Tire size calculator
Compare the size on your car with the one you are considering: diameter, sidewall, revolutions per kilometre, and what the speedometer will read.
Compare two metric tire sizes: overall diameter, sidewall, width, revolutions per kilometre and speedometer error. Staying within about 3% of the original diameter is a tire maker’s guideline (BFGoodrich), not a law. The size your vehicle maker specifies is on the tire information label, usually on the driver’s door or door frame. Sources checked .
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How to read a tire size
The size is moulded on the tire’s sidewall. Taking 225/65R17 apart, the way Transport Canada and the U.S. Tire Manufacturers Association describe it:
- P or LT in front, when there is one: a passenger or a light-truck tire. A size with no letter is a metric (Euro-metric) passenger size.
- 225: the section width, sidewall to sidewall, in millimetres.
- 65: the aspect ratio. The sidewall is 65% as tall as the tire is wide, which here is 146 mm.
- R: radial construction.
- 17: the rim diameter, in inches.
- After the size, a number and a letter such as 102H: the load index and the speed rating.
The overall diameter is twice the sidewall height plus the rim diameter. For 225/65R17 that is 2 × 146 mm + 432 mm (17 in) = 724 mm, or about 28.5 in. The calculator takes metric sizes only. Flotation sizes such as 31x10.50R15 give the diameter and width in inches, so they have no aspect ratio to work from.
The ±3% rule of thumb
A tire with a different overall diameter covers a different distance each time it turns. The USTMA’s service manual lists what that can affect: the speedometer and odometer, ABS, the tire pressure monitoring system, all- and four-wheel drive, traction control and other electronic systems, and how the vehicle handles.
BFGoodrich’s guideline is to stay within about 3% of the original diameter. Past that point, it says, the speedometer and odometer drift far enough to matter, and the calibration of ABS and stability control suffers. It is a rule of thumb, not a law and not your vehicle maker’s figure. What governs is the vehicle maker’s tolerance: the USTMA manual says all four tires have to stay inside it, and that plus sizing (a bigger rim with a lower, wider tire) sets out to keep the original diameter.
The calculator checks geometry, not fit. The same manual says a different size needs at least the load capacity of the original at the placard’s pressure, a speed rating at least as high if the vehicle’s speed capability is to be kept, a rim of an approved width, and room in every wheel well at full lock and full suspension travel.
Sizing a winter set
Consumer Reports notes that tire sellers often suggest putting winter tires on a smaller wheel with a taller sidewall, keeping the overall diameter about the same: the longer footprint can help in snow, while a wider tire hurts snow traction. Transport Canada also says wide, high-performance tires that are not winter tires do not suit snow-covered roads. Going down one inch, with the diameter kept:
| Measurement | 225/45R18 | 215/55R17 | Difference |
|---|---|---|---|
| Section width | 225 mm | 215 mm | −10 mm |
| Sidewall height | 101 mm | 118 mm | +17 mm |
| Rim diameter | 18 in | 17 in | −1 in |
| Overall diameter | 660 mm (26.0 in) | 668 mm (26.3 in) | +8 mm (+1.3%) |
| Circumference | 2,073 mm | 2,100 mm | +27 mm |
| Revolutions per kilometre | 483 | 476 | −7 |
The smaller wheel still has to clear the brakes, which Consumer Reports warns not every wheel does, and the size has to be one your vehicle maker allows. Fit the same size on all four: Transport Canada says a car handles best when all four tires match in type, size, speed rating and load index. For when to put them on, see the winter tire dates for Calgary and the Calgary winter tire guide.
Start from the placard
The size your vehicle maker chose is on the tire information label, which Transport Canada says is usually fixed to the edge of the driver’s door or to the door post, and in the owner’s manual. On a used car the tires fitted now may not be that size, and Michelin treats the placard as the one to go by. Vehicle makers sometimes approve other sizes as well; the Tire and Rubber Association of Canada suggests asking a tire technician and the manufacturer before changing. For prices on the size you settle on, ask the tire shops listed in Alberta for quotes.
Frequently asked questions
What does a tire size like 225/65R17 mean?
The tire is 225 mm wide from sidewall to sidewall, the sidewall is 65% as tall as the tire is wide (the aspect ratio), R means radial construction, and it fits a 17-inch rim. A P in front marks a passenger tire and LT a light-truck tire. The number and letter after the size, such as 102H, are the load index and speed rating.
Is the 3% rule a law?
No. Keeping a new size within about 3% of the original overall diameter is a tire maker’s guideline (BFGoodrich). The limit that counts is your vehicle maker’s: the U.S. Tire Manufacturers Association’s service manual says all four tires have to stay inside the vehicle maker’s tolerance for their outside circumference, and so for their diameter.
How does a bigger tire change my speedometer?
A tire with a larger overall diameter travels farther each turn, so a speedometer set up for the original size reads lower than your real speed, and the odometer counts fewer kilometres than you drive. A smaller tire does the opposite. Systems such as ABS, stability control and tire pressure monitoring can be affected too.
Can I use smaller rims for my winter tires?
Often, if the vehicle maker allows it. Going down a rim size with a taller sidewall keeps the overall diameter close to the original, and a narrower tire suits snow better than a wide one. The smaller wheel must still clear the brakes, and all four tires should be the same size.
Does the calculator check load index and speed rating?
No, it compares the geometry only. A replacement tire needs at least the load capacity of the original size, and the same or a higher speed rating unless your owner’s manual allows less, as some do for winter tires. The original tires carry both on the sidewall, just after the size.
Sources
- Transport Canada, Riding on Air: critical safety and environmental information for you and your tires (TP 2823 E). How to read a sidewall; replacement tires; where the tire information label is. Checked October 1, 2026.
- U.S. Tire Manufacturers Association (copy hosted by the Tire and Rubber Association of Canada), Care and Service of Passenger and Light Truck Tires (PDF). Size designations, overall diameter, and plus sizing (chapters 1 and 3). Checked October 1, 2026.
- BFGoodrich Tires, Changing tire size on a car. The source of the ±3% guideline. Checked October 1, 2026.
- Michelin, Choosing the right tire size for your vehicle. Checked October 1, 2026.
- Tire and Rubber Association of Canada, What you need to know when replacing your tires. Checked October 1, 2026.
- Consumer Reports, How to choose a different-sized wheel and tire combination. Checked October 1, 2026.
- Transport Canada, Using winter tires. Checked October 1, 2026.
Every figure on this page is worked out from the sizes themselves. No tire maker, association or government body named here endorses GarageMap.