TireGeoCalcCan You Put Two Different Tire Sizes on an AWD Vehicle?
Usually not. Two tire sizes on an all-wheel-drive (AWD) vehicle make the wheels turn at different speeds all the time, and the drive system has to make up the difference on every mile. For example, 275/65R18 and 285/65R18 differ by 1.61 inches of circumference, while Subaru, the one maker that publishes a number, allows about 1/4 inch.
Check your owner's manual before you mix sizes, or before you replace only two tires. This guide shows how to calculate the difference yourself.
Why does a size difference matter on AWD?
On an AWD vehicle the engine drives the front and rear wheels together through a center differential or a clutch coupling. In a turn, the wheels travel different distances for a short time, and the system is built to allow that.
A tire with a larger circumference covers more road per turn, so it turns more slowly than the others at the same speed. With mismatched tires this difference never stops, even on a straight road. Vehicle makers set tire-matching rules for AWD models for this reason. The exact rule for your vehicle is in the owner's manual.
Subaru publishes its limit: the overall circumference of the tires should be within about 1/4 inch (6 mm) of each other (Subaru service bulletin 03-75-15, filed with NHTSA as SB-10093066). That is a Subaru rule. Do not apply it to other makes without checking their own manual.
How do you calculate the difference between two tires?
- Overall diameter (mm) = rim x 25.4 + 2 x width x aspect / 100.
- Circumference = 3.1416 x diameter.
- Subtract the two circumferences.
Example: 275/65R18 and 285/65R18
- 275/65R18: 457.2 + 2 x 275 x 0.65 = 457.2 + 357.5 = 814.7 mm.
- 285/65R18: 457.2 + 2 x 285 x 0.65 = 457.2 + 370.5 = 827.7 mm.
- The diameters differ by 13.0 mm. The circumferences differ by 3.1416 x 13.0 = 40.8 mm, or 1.61 inches.
- That is 1.60% of the 275/65R18 diameter, and more than six times Subaru's 1/4-inch figure.
You can check any pair with the tire size calculator.
How big is the difference for common size pairs?
Table 1 uses nominal sizes. Real tires differ by brand and model.
| Tire A | Tire B | Diameter difference | Circumference difference |
|---|---|---|---|
| 225/45R17 | 235/45R17 | 9.0 mm (+1.42%) | 28.3 mm (1.11 in) |
| 225/60R17 | 235/55R17 | -11.5 mm (-1.64%) | -36.1 mm (-1.42 in) |
| 225/65R17 | 235/65R17 | 13.0 mm (+1.79%) | 40.8 mm (1.61 in) |
| 275/65R18 | 285/65R18 | 13.0 mm (+1.60%) | 40.8 mm (1.61 in) |
| 225/60R18 | 225/65R18 | 22.5 mm (+3.09%) | 70.7 mm (2.78 in) |
Calculated dimensions
- Tire A
- 225/45R17
- Tire B
- 235/45R17
- Diameter difference
- 9.0 mm (+1.42%)
- Circumference difference
- 28.3 mm (1.11 in)
- Tire A
- 225/60R17
- Tire B
- 235/55R17
- Diameter difference
- -11.5 mm (-1.64%)
- Circumference difference
- -36.1 mm (-1.42 in)
- Tire A
- 225/65R17
- Tire B
- 235/65R17
- Diameter difference
- 13.0 mm (+1.79%)
- Circumference difference
- 40.8 mm (1.61 in)
- Tire A
- 275/65R18
- Tire B
- 285/65R18
- Diameter difference
- 13.0 mm (+1.60%)
- Circumference difference
- 40.8 mm (1.61 in)
- Tire A
- 225/60R18
- Tire B
- 225/65R18
- Diameter difference
- 22.5 mm (+3.09%)
- Circumference difference
- 70.7 mm (2.78 in)
Every pair in the table is well above 1/4 inch (about 6 mm) of circumference.
Does tread wear count too?
Yes. A worn tire is shorter than a new one of the same size. Tread depth is measured in 32nds of an inch, and it counts twice in the diameter, once at the top and once at the bottom.
- New tire with 10/32 in of tread next to a worn tire with 4/32 in: the diameters differ by 12/32 in (0.375 in). The circumferences differ by about 1.18 in (30 mm).
- Even 10/32 next to 8/32 gives about 0.39 in (10 mm) of circumference difference.
This is why replacing only two tires on an AWD vehicle can cause the same problem as mixing sizes. Some tire shops offer tire shaving, which cuts the new tire's tread down to match the worn ones.
What should you do instead?
- Replace all four tires with the same size, brand and model, if the others are worn.
- If only one tire is damaged, ask the shop to measure the tread on the other three and compare it with the limit in your manual.
- If you want a different size, change all four and keep the new diameter close to the original. The tire size alternatives tool lists sizes near your current one.
- A size change also shifts the speedometer and odometer. See do bigger tires change your odometer reading.
Frequently asked questions
Is a 1.6% difference fine because it is under 3%?
No. The 3% figure is a common industry guideline for comparing tire sizes and speedometer error. It is not a law and not an AWD tolerance. AWD limits are much tighter where a maker publishes one.
Can I put the two new tires on the rear and keep the old ones on the front?
On an AWD vehicle, a tread difference between axles is still a circumference difference. Whether it is allowed depends on your owner's manual and the measured tread depths.
Some AWD cars come with different front and rear sizes. Why is that allowed?
The vehicle maker chose those sizes and set up the drive system for them. Copying the idea with sizes the maker did not list is not the same thing.