Car wheel size can be determined by several methods. All of these methods are correct and can be used when ordering tires. Measuring your tires with any one of the methods below will give you a number that a salesperson at any tire shop should be able to use to get you the size tire you need.
Doing just one of these methods is sufficient but several have been presented so that you know all your options.
Decide which measurement you need:
Rim diameter is from the left edge of the rim to the right.
Tire diameter is from the left outside edge of the tire to the right outside edge of the tire.
Tire radius is a measurement from the center of the rim to any outside edge of the tire. This can also be calculated as tire diameter divided by 2.
Tread width is the width of the tire that would be contacting the road.
Rim width is the width of the rim from outside flange to inside.
To measure rim diameter:
Start by placing the edge of the tape measure onto the lip of the rim just as it meets the tire on the left hand side. Then measure to the lip directly across on your right hand side.
To measure tire diameter:
Place the edge of the tape measure onto the left side of the tire, preferably where the tread is sticking out at its farthest. Then measure to the right hand side of the tire, again preferably to where the tread is sticking out.
Take care to measure this equally, if you measure the left side from where the tread begins (commonly, the tape measure tab usually likes to "stick" here), make sure to measure to this same point on the right side of tire (In this case, where the tread sticks out).
To measure tire radius:
Hold the tape measure end in the center of the rim and measure to the right side of the tire. Again, preferably measure to the outside tread.
Or
Simply divide the tire diameter by 2.
To measure tread width:
Place the end of the tape measure all the way back into the wheel well, let it snag on the backside of the tire tread. You normally will not be able to see this point but trust that it has snagged on the tread correctly. Measure to the other side of the tire tread that is closest to you.
To measure rim width:
This is most easily accomplished if the tire is dismounted from the rim. Measure from the
inside tire seat to the outside tire seat. Do not measure the width between the two ridges as this will often be very inaccurate.
If this is not an option due to tool considerations then you can read the tire, you will find a code similar to: "P205/40R16". The "R16" part is the rim width.
Record your measurements in a tablet or small notebook. These measurements are often referenced and should be kept on hand.
