How do I read a tire size?

In 225/45R17, 225 is the width in millimetres, 45 is the sidewall height as a percentage of width, R means radial, and 17 is the wheel diameter in inches.

Can I put a different tire size on my car?

Often, if the overall diameter stays close to the original (commonly within about 3%), the load index and speed rating are equal or higher, and the tire clears the suspension and bodywork. Check with a tire professional.

Do bigger tires affect the speedometer?

Yes. A larger overall diameter makes the speedometer read lower than your true speed; a smaller one makes it read higher.

Where is my car's correct tire size?

On the placard inside the driver's door jamb, in the owner's manual, and on the sidewall of the tires currently fitted.

What do the load index and speed rating mean?

The load index is the weight each tire can carry; the speed rating is the maximum sustained speed. Replacement tires should match or exceed the originals.

Tire Size Calculator: Compare Sizes (2026) / CarO

Written by Muneeb Rana · Last updated 21 October 2026

Quick answer: A tire size calculator turns a size like 225/45R17 into real dimensions. Width is 225 mm, sidewall height is 45% of that (101 mm), and the wheel is 17 inches, so overall diameter is 17 + 2 × 101 ÷ 25.4 ≈ 25.0 inches. Comparing two sizes shows the diameter change and speedometer error; most fitters keep it within about 3%.

TL;DR

Enter the current and the new tire size.
Sidewall = width × aspect ratio ÷ 100.
Keep diameter change small (commonly within about 3%) to protect speedometer accuracy.

This tire size calculator explains any tire size and compares two sizes side by side, showing the change in diameter, sidewall, width and speedometer reading. It's handy when you're considering a plus-size wheel, winter tires on smaller rims, or a different size than the one fitted.

How to use the tire size calculator

Enter your current size from the tire sidewall or the door-jamb placard, for example 225/45R17.
Enter the size you're considering.
Read the differences in diameter, sidewall height, width and revolutions per mile.
Check the speedometer difference: at an indicated 60 mph, what is your true speed?
Confirm load index and speed rating with a tire professional before buying.

How the tire size calculator works

sidewall_mm = width_mm * aspect / 100
diameter_in = rim_in + 2 * sidewall_mm / 25.4
circumference_in = diameter_in * 3.14159
revs_per_mile = 63360 / circumference_in
speed_error_pct = (new_diameter / old_diameter - 1) * 100

In a size like 225/45R17 94W: 225 = width in mm, 45 = aspect ratio (sidewall as a % of width), R = radial construction, 17 = rim diameter in inches, 94 = load index, W = speed rating. Bridgestone's sidewall guide explains every marking.

Worked example

Current 205/55R16: sidewall 112.8 mm, diameter 24.9 in. New 225/45R17: sidewall 101.3 mm, diameter 25.0 in. The difference is about +0.4%, so at an indicated 60 mph you'd be doing about 60.2 mph. That's a close match.

How much size difference is safe?

Many fitters keep the overall diameter within about 3% of the original to avoid speedometer error, ABS and stability-control issues, and rubbing. Always match or exceed the original load index and speed rating, and check clearance at full steering lock. Tire Rack's sizing guide covers plus-sizing in detail.

Don't forget pressure and tread

A new size can need a different pressure; follow the placard or the tire maker's load tables. Use the tire pressure guide and learn when to replace tires and how often to rotate.

Log your tires

Record the size, brand, purchase date and odometer when you fit new tires, so you can track tread life and tire age. CarO's maintenance log and app keep this with the car's other records. See also wheel alignment.

Download CarO free on the App Store or Google Play. Free with ads; optional in-app purchases.

Sources

Bridgestone: How to Read a Tire Sidewall: www.bridgestonetire.com (accessed 21 October 2026)
Tire Rack: Tire Size Conversion and Sizing Basics: www.tirerack.com (accessed 21 October 2026)
NHTSA: Tire Safety (TireWise): www.nhtsa.gov (accessed 21 October 2026)

Disclaimer: CarO and this website give general car-care information, not professional advice. Maintenance intervals and specifications vary by vehicle, so always follow your owner's manual and a qualified mechanic. Mileage and cost figures are estimates; check local tax rules before using any log for deductions.

Tire Size Calculator