Here's a quick formula, for those who want to compare tire sizes. I only have to do this about a zillion times a day. It'll give you a quick estimate of the differences in tires diameters.
Fist off, you need to understand the numbers on metric sized tires. We will go with a 275/65R18.
275 is the section width. This is the cross-section width of the tire (not tread width) in millimeters. The tire is 275 millimeters wide when measured at it's widest bulging parts when inflated to test pressures on the nominal width wheel for the size.....and at it's nominal load. So, you can look at the tire, when loaded, and the distance between the two bulges near the ground will be 275 millimeters.
65 is the height to width aspect ratio. In simple terms you can say the distance from the ground to the wheel (or sidewall height) when the tire is loaded, is 65% of the section width of 275 millimeters.
The R means it is a radial constructed tire (the body plies run across the radius of the tire).
The 18 means the tire fits a 18" diameter rim. This really a misnomer as the cross-section numbers beforehand were metric, but the rim size is imperial. Oh well, can't make it too easy.
To get a good estimate of the actual diameter, where we take these numbers and do some conversions from metric to imperial is as follows:
Multiply the cross section width by the aspect ratio, then by 2 (which gives both sidewalls height.)
275 X .65 X 2 = 357.5 mm
Now divide 357.5 mm by 25.40 (cause there are 25.40 mm to an inch)
357.5/25.40 = 14.074 inches.
We now know there are 14.074 inches in the two sidewalls, but still need to add the hole in the middle of the tire (the rim) back to it, to get the complete diameter.
So 14.074 + 18 = 32.074 inch diameter tire which you may see the spec charts round off for 32.1
You can also do away with all the decimal places and just use the following whole numbers.
Section width X Aspect ratio X 2
------------------------------ + rim diameter = app. diameter
2011 Flagstaff 831 RLBSS
I used to be addicted to time travel, but that's all in the past now.