How to Measure Car Rims

To measure the diameter of the tire, in other word, what size tire you're looking for, you would measure the inside of the rim across using an inside diameter reading. The actual reading would be where the bottom of the bead of the tire sits, but that's hard to measure considering the ridge at the back of the rim prohibits this. The bead ridge of a rim is approximately 3/4 of an inch. If you measure the inside diameter of the rim, you're going to get a pretty accurate reading for steel wheels and you could add about 1/2 inch for thicker aluminum wheels. It's safe to say that although on trucks and SUVs you can place a larger size tire on them, for example moving up from a 15-inch tire to a 16-inch tire, you're going to be much more limited by trying to do that with a car that will have less tolerant wheel well room. So if 15-inch tires came with the car and you just want a wider rim, it's still going to be a 15-inch diameter tire that's going to fit on the new rim no matter what the measurement is. It is not uncommon to see diameter measurements contradict the actual size of the tire that fits it by a half inch to an inch in either direction. Most all rims have their measurement stamped or cast into the rim on the back side.

Measure the width of the tire by placing the measuring tape on the inside bead lip and across to the opposite inside bead lip. So if it measured 7 inches and the diameter was 15 inches, it would be a 15-by-7-inch rim.

Measure the bolt patterns of the rim. This is where the rim sizing starts to get a little tricky. The first thing you want to compare is how many lug nuts or studs are on your vehicle and then how many are on the rim. If the numbers match, that does not necessarily mean the bolt pattern will fit. Measuring the hub of the rim (the hole in the center), how many bolt holes in the rim, and then how many inches apart the bolt holes are from one another is how to achieve an accurate bolt hole pattern measurement. Take the measuring tape and place it on the edge of the inside of one bolt hole and then measure across to the farthest opposite bolt hole to the inside of that hole. Let's say it was 4 1/2 inches. So far this rim would be a 15x7x4.5. Even bolt hole numbers are measured from one bolt hole to the exact opposite one and odd-numbered bolt holes are measured to the furthest one away across at the slight angle.

The next measurement to consider is the backspacing measurement of the rim. This is the distance of the face of the wheel to the inside edge of the wheel, which determines clearance. You will measure the backspace by running a straight edge across the rim bead, drop another straightedge down to the wheel face and measure that distance. That measurement would be your backspace.

The last measurement to consider is the offset of the rim. This determines where the rim sits against the hub of the vehicle and how far it will protrude in inset while bolted securely. There are three offset positions and they will determine how the wheel and tire will sit on the vehicle when secured. A positive offset will have a deeper face on the rim and extend the tire outward from the vehicle. Considerations with a positive offset are whether state laws allow a tire to pass the wheel well of the vehicle or not and if so, but how much. A negative offset has a shallower face and seat the tire and rim deeper into the wheel well. Considerations with a negative offset is whether the rim or tire will properly fit over the caliper and not incur damage to suspension components or rub against the interior of the wheel well, especially when turning. A zero offset is the face of the rim perfectly centered in the middle of the rim width. Considerations with this option are only comparison to your stock rim, which might have a positive or negative offset, and how that will manipulate the new tire.