The 20% Rule: Should You Replace 1, 2, or All 4 Tires?

You hit a pothole on Crowchild, ruined one tire, and now the shop is asking whether you want to replace one, two, or all four. The answer matters for your wallet and for how the car drives. The "20% rule" is the industry shorthand most reputable shops use to make this call — but the rule is more nuanced than it sounds, and some Calgary-specific factors make the decision different here than in the textbook. Here's how to know whether one, two, or all four tires need replacing after a single-tire failure, what the 20% rule actually says, and what to do when budget and safety pull in different directions.

Table of Contents

• The 20% Rule Explained
• When Replacing 1 Tire Is OK
• When You Need to Replace 2 (an Axle Pair)
• When You Need to Replace All 4
• Calgary Tire Replacement FAQs

The 20% Rule Explained

The 20% rule says: if your existing tires have lost more than 20% of their original tread depth, replacing only the damaged tire creates a meaningful tread-depth mismatch — and that mismatch causes handling, drivetrain, and traction problems. In that case, you need at least an axle pair (two tires), and on AWD vehicles all four.

The math: a typical new passenger tire has 10 to 11/32" of tread. 20% of that is roughly 2/32". So if your other tires have worn from 10/32" to 8/32" or less, a brand-new tire on the same axle creates over 2/32" of difference — and that's the threshold where ABS, traction control, and AWD systems start to behave unpredictably.

Most Calgary tires hit the 20% wear mark within the first 30,000 km. Daily commuters running Deerfoot or Glenmore hit it sooner. Our breakdown on how long do tires last in Calgary covers what's typical here. The toonie test is the easiest at-home way to check.

When Replacing 1 Tire Is OK

Single-tire replacement is fine if all of these are true:

• Your other three tires have lost less than 20% of their tread (so they're at 8/32" or above)
• Your other three are the same brand and model as the replacement you're buying
• The vehicle is front-drive, rear-drive, or 2WD — not AWD or 4WD
• The new tire is the exact same size, speed rating, and load index

If all four boxes check, one new tire is the right and economical answer. The new tire goes on the rear axle and the existing rear tire from that side moves to the front to replace the damaged one.

When You Need to Replace 2 (an Axle Pair)

Replace two tires (matched as an axle pair) when:

• Your other tires are between 4/32" and 7/32" of tread (i.e., past the 20% threshold but not yet down to legal minimum)
• You can't source the same brand/model as the existing tires for the single replacement
• The damaged tire is on a vehicle where the rear pair is more worn than the front (or vice versa) and replacing one would create an even larger mismatch

The two new tires always go on the rear axle. The existing rear pair (still serviceable) moves to the front. This is counter-intuitive for many drivers, but the rear is where you lose grip first in wet, snow, or panic-braking conditions, and you want the deeper-tread tires there. This is true for front-wheel drive too.

For more on why mismatched tires are dangerous, see our mismatched tires breakdown.

When You Need to Replace All 4

All-four replacement is required (or strongly recommended) when:

• The vehicle is AWD or 4WD and any tire is more than 2/32" different from the others. Mismatched diameters generate continuous heat that destroys the drivetrain over months.
• All four tires are within 6 months of replacement age (six years from manufacture). See how to read a tire's DOT date.
• The remaining tires have less than 4/32" of tread. Below 4/32" you're approaching the legal minimum.
• You drive winter tires and the failed tire is in the winter set. Winter tires need matched tread depth for snow performance.

For Calgary AWD owners (Subaru, RAV4, CR-V, BMW X-anything, Audi Q-anything, Mercedes 4MATIC), the four-tire replacement after a single failure is non-negotiable on a tire set that's already 1+ years old.

Calgary Tire Replacement FAQs

Why do new tires go on the rear, even on a front-drive car?
Because the rear is where you lose control if you understeer or get caught in wet/snow conditions. Deeper tread on the rear keeps the back of the car planted. Counter-intuitive, but every tire manufacturer agrees.

Can I just buy one used tire to replace mine if the others are worn?
Yes if you can find a matching brand, model, size, and similar tread depth. Read our used tire buying guide for what to check.

What if the dealership says I need all four for my AWD even though only one is damaged?
On AWD, they're usually right. The math is unforgiving.

How much does it cost to replace one tire vs four in Calgary?
One installed: $200-$400 typical. Four installed: $800-$1,800 typical. See our honest price guide.

Will my insurance cover pothole-damaged tire replacement?
Comprehensive coverage often does, but the deductible usually exceeds the cost of one tire. Check before claiming.

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Honest call on whether you need 1, 2, or 4

If you've damaged a tire and you're not sure how many to replace, bring the car in. We'll measure tread on all four, look at your drivetrain type, and give you the straight answer — including when one is enough.

Prince Tires
111 42 Ave SW, Calgary, AB
(403) 452-4283
Book your appointment