PC Tires Tools

Tire Size Calculator

Compare your current size against a new one. See the difference in diameter, sidewall and speedometer reading, get a fit verdict, and shop the size if we stock it.

A

Current size

or pick it
/ R
Find it on the sidewall or the sticker inside the driver’s door jamb.
B

New size

or pick it
/ R
The size you’re thinking about. Works with P, LT and 33x12.50R18-style sizes too.

How to Read a Tire Size

A size like 225/60R17 99H breaks down as: 225 = section width in millimetres, 60 = aspect ratio (the sidewall is 60% of the width, so 135 mm tall), R = radial, 17 = rim diameter in inches, 99 = load index (775 kg per tire) and H = speed rating (210 km/h). Overall diameter is the rim plus two sidewalls: 17 × 25.4 + 2 × 135 = 701.8 mm, or about 27.6 inches. Truck sizes like 33x12.50R18 state the overall diameter and width in inches directly.

The 3% rule
Keep the new overall diameter within 3% of original, and ideally within 1.5%. Past that, the speedometer and odometer drift, ABS and stability control can throw codes on some vehicles, and the tire can rub at full lock or over bumps.
Speedometer error
Your speedometer counts wheel revolutions. A taller tire covers more ground per turn, so the car is going faster than the needle says. A 3% taller tire at an indicated 100 km/h is really doing 103.
Rim width matters
Every tire width has an approved rim-width range about 2 inches wide (a 225 wants roughly 6–8"). Too narrow a rim pinches the tread; too wide stretches it. Changing width by more than 10–20 mm usually means checking the rim.
Load index and speed rating
Dimensions are only half the fitment. The new tire’s load index and speed rating must meet or beat what’s on your door-jamb placard. This matters most on SUVs, half-tons and anything that tows.
Going down a size for winter? A narrower tire on a one-inch-smaller steel wheel is the classic Ontario winter setup: cheaper tires, better bite in snow, and your alloys stay out of the salt. Use the Plus & Minus Sizing tool to find a winter size that keeps your speedometer accurate.

Frequently Asked Questions

Can I put a different size tire on my car?
Usually yes, within limits. Keep the overall diameter within 3% of the original size (ideally 1.5%), keep the width inside the range your rims support, and make sure the load index and speed rating meet or exceed the door-jamb placard. If you are changing rim diameter you also need wheels with the right bolt pattern, offset and centre bore. When in doubt, text us the size and your vehicle and we will check it.
What does a 3% diameter difference actually do?
At an indicated 100 km/h a 3% taller tire is really doing about 103 km/h and the odometer under-counts by the same 3%. Beyond that, some vehicles log ABS or stability-control faults because the wheel speed no longer matches what the computer expects, and clearance to fenders and struts gets tight.
Does a bigger tire make my speedometer read faster or slower?
Slower. A larger overall diameter means fewer revolutions per kilometre, so the speedometer under-reads: you are going faster than it says. A smaller tire does the opposite and the speedometer over-reads.
How do I find my current tire size?
Look at the sidewall of your tire for a code like 225/60R17, or check the tire-and-loading sticker inside the driver’s door jamb, which lists the factory size. Our guide on how to find your tire size walks through it with pictures, and you can also enter your VIN on our home page.
Does this calculator cover LT and 33x12.50R18 truck sizes?
Yes. Enter P-metric, LT-metric or flotation sizes (for example LT265/70R17 or 33x12.50R18) in either box. Flotation sizes are converted from inches so you can compare them directly against metric sizes.
Can PC Tires install the new size?
Yes. Every size we list online ships to our Pain Court shop and we install it, including TPMS service and an alignment check recommendation if the size change warrants it. If the size shows as not listed online, call or text 519-397-4686 and we can usually order it.

More Tire Tools

Plus & Minus Sizing
Going up a rim size, or down for winter steelies? Get sizes that keep your speedo honest.
Tread Depth Calculator
How many kilometres and months are left on your tires, and when to replace.

Calculations use the standard metric tire-size formula (sidewall = width × aspect ÷ 100; overall diameter = rim × 25.4 + 2 × sidewall). Actual mounted dimensions vary by brand, rim width, load and pressure, typically within about 1%. These tools are a starting point, not a fitment guarantee: always confirm load index, speed rating and clearance against your vehicle's door-jamb placard and owner's manual. Questions? Call or text 519-397-4686 and we'll check it for your vehicle.

Found Your Size?

Shop by size or vehicle, pick an install time at checkout, and we'll have your tires ready in Pain Court.

Shop Tires 📞 Call or Text 519-397-4686