Guides · Field note
Why AWD and 4WD vehicles need four matching tires
On a two-wheel-drive car, replacing one damaged tire is often a simple parts decision. On an all-wheel-drive or four-wheel-drive vehicle it is not. The four tires are part of the driveline. If one is a different rolling circumference, it turns at a different speed in a straight line, and the coupling that joins the axles has to slip to make up the difference. Vehicle makers publish that as a damage risk, not as a sales suggestion.
What a mismatch does
In a straight line, four tires of the same circumference rotate at the same speed. AWD systems that permanently connect the front and rear — a centre differential, a viscous coupling, or a multi-plate transfer clutch — are built around that assumption. A shorter tire has to complete more revolutions to cover the same ground. The coupling then works as if the vehicle were in a shallow turn all the time.
Subaru of America’s 2021 technician bulletin on the CVT transfer clutch is unusually direct. The transfer clutch, at the rear of the CVT, applies power to the rear wheels. The bulletin restates a December 2010 TechTIPS note:
The tire circumference difference should be no more than 6mm or ¼" of each other. This measurement is different than tread depth.
The same bulletin says a deformed or severely worn tire can be smaller than the others and “could result in transfer clutch damage.” Mixed brands, mixed sizes, or even mixed models of the same brand can cause the same binding. Subaru’s example is an Ascent on factory Falken Ziex tires with one or two Falken WildPeak replacements of the same marked size.
That is why “they’re the same size on the sidewall” is not the test. Marked size is a nominal designation. Circumference is what the driveline sees.
What the vehicle makers publish
Subaru
Owner’s manuals for Subaru models, including the 2023 BRZ, list the same eight items that must match on all four tires: size, circumference, speed symbol, load index, construction, manufacturer, brand (tread pattern), and degrees of wear. If they do not, the manuals warn of serious mechanical damage to the drivetrain, plus effects on ride, handling, braking, speedometer calibration, and body clearance. The 2021 TechTIPS bulletin is the document that puts a number on circumference: 6 mm (1/4 inch), measured with a cloth tape at equal pressures from the door-jamb placard — not inferred from tread depth.
Audi (quattro / four-wheel drive)
Audi’s A4 owner’s information says all four wheels should carry tires of the same type and size (rolling circumference). On four-wheel-drive vehicles the language is stricter:
On vehicles with four-wheel drive, all four tyres must always be fitted with tyres of the same type, make and tread pattern, as otherwise the driveline can be damaged by continuous variations in the wheel speeds.
The B9 A4 manual repeats that for all-wheel drive: same brand, construction, and tread pattern “so that the drive system is not damaged by different wheel speeds,” and a spare used in an emergency should match the regular tires’ circumference. Audi also notes that two tires with the same size marking can still have different actual contours.
Honda
Honda’s 2024 CR-V owner’s manual tells owners to replace tires with radials of the same size, load range, speed rating, and maximum cold inflation rating. Different size or construction can make ABS and Vehicle Stability Assist work incorrectly. Honda’s replacement advice is: “It is best to replace all four tires at the same time. If that isn’t possible, replace the front or rear tires in pairs.” Honda does not publish a millimetre circumference limit in that section. The published preference is still a set of four, then an axle pair — not a single unmatched tire.
Toyota
Toyota’s four-wheel-drive language is consistent across models. The 2005 RAV4 manual: when replacing a tire, use the same size and construction; “on four-wheel drive models, all the tires must be the same brand and have the same tread patterns.” It also warns against mixing brands, sizes, construction, or tread patterns on 4WD because of dangerous handling. The 2021 Highlander manual tells owners to make sure all tires are the specified size, brand, tread pattern, and load capacity, and that ABS, traction control, and VSC will not function correctly if different tires are installed. Toyota recommends replacing all four, or at least both tires on an axle, as a set.
Those manuals do not all publish the same millimetre or 32nds figure. The common instruction is: same construction, same brand and pattern, same rolling size, and do not treat one new tire as interchangeable with three worn ones.
Circumference, not a casual tread-depth guess
Shops often talk about a 2/32 inch tread-depth spread. That number is a common proxy, not the measurement Subaru published. Subaru’s technician bulletin says the 6 mm / 1/4 inch limit is circumference, and that this “is different than tread depth.” Tread depth is easier to check with a gauge. Circumference is what you measure with a tape around the inflated tire.
A new passenger tire often starts around 10/32 inch of tread. Three tires that have worn 4/32 inch are already a different radius. Whether that difference exceeds a given vehicle’s limit is a measurement, not a rule of thumb. The owner’s manual for the vehicle in front of you is the document that governs.
When shaving a new tire is the alternative
If three tires still have a lot of life and one is destroyed, some specialty retailers will shave (buff) a new tire on a lathe so its tread depth — and therefore its rolling circumference — matches the three that stay on the car. Consumer Reports describes that service: buy the replacement, have it machined to the remaining depth of the others, and keep the set matched. Tire Rack publishes the same offer.
Shaving is not a driveway job. It needs a tire lathe and a measured target depth. Most mobile operators do not carry that equipment and cannot offer it on site. The honest alternatives in that case are a matched set of four, or sending the new tire to a shop that can shave it before it is mounted. Shaving also uses up tread life on purpose; it only makes sense when the remaining three tires still have substantial depth.
What this means on a driveway
If the vehicle is AWD or 4WD and one tire is finished, the question is not “can I mount a new one.” The question is whether the new tire’s circumference will stay inside the maker’s match rule. If the other three are close to new, one replacement of the same brand, model, and size may be inside the limit. If they are half worn, it usually is not. Replacing all four, or shaving the new tire, is how shops keep the driveline from seeing a constant speed error.
That recommendation is in the owner’s manual and in manufacturer service notes. It is not a registry rule, and it is not a verification of any operator. Check the manual for the specific vehicle. The pages linked below are the statements this article relies on.
Sources
- Subaru of America, TechTIPS special edition, “Transfer Clutch System Diagnostics Consolidated,” 2021 (NHTSA-hosted copy, MC-10205932-0001). Circumference limit 6 mm / 1/4 inch; mixed brand or model can bind the transfer clutch. https://static.oemdtc.com/TechTIPS/MC-10205932-0001.pdf
- Subaru BRZ 2023 owner’s manual, tires and wheels: all four tires must match size, circumference, manufacturer, brand, construction, and degrees of wear, or the drivetrain can be damaged. https://www.carmanualsonline.info/subaru-brz-2023-owners-manual/38
- Audi A4 owner’s information, replacing wheels and tyres: four-wheel drive requires the same type, make, and tread pattern or the driveline can be damaged by continuous wheel-speed variation. https://www.instruccionesaudi.com/A4/en_GB/onboard/o_m00489.htm
- Audi A4 (B9) owner’s manual, new tires or wheels: all-wheel drive, same brand, construction, and tread pattern so different wheel speeds do not damage the drive system. https://www.audia4b9.com/new_tires_or_wheels-1978.html
- Honda, 2024 CR-V owner’s manual, tire and wheel replacement: best to replace all four; otherwise replace an axle pair. https://techinfo.honda.com/rjanisis/pubs/OM/AH/A3A02424IOM/enu/details/131240047-14857.html
- Toyota RAV4 2005 owner’s manual: on four-wheel-drive models, all tires must be the same brand and have the same tread patterns. https://www.carmanualsonline.info/toyota-rav4-2005-xa30-3-g-owners-manual/?srch=tire+type
- Toyota Highlander 2021 owner’s manual: all tires the specified size, brand, tread pattern, and load capacity; ABS/VSC will not function correctly with different tires. https://www.carmanualsonline.info/toyota-highlander-2021-owners-manual-in-english-2/?srch=wheel+torque
- Consumer Reports, “Do You Really Need to Replace All Four Tires on an AWD Car?” (tire shaving as an alternative to a four-tire set). https://www.consumerreports.org/cars/tires/tire-shaving-awd-car-no-need-to-replace-all-four-tires-a4235415952/
Questions that come up
Is replacing all four just an upsell?
Not on AWD or 4WD. Subaru, Audi, Honda, and Toyota publish matching rules because a circumference difference is seen by the driveline as constant slip. The owner’s manual is the document to read, not a sales script.
What is Subaru’s actual limit?
Subaru’s 2021 technician bulletin: no more than 6 mm (1/4 inch) of circumference difference, measured at equal placard pressures. The bulletin says that is not a tread-depth measurement. Owner’s manuals also require matching brand, pattern, construction, and wear.
Can I replace just two tires?
Honda explicitly allows an axle pair if four are not possible. Toyota recommends all four, or at least both tires on an axle. Audi prefers pairs rather than a single tire, and requires all four on four-wheel drive to share type, make, and pattern. A pair still has to stay inside the vehicle’s circumference match.
What is tire shaving, and can a mobile operator do it?
Shaving is machining tread off a new tire so its depth matches the tires that stay on the car. Consumer Reports and Tire Rack describe it as a way to replace one tire on AWD without buying four. It needs a lathe. Most mobile vans do not have one.