Different size tires means different diameter

In this morning’s newspaper, you replied to the buyer of a 1966 VW camper who discovered that the vehicle had 205/75-14 tires on the front and 185/75-14 tires on the back. You told her “And since your tires are all the correct diameter (they have the same inner opening, 14, which is the wheel size, and aspect ratio, 75, which essentially is the height of the sidewall), your gauges shouldn’t be affected at all.”

Not so! The sidewall height is the width multiplied by the aspect ratio. The difference in widths is 20 mm. With an aspect ratio of 75, the means a difference of 15 mm in height and an equal difference in the rolling radius of the tire. The difference in diameter is 30 mm which is 1.18 inches. To check my calculation, I looked up the diameters of Kumho tires with these sizes on TireRack’s web site. They give diameters of 26.1 and 25 inches for a difference of 1.1 inches.

Maybe you guys were really tired and hung over after an all night party on your boat.

That is about a 4% difference, Tyre wear will likely be more than that. In any case few speedometers are that accurate.

I emailed this exact same thing. The dis- and mis-information about tire sizes out there is astounding. Using an online calculator, I found it was about 2.3% difference on the speedo (says 60, actually going 61.4) and would also register fewer miles on the odo.

I just sent them the same message. Yes, it’s a small difference, but it is different. Surprising that they made this obvious mistake.