Hey guys, I never got used to tire and wheel sizes for cars. I remember years ago when I use to drive on 35" swammpers and everything was so big and easy to calculate. My tire sizes say R14, but I measure the rim from top to bottom, the diameter? And they are 15" I thought they were 14's, also how Do I measure the width on the wheels? And what exactly is backspacing? tongue.gif
Fill me up with info! Thanks.....

The 14 or 15 is the size of the rim.

The largest number (215-275) is the width of the tire in millimeters.

The last number (70-35) is the height of the tires side wall, in percentage of the width.

The rim diameter is measured to where the tire seats. There is a lip, which is why you measure more than 14 inches...

If you were to measure the inside of the TIRE opening before it was mounted on the rim, then it would measure 14-inches. The same goes for the wheel. If you were to measure the diameter where the tire seats (inside the lip, inaccessible when the tire is on and inflated), you would also measure 14-inches...

That lip keeps the tire from sliding off the rim...

Wheel width: This is measure from the INSIDE of the above lip to the INSIDE of the opposite lip.

On steel wheels, this lip is often close to 1/2 inch wide. Therefore an 8-inch wide rim is really 9-inches overall width. BUT, its 8-inches where the tire actually seats...


Backspacing is measured from the OUTSIDE of the
lip (same lip as above, but not where the tire seats) to the wheel mounting surface... Of course, this is measure on the backside of the rim, so use that lip...

Example: Say you have a 15*8 inch wheel... Its really 9 inches overall but 8-inches between the lips, so 4.5-inch Backspacing will center the wheel about the mounting point because 4.5 is half of the total 9-inch width (and NOT 4-inch BS).