Speedometer Calibration

[Duke Yugo]

Use this site or look up any other for the tire size comparison:

Tire Size Comparison

Tire size comparison the easy way. Use our tire size comparison calculator to compare diameter, width, sidewall, circumference and revolutions per mile.

tiresize.com

You will find that the difference is minimal; this tire will be an inch "taller".
The drive/ride is actually better since there is more rubber to absorb impacts.
Look up GOODYEAR Assurance MaxLife. 85,000 miles tread life!
And note the difference in the speedometer reading.
See the image I attached.

 

[Corvetteboy]

Thanks Duke! Do you feel the handling got too sloppy/slow?

And no clearance issues?

 

[GT4GT]

Mystery solved!

Last year I changed tires on my CX-9 and for some unknown reason, I ordered 255/55R20 to be installed.
OEM tires are 255/50R20. In the size 55, the speedometer reading is about 3% off showing slower speeds. That is exactly the "issue" I had all along. I don't know why I made the mistake I did, but others should also check on the size of their tires if they have any speed reading problems.
I can't believe it took me 6 months to figure this one out.

A belated thank you from the OP. I’m still running the original OEM tires in size 50, and my speedometer continues to be off. If I understand you correctly, switching to 55s will worsen the problem.

 

[PTguy]

Larger diameter tires will be a bit like driving in a higher gear. The drivetrain will turn a bit more slowly, thus whatever picks up the drivetrain speed and translates that to the speedometer will read a bit low at the same actual speed.