Speedo correction for larger tires?

ah64id wrote:
Many people have mentioned calibrating the speedometer, but I prefer to calibrate the odometer.

I have never driven a rig where the speedo and odometer match, some are off by 2-3 mph. My DD VW is off by 2-3 mph at 70 but the odo is dead accurate.

For calculating mileage, maintenance, and true odometer readings I suggest calibrating the odometer over a 100-200 mile stretch and learning the small speedo error.

My sig truck is calibrated within 0.3%, and reads about 1mph fast at 70. It's more accurate IMHO, and keeps the wife from speeding and I just set it based on the speedo and I'm not going as fast.

Many manufacturers post rev/mile data, this is far more accurate to start with than tire math.

wintersun wrote:
Check your speed with a GPS or smartphone with GPS and see how accurate your speedometer/odometer are with the stock tires. I did not realize it but with a camper load the actual speed was 2 MPH less than indicated thanks to the tires squatting and not being as tall. Only takes a 1/2" of squat to affect the speedometer accuracy by 4%.

If you are getting a 1/2" of squat your tires are overloaded or under inflated.

I will see a small change empty to loaded, but never even 1%.

You really think that the speedometer and the odometer function differently and that one uses the rotation of the tires and the other uses what instead? Talk about magical thinking.

To have 3,000 lbs. of weight on a tire with a 3,750 load rating is not overloading it. At 80 PSI the weight results in flexing of the sidewall. Why do you think that the air temperature inside tires increases as the vehicle is driven? It is the flexing that generates heat which warms up the air inside the tire. Not rocket science but common knowledge that somehow escaped your notice.