my daughters drive appears to be accurate as well. My daily driver/commuter car is a consistent 3 mph high at all speed ranges. Must be that needle on the speedometer got mounted "off". The Toyota dealer won't fix it under warrantee.

- Mark

 

The problem with your math is that you are assuming the speedometer is calibrated based on either the maximum or the minimum diameter of the tire.

The OE rear tires on my 09 Z06 are 26.7" in diameter and have 8/32" (.25") tread depth when new.

If you calibrate the speedometer(at the factory) to be dead nuts on at 50% thread depth then the tire's mean diameter would be 26.45". Then, when the tires are new, the speedometer would read slower by 1% and when the tire is worn out, the speedometer would read faster by 1%.

That is not a 2% error but a +/- 1% error.

That means that when the tires are new and your true speed is 60 MPH, the speedometer would show 59.4 MPH and when your tires are half worn out, and your true speed is 60 MPH, the speedometer would read 60 MPH and when your tires are fully worn out, and your true speed is 60 MPH, the speedometer would show 60.6 MPH.

If I'm doing a true 60 MPH according to my GPS, then my speedometer would show 59 MPH(rounded off) with new tires and would show 61 MPH(rounded off) with worn out tires.

So, all the people that posted that their speedometers were close to 1 MPH of what their GPS shows, are correct. But, then you also have to factor in what the tires loaded tire radius is. That could change depending on the tires temperature and the tires loading. A cold tire reading 30 psi will have a smaller radius then a hot tire reading 35 psi. A car with a 100 pound driver will have a loaded tire radius larger then a car with a 300 pound driver plus another 300 pound passenger(or luggage).

I'm assuming the engineers factored in all those variables when they designed the speedometer's calibration and selected a mean tire revolutions/mile based not on the two extremes, but the mean.