Originally Posted by jetmechoh
I thought the difference between the 205 and 225 had to do with width of tire. Your saying it’s changes the height also?
In the metric tire sizing system, it is relative to the aspect ratio:
I copied this from a previous post I had:
OK, just going to go into a basic tire tutorial which you may already know. In the tire size designation of 225/75R15:
the 225 is the section width in millimeters of the tire (which will be the fattest bulge of the tire when looking directly at the tread of the tire, when it's loaded at it's nominal carrying capacity.
the 75 is the aspect ratio, which is the height of the sidewall of the tire in comparison to the section width (which will be 75% for this one). This means that the sidewall of the tire when measured from the rim to the tread is 75% of the section width
The R means it's of radial construction type, which means the body plies run at a 90 degree angle to the beads
the 15 is the rim diameter in inches. You have millimeters for the section width, but inches for the rim diameter. Kind of a mashup of two measuring systems, but that is cause of the U.S. and our reluctance to go all in on the metric system. Ford and Michelin got into an unholy alliance back in the eighties and did make metric size wheels, but I digress.
There is a formula that will let you approximate the overall diameter of a tire, which will change it all to inches. Each manufacturer can measure their tire a little different by regulations and there can be some slight differences per manufacturer. They also sometimes round off the diameters in the documentation.
It's section width (225) multiplied by aspect ratio (75) multiplied by two (since you have a bottom and top sidewall from the middle of the tire)
then this number is divided by 2540 (there are 25.4 mm to an inch) . We move the decimal one more place at this step to make keep from doing it later.
We then add the rim diameter to this final number, which is already in inches.
225/75R15 equates to
225X75X2 divided by 2540 which is
33750/2540= 13.287 inches
13.287 inches + 15 inch rim diameter= 28.287 inch approximate total tire diameter.
Here is a spec chart I downloaded from towmax on their ST trailer tires that you can see the diameters and load carrying capacities. As I said earlier, each manufacturer can vary a little, but will all be close.