Tire Size Conversion Chart: Metric to Inches

Metric sizes like 275/70R18 and flotation sizes like 33x12.50R18 describe the same thing in different languages. This page converts between them and shows how to pick a size that keeps your speedometer honest.

The 3 percent rule

Unless you are lifting the vehicle and correcting the electronics, keep overall tire diameter within about 3 percent of the factory size. Inside that window the speedometer stays close enough, the transmission shifts near its calibration, and ABS and stability control read wheel speeds they can work with.

Outside that window things start to drift in ways that are more than cosmetic. On all-wheel drive vehicles, a diameter mismatch between axles means the driveline is being asked to absorb a permanent speed difference, which loads the center differential or transfer case continuously.

How to calculate overall diameter

Overall diameter (in) = ((Section width mm x Aspect ratio / 100) / 25.4 x 2) + Wheel diameter

Worked example with 275/65R18: 275 x 0.65 = 178.75mm of sidewall. Divided by 25.4 gives 7.04 inches. Doubled is 14.08 inches. Plus the 18 inch wheel gives 32.08 inches overall. That is why 275/65R18 is commonly called a 32 inch tire.

16 and 17 inch wheel equivalents

Flotation size	Metric equivalent	Overall diameter
31x10.50R15	265/75R15	30.6 in
32x11.50R15	285/75R15	31.8 in
30x9.50R16	245/75R16	30.5 in
31x10.50R16	265/75R16	31.6 in
32x11.50R16	285/75R16	32.8 in
33x12.50R16	305/70R16	32.8 in
31x10.50R17	255/75R17	32.1 in
32x11.50R17	285/70R17	32.7 in
33x12.50R17	285/75R17	33.8 in
34x11.50R17	295/70R17	33.3 in
35x12.50R17	315/70R17	34.4 in
37x12.50R17		37.0 in

18 inch wheel equivalents

Flotation size	Metric equivalent	Overall diameter
31x10.50R18	265/70R18	32.6 in
32x11.50R18	275/70R18	33.2 in
33x12.50R18	285/70R18	33.7 in
34x12.50R18	305/65R18	33.6 in
35x12.50R18	325/65R18	34.6 in
37x12.50R18		37.0 in

20 inch wheel equivalents

Flotation size	Metric equivalent	Overall diameter
31x10.50R20	255/60R20	32.0 in
32x11.50R20	275/60R20	33.0 in
33x12.50R20	295/60R20	33.9 in
34x12.50R20	305/60R20	34.4 in
35x12.50R20	325/60R20	35.4 in
37x13.50R20		37.0 in

22 inch wheel equivalents

Flotation size	Metric equivalent	Overall diameter
33x12.50R22	305/45R22	32.8 in
34x12.50R22	325/50R22	34.8 in
35x12.50R22	345/45R22	34.2 in
37x13.50R22		37.0 in

Calculating speedometer error

When you fit a larger tire the vehicle travels farther per wheel revolution than the speedometer expects, so the displayed speed reads low. The error percentage is the same as the diameter change percentage.

Actual speed = Indicated speed x (New diameter / Original diameter)

Stock diameter	New diameter	Change	Indicated 70 mph = actual
32.0 in	33.0 in	+3.1%	72.2 mph
32.0 in	34.0 in	+6.3%	74.4 mph
32.0 in	35.0 in	+9.4%	76.6 mph
32.0 in	37.0 in	+15.6%	80.9 mph
33.0 in	35.0 in	+6.1%	74.2 mph
33.0 in	37.0 in	+12.1%	78.5 mph

Most modern trucks can be recalibrated with a handheld programmer that lets you enter the new tire size. Doing this restores correct speedometer and odometer readings and, importantly, restores correct transmission shift points, which is why a truck on oversized tires often feels better after recalibration than the tire change alone suggests.