Tire math and the odometer
 

Just been doing some math based on the adjustment factor in speedo reading vs tire size and it occurs to me that one could intentionally understate the actual odometer reading by running oversized tires...or am I missing something?

If factory tire size revolutions and odometer are calibrated to 1 rev = "D" (distance travelled), then a bigger diameter tire would travel farther in one rev...or "D+X", where x is the difference in circumference.

The odometer only knows that the wheel has turned one rev.

Considering how many enthusiasts oversize their tires, it seems like there might be a whole bunch of understated odometer readings out there...or am I missing something? :-huh

You have the math right. You just left out the fact that most of us also run some sort of programmer which allows us to enter the tire size and recalibrate the speedo

Nope you are right. My truck came with 275 55r20 tires and now has 275 65r20 tires. Not sure of the circumference difference, but now when the speedo shows 100 kilometers per hour I am really going about 109 kph. Tire rack or 1010tire.com are two of the better tire size calculators out there to compare sizes. Most people don't nessesarily put bigger tires on because of that reason though, it's because the look of a bigger tire is more appeasing to the eye and fills the wheelwell better.:-wink

Yup. Circumference / 1609344 = revolutions per mile. Millimeters in my formula. From there I calculated the percent difference from stock and plugged that number into my ScanGauge for a corrected display. I am not able to correct the OEM speedometer with the ScanGauge. For some reason my newer F250 with stock tire size over-states the speed by about 2.5mph, confirmed by GPS and OBD2 data. Why the OEM speedo gauge and OBD2 data are not the same escapes me.