The Tire Size Comparison Visualizer, explained
Everything you need to know about reading a tire size, swapping to a different size safely, and how the math works behind the scenes.
What does a tire size mean?
A modern metric tire size like 275/70R18 tells you four things at once. The first number (275) is the section width in millimeters. The second (70) is the aspect ratio — the sidewall height as a percentage of the width. The R means radial construction. The final number (18) is the wheel diameter the tire fits, in inches.
From those four pieces of info you can derive the overall diameter, circumference, revs per mile, and the actual sidewall height. The calculator above does the math for you, but it’s worth understanding the relationship: if you keep the width and ratio the same and bump the wheel diameter up an inch, your overall diameter goes up by 1″. If you keep the wheel and width the same and drop the ratio, the sidewall gets shorter and the tire is less cushioned.
Why upsize or downsize?
People change tire sizes for a handful of practical reasons. Upsizing to a taller tire raises ride height, clears bigger brake calipers, and looks more aggressive on trucks and SUVs. Going wider improves dry grip and gives a planted look. Going to a shorter sidewall (lower aspect ratio) sharpens steering response on sport sedans.
Downsizing or running a +0 plus-size for winter tires saves money, gives you more sidewall for pothole protection, and lets you mount dedicated snow tires on a cheaper steel wheel. Whatever the reason, the visualizer above lets you preview the change before you spend the money.
What is the ±3% rule?
The industry rule of thumb is to keep your overall tire diameter within ±3% of the factory size. Inside that window, your speedometer, ABS, traction-control, and stability-control systems stay accurate, and you usually clear the fender and suspension without lifting or trimming.
Between ±3% and ±5% you’ll see a noticeable speedo error and may need to check clearance under articulation. Anything beyond ±5% should be paired with a speedometer recalibration — and on 4WD trucks, a gear-ratio change to restore your engine RPM and fuel economy.
How does tire size affect my speedometer?
Your speedometer reads off the transmission output, not the wheels. It assumes a specific tire diameter (the OEM size). When you go to a taller tire, each wheel rotation covers more ground, so the actual speed is higher than what the dash shows. A shorter tire is the opposite.
The Speedometer Reading table above does this math live. Set Size 1 to your OEM size and Size 2 to the new size, and the grid shows what your true speed is at 20-90 mph. A 4% diameter jump means your speedo reads about 4% slow at every speed.
How do I read load index and speed rating?
The numbers after the size (e.g. 102V) are the load index and speed rating. 102 means each tire can carry up to 1,874 lbs at full pressure. V means it’s rated for sustained speeds up to 149 mph. Truck and SUV tires also use a load range letter (B/C/D/E/F) that maps to a max inflation pressure.
Always replace at or above your factory load index and speed rating — never below. The Education tab in the calculator has the full lookup tables.
What’s the difference between metric and LT (inch) sizes?
Metric (P-metric and Euro-metric) sizes use millimeters and a percentage for the sidewall: 285/75R16. LT (Light Truck) inch sizes spell out the overall diameter and width in inches: 33×12.50R17 LT E. Both formats coexist on truck and Jeep platforms.
To convert metric to inches, divide the width by 25.4, multiply by the aspect ratio to get the sidewall, double it, and add the wheel diameter. The Education tab walks through a full worked example.