AWD Tire Circumference Tolerance: How to Match All Four Tires on an All-Wheel-Drive Vehicle

Short answer: On an all-wheel-drive (AWD) vehicle, all four tires should have nearly the same rolling circumference. That means the same size designation on the sidewall and closely matched tread depth. The exact acceptable difference is set by each vehicle manufacturer, so the owner's manual or the manufacturer's service information is the authority for your vehicle. Commonly cited AWD guidance limits tread depth differences to roughly 2/32 to 4/32 inch between tires, or limits the circumference difference to a fraction of an inch. A 2/32 inch tread depth difference changes a tire's circumference by about 0.39 inch (10 mm), whatever the tire size. This article explains how to get from the sidewall numbers to those figures and what the figures do and do not tell you.

What Do the Numbers on the Tire Sidewall Mean?

A tire size such as 225/60R17 99H combines three dimensions, one construction code and two performance ratings. Only the dimensions determine a tire's calculated diameter and rolling circumference.

Dimensions: width, aspect ratio and rim diameter

• 225 is the section width in millimetres. It is the nominal width of the tire from sidewall to sidewall, measured on a specified rim.
• 60 is the aspect ratio, a percentage rather than a length. The sidewall height equals 60% of the section width. A larger number means a taller sidewall relative to the width.
• R is the construction type. It means radial construction. It is not a dimension.
• 17 is the rim diameter in inches. It is the wheel diameter the tire is designed to fit.

One size code mixes two units: the width is in millimetres and the rim diameter is in inches. Any calculation has to convert them to the same unit first. One inch equals 25.4 mm.

Performance ratings: load index, speed rating and others

• 99 is the load index. It is a code for the maximum load one tire can carry at its specified inflation pressure. It says nothing about size.
• H is the speed rating. It is a letter code for the maximum sustained speed the tire is certified for under test conditions.
• UTQG treadwear, traction and temperature grades appear on many passenger tires. The treadwear number compares wear rates under a standardized test. It does not state the tread depth in millimetres or 32nds of an inch.
• The DOT code identifies the manufacturer and plant, and its last four digits give the week and year of production.
• Prefixes such as P (passenger) or LT (light truck) indicate the service type and load standard. They are not dimensions.

Tread depth is not printed on the sidewall. You measure it with a tread depth gauge. In the United States it is usually given in 32nds of an inch; elsewhere it is often given in millimetres.

How Do Width, Aspect Ratio and Rim Size Determine Circumference?

A tire's calculated overall diameter equals the rim diameter plus two sidewall heights. Rolling circumference is that diameter multiplied by pi (about 3.1416). The calculation works in three steps.

1. Sidewall height = section width × aspect ratio ÷ 100. For 225/60R17: 225 × 0.60 = 135 mm.
2. Overall diameter = (rim diameter × 25.4) + (2 × sidewall height). For 225/60R17: 431.8 mm + 270 mm = 701.8 mm (about 27.6 inches).
3. Circumference = overall diameter × 3.1416. For 225/60R17: about 2,205 mm (about 86.8 inches). That works out to about 730 revolutions per mile (1,609,344 mm ÷ 2,205 mm).

Width affects diameter only through the aspect ratio. A wider tire with the same aspect ratio has a taller sidewall, because the sidewall is a percentage of the width.

Example: comparing three 17-inch sizes

Size	Sidewall height	Calculated diameter	Calculated circumference	Difference vs 225/60R17
225/60R17	135.0 mm	701.8 mm	2,204.8 mm	Reference
235/55R17	129.3 mm	690.3 mm	2,168.6 mm	−36 mm (−1.6%)
225/65R17	146.3 mm	724.3 mm	2,275.5 mm	+71 mm (+3.2%)

235/55R17 is wider than 225/60R17 but has a smaller overall diameter, because its lower aspect ratio more than cancels the extra width. The difference between these nominal sizes is much larger than the difference caused by a few 32nds of tread wear. This is why mixing size designations on an AWD vehicle is a bigger problem than modest tread wear differences.

Calculated geometry vs actual tire dimensions

Sidewall calculations give a nominal diameter, not the real measurement of a particular tire. The real dimensions can differ for several reasons:

• Brand and model: two tires with the same size code can differ slightly in actual overall diameter.
• Load: the rolling radius under the vehicle's weight is smaller than the unloaded radius. For this reason, manufacturers publish a measured revolutions-per-mile (or per-kilometre) figure, which differs from the simple geometric calculation.
• New tread depth: tires start with different tread depths depending on the model and type, such as winter, all-season or all-terrain.
• Inflation pressure: lower pressure lets the tire deflect more and reduces the loaded radius.

For the most accurate comparison between specific tires, use the overall diameter and revolutions-per-mile figures from the tire maker's specification data. Fitment is a separate question. Whether a size clears the suspension and bodywork, suits the wheel width and meets load requirements depends on the vehicle manufacturer's specifications, not on circumference alone.

What Is AWD Tire Circumference Tolerance?

AWD tire circumference tolerance is the maximum difference in rolling circumference between the tires that a vehicle's drivetrain is designed to accept without excessive internal slip. Some manufacturers express it as a circumference difference, others as a tread depth difference, and some simply require all four tires to be the same brand, model, size and wear level.

Why rolling circumference matters to AWD drivetrain compatibility

A tire with a smaller circumference must turn more times to cover the same distance. When the front and rear tires differ, the front and rear driveshafts are forced to turn at different speeds even when driving straight. How the vehicle copes depends on the drivetrain:

• Full-time AWD with a center differential lets front and rear axles turn at different speeds, but a constant speed difference keeps the center differential working at all times. Any limited-slip device inside it, such as a viscous coupling or clutch pack, may also be under constant load.
• On-demand AWD with an electronically controlled coupling may interpret a constant speed difference as wheel slip and engage the rear (or front) axle more than intended.
• Part-time four-wheel drive with a locked transfer case has no center differential when 4WD is engaged. It mechanically ties front and rear driveshafts together, so a front-to-rear rolling circumference mismatch has to be absorbed by tire slip and stress in the driveline.

Constant internal slip generates heat and wear in these components. That is the reason manufacturers set tire-matching requirements. How much tolerance a given system has is a design matter, and only the vehicle manufacturer specifies it.

How Much Tread Depth Difference Is Acceptable on an AWD Vehicle?

The acceptable tread depth difference on an AWD vehicle is whatever the vehicle manufacturer specifies. Limits commonly cited in AWD tire guidance fall roughly between 2/32 and 4/32 inch (about 1.6 to 3.2 mm), and some manufacturers state the requirement as a circumference limit instead. There is no single universal figure for maximum tread depth difference across all AWD vehicles. Check the owner's manual, the tire section of the maintenance information, or a dealer for your exact model.

How tread depth difference converts to circumference difference

Tread depth reduces a tire's radius. Because the diameter includes the tread on both sides of the tire, every 1/32 inch of tread depth difference changes the diameter by 2/32 inch. The calculated circumference difference is:

Circumference difference = 2 × π × tread depth difference

In inches or millimetres, this result does not depend on tire size. As a percentage, it does depend on tire size. The table uses 225/60R17 (about 2,205 mm calculated circumference) for the percentages.

Tread depth difference	In millimetres	Circumference difference	Percentage on 225/60R17
1/32 inch	0.79 mm	about 0.20 inch (5 mm)	about 0.23%
2/32 inch	1.59 mm	about 0.39 inch (10 mm)	about 0.45%
3/32 inch	2.38 mm	about 0.59 inch (15 mm)	about 0.68%
4/32 inch	3.18 mm	about 0.79 inch (20 mm)	about 0.90%
6/32 inch	4.76 mm	about 1.18 inch (30 mm)	about 1.36%

If a manufacturer gives a circumference limit of 1/4 inch, the equivalent tread depth difference is about 0.04 inch, a little over 1/32 inch (0.25 ÷ 2π ≈ 0.040 inch). A circumference-based limit can therefore be stricter than it first sounds.

Why replacing a single tire on an AWD vehicle causes a tread depth mismatch

Fitting one new tire next to three worn tires creates the largest likely tread depth mismatch. Passenger tires often start with tread in the range of roughly 10/32 to 11/32 inch, although new depth varies by model. Three worn tires at 5/32 inch next to one new tire at 10/32 inch differ by 5/32 inch, which is about a 1-inch (25 mm) circumference difference. That exceeds the commonly cited AWD tread depth limits.

How to Check Whether Your Four Tires Are Matched

You can check AWD tire matching with a tread depth gauge and the information on the sidewalls.

1. Read every sidewall. Confirm that all four tires have the same size designation. Unless the vehicle is designed for different sizes front and rear, they should also be the same brand and model.
2. Find the manufacturer's limit. Look in the owner's manual for the AWD tire-matching requirement, whether it is a tread depth or a circumference figure.
3. Measure tread depth on each tire. Measure in the main grooves at the inner, center and outer positions, avoiding the raised wear indicator bars. Record the readings in 32nds of an inch or in millimetres, using the same unit for all four tires.
4. Calculate the largest difference. Subtract the shallowest tire's depth from the deepest tire's depth. Uneven wear across one tire is a separate issue, but note it as well.
5. Convert if needed. If your limit is stated as circumference, multiply the tread depth difference by 2π (about 6.28) to get the circumference difference.
6. Compare with the limit. If the difference is within the manufacturer's limit, the tires are matched for that requirement. If not, matching options include replacing all four tires or having a new tire's tread shaved down to the depth of the others, a service some tire shops offer. Which options are acceptable depends on the vehicle manufacturer's guidance.
7. Check inflation. Set all tires to the pressure on the vehicle's placard, because pressure differences also change the loaded rolling radius.

Can You Run Two Different Size Tires on Front and Rear of a 4x4 or AWD Vehicle?

Two different tire sizes front and rear are acceptable only when the vehicle manufacturer specifies that combination for the model. Fitting different sizes on a 4x4 or AWD vehicle that was designed for one size all round creates a front-to-rear rolling circumference mismatch the drivetrain was not designed for.

Factory staggered fitments: different widths, similar diameter

Some AWD performance vehicles are fitted from the factory with wider rear tires. These combinations are usually chosen so that the overall diameters stay close. For example, on 19-inch wheels:

• 245/40R19: sidewall 98 mm, calculated diameter 678.6 mm, circumference about 2,132 mm.
• 275/35R19: sidewall 96.3 mm, calculated diameter 675.1 mm, circumference about 2,121 mm.

The calculated circumference difference is about 11 mm (0.5%), similar in size to a 2/32 inch tread depth difference. Even so, calculated numbers do not prove compatibility. The actual dimensions of the chosen tires and the vehicle manufacturer's approved sizes decide whether a staggered set is acceptable.

Non-matching sizes on a vehicle designed for square fitment

On a vehicle designed for the same size at all four corners, a size mismatch such as 225/60R17 on one axle and 225/65R17 on the other produces a calculated circumference difference of about 71 mm (about 2.8 inches). That is several times larger than the commonly cited AWD tread depth limits allow. In a part-time 4WD system with the transfer case locked, the mismatch has nowhere to go except into tire scrub and driveline stress. In a full-time AWD system, the center differential or coupling is kept continuously slipping.

What the Numbers Can and Cannot Tell You

Sidewall calculations and tread depth measurements show whether tires are dimensionally similar. They do not confirm that a tire combination is approved for a particular vehicle.

• The numbers can tell you: nominal sidewall height, overall diameter and rolling circumference; how a tread depth difference translates into a circumference difference; and how two size designations compare in percentage terms.
• The numbers cannot tell you: the exact loaded rolling radius of a specific tire on your vehicle, how much continuous slip your AWD system tolerates, or whether a size physically fits and carries the required load.

For AWD tire circumference tolerance, use the calculations to understand and compare, and use the vehicle manufacturer's tire-matching requirements to decide.