I have two sets of tyres. Copper AT3 265/60/18s with a near new 265/60/18 Cooper HT spare, and 255/55/19 HTs. I recently swapped over the 18s for the 19s and I thought I would show the deference in actual diameter.

To demonstrate the diameter difference I used 2 ATs and the comparison tyre in the middle. I sat a piece of timber on top of the ATs to compare their height to the other tyre. Photos with rule below.

Comparison between Cooper AT 18" and HT 19" tyres. There is about 25mm difference in height.

Is the difference between 18's & 19's a surprise ?

There's loads of web sites like this one that will tell you the difference in diameter between 265 60 18 & 255 50 19 is half an inch which is 15mm....

At 30 mph it's only a 0.5mph difference in speed though.

Some tyres bow a bit more in the middle when not under load - plus different tread depths & tyre pressures could account for some more.....

I was particularly surprised by the difference between the two Coopers of the same size. I was expecting it to be a difference of about 5mm.

It was really just a general information post to illustrate how two tyres with the same nominal size by the same maker may have different actual and so rolling diameters. ______________________________
Vogue tdv8 08MY poverty pack - wow

It's a fair point - and one of the many reasons I never mix tyres