Sidewall height. Let's translate the numbers, shall we?

235/75r15

235 is the tread width in millimeters. 75 is the aspect ratio of the sidewall to the tread width. The sidewall in this case is 75% of the tread width.

.75(235)= 176.25mm.

The third number is the diameter of the rim.

Using these numbers, you can find the overall diameter of the tire. You divide the tread width by 25.4 to convert to inches, multiply by the aspect ratio, double it because you have sidewall at the top and bottom of the rim and add the diameter of the rim.

235/25.4 x .75 x 2 + 15 = 28.9 inches

The other tire, 235/70r15 is 27.9 inches

If you have electronic wheel speed sensors on your vehicle, you'll not want to combine the two tire sizes or install the wrong tire size. This will not only throw of your speedometer but it will cause your antilock brakes and traction control to operate erratically and unsafely.

235/75r15

235 is the tread width in millimeters. 75 is the aspect ratio of the sidewall to the tread width. The sidewall in this case is 75% of the tread width.

.75(235)= 176.25mm.

The third number is the diameter of the rim.

Using these numbers, you can find the overall diameter of the tire. You divide the tread width by 25.4 to convert to inches, multiply by the aspect ratio, double it because you have sidewall at the top and bottom of the rim and add the diameter of the rim.

235/25.4 x .75 x 2 + 15 = 28.9 inches

The other tire, 235/70r15 is 27.9 inches

If you have electronic wheel speed sensors on your vehicle, you'll not want to combine the two tire sizes or install the wrong tire size. This will not only throw of your speedometer but it will cause your antilock brakes and traction control to operate erratically and unsafely.