Stock Ride Height of Tires

[EquatorHeat00]

Whats the stock ride height for tires. Is it 26.00 inches?
At tire rack it says 255-50-16 diameter is 26.1
235-55-16 diameter is 26.1
Exactly the same, so why is my speedometer off on the high side by 2 to 3 mph at 30 mph? I have a couple of those speed trap things in town that show how fast you are going, and both said I was going 27 mph when the car speedometer said 29, or 30 mph. I assume at 60 mph this doubles to being about 5, or 6 mph off on the fast side right?
My hypertech programmer can raise it up a quarter of an inch, but then it might be more off. That might put it at 26.25 tire height which would be off slighly more.

[Injuneer]

The small differences you are asking about in the tire diameter would not account for your statement that the speedometer is reading as much as 10% higher than the actual speed (speedo says 30mph when radar indicates 27mph). That would take a 2.6" change in diameter to accomplish. And, if the speedo is reading higher than actual speed, it means the tires are smaller than the PCM is programmed for.

If you do the math on the tire sizes (actual manufacturers diameter will vary, and the amount of wear on the tread will make a difference of up to 1/2"), you get:

255/5016 = 26.04"
235/55-16 = 26.18"

The best way I have found to check the speedo is to get on an interstate with 0.1 increment mile markers, hold it precisely at 60mph on the speedo for at least 5 miles, and use a stop watch to measure how much time it took to cover that distance. If it took exactly 300 seconds, your speedo is accurate. If it took more than 300 seconds, your speedo is reading higher than actual, if it took less than 300 seconds your speedo is reading lower than actual. You can convert the "error" into miles per hour.

You might also want to put a scanner on it, and compare the PCM speed readout with the speedometer, to see if the error lies somewhere between the PCM and the speedometer.