Speedometer Calibration With Larger / Bigger Tires?

[RSH]

Modern speedometers are plenty accurate as long as they know the actual circumference of the wheels they're measuring. You'll experience variation as the temperature of your tires changes (ideal gas law), but I speculate temperature changes will have a greater impact on pressure than volume, given the steel cables in the tire. So I imagine it's reasonable to say you're calibrating your speedometer for a given tire pressure, and that can be within +/-1 MPH at typical highway speeds. Though again, I'm just speculating. I haven't studied tires. But personal experience seems to support my theory.



Nominally, that's roughly a 10% increase in circumference. So you're correct at 20 MPH. At 65 MPH that becomes a difference of 6.5 MPH.

You are correct, I had a typo in my post, I typed 245 75 17 instead of 245 65 17.
In my case I do have 245 65 17's on my Badlands.

The calculated speedo error at 65 mph would be 2.3 mph when compared to standard Bronco Sport 225 65 17's or 2.01 mph compared to the 225 60 18 tire size, which for me really isnt a big deal.

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