Tire size calculator and tire life checker
Compare your current tire size with a new one to see how the diameter, sidewall and speedometer change. Or switch tabs and check how much tread you have left and roughly how many miles until you need new tires.
How to read a tire sidewall
Take P225/45R17 94V. The P means passenger car. LT means light truck, and no letter usually means a Euro metric size. 225 is the section width in millimeters. 45 is the aspect ratio: the sidewall is 45% as tall as the tire is wide, so about 101 mm here. R means radial construction. 17 is the wheel diameter in inches. 94 is the load index and V is the speed rating.
Off-road and lifted trucks often use flotation sizes like 33x12.50R17. Those are already in inches: 33 inches tall, 12.5 inches wide, on a 17 inch wheel.
Further along the sidewall is the DOT number. Its last four digits are the build date. NHTSA's example: 0308 means the third week of 2008.
Plus sizing vs minus sizing
Plus sizing means a bigger wheel with a lower profile tire, so the overall diameter stays about the same. Going from 225/45R17 to 225/40R18 is a plus one. Tire Rack's rule of thumb is to drop the sidewall height 5 to 10 percent for each inch of wheel you add. You get sharper steering and a firmer ride, and the shorter sidewall is easier to bend on potholes.
Minus sizing goes the other way: a smaller wheel with a taller sidewall. People do it for winter tires because they're cheaper in smaller sizes and the taller sidewall soaks up rough roads. Make sure the smaller wheel still clears your brake calipers.
Whichever way you go, the goal is the same overall diameter. The Tire Industry Association's service training still uses the 3% guideline, which grew out of tire makers asking for plus-size fitments to stay within 3% of the original diameter. Load index and speed rating should be at least what your door jamb placard lists.
What a diameter change does to your speedometer and ABS
Your car counts wheel turns to work out speed. A taller tire covers more ground per turn, so the speedometer reads low: it might show 60 mph when you're really doing 62. A shorter tire makes it read high. The odometer is off by the same percentage, which matters for warranty miles and service intervals.
ABS, traction control and stability control use the same wheel speed sensors. Tire Rack notes that big diameter changes can affect how well those systems work. That's why the fitment goal is a new size within about 3% of the original, and closer is better.
When to replace your tires
- 2/32" or less: Worn out. NHTSA says to replace tires at 2/32", and that is the legal limit in most states. Built-in tread wear bars sit level with the tread at this point.
- 4/32": Consumer Reports suggests most drivers replace tires here, because its testing showed hydroplaning resistance and snow traction drop off well before the legal limit.
- Age: NHTSA notes that some vehicle and tire makers recommend replacing tires that are 6 to 10 years old no matter how much tread is left. You can't see rubber aging from the outside.
- Damage: Bulges, cracks, cuts in the sidewall or cords showing mean replace now.
The penny and quarter tests
Put a penny in the shallowest groove with Lincoln's head upside down and facing you. If you can see the top of his head, you're at about 2/32" and need tires now. For the quarter test, do the same with Washington's head. If you can see all of his head, you're at about 4/32" or less and should start shopping. Check a few spots across each tire, since uneven wear often points to alignment or pressure problems.
Tire repair vs replacement
The US Tire Manufacturers Association (USTMA) says a puncture can be repaired only if it is in the tread area and no bigger than 1/4 inch (6 mm) across. Damage in the sidewall or in the shoulder near the belt edge can't be fixed safely, so that tire gets replaced.
A proper repair means taking the tire off the wheel, checking the inside, filling the hole with a rubber stem or plug and sealing the inner liner with a patch. USTMA says a plug alone is not an acceptable repair. Repairs can't overlap, and some tires, like many run flats, have stricter limits from the maker. If you're driving on a plug from the roadside kit, treat it as a way to get to a shop.
Where these numbers come from
The size math uses the standard formulas: sidewall = width × aspect ratio ÷ 100, overall diameter = wheel diameter × 25.4 + 2 × sidewall, and revolutions per mile = 63,360 ÷ (π × diameter in inches). Speedometer readings assume it is calibrated for your current size. Fitment and tread guidance comes from these sources, reviewed in October 2026:
- NHTSA: Tires (2/32" replacement point, penny test, tread wear indicators, 6 to 10 year age guidance, DOT date code)
- Tire Rack: Tire Tread Depth (new car tires typically 10/32" to 11/32", state minimums)
- Tire Rack: Plus Sizing and What Are Plus Size Wheels and Tires? (keep overall diameter the same, 5 to 10% sidewall rule, effects on speedometer, ABS and stability control)
- Tire Review: New Consumer Reports tests show need for 4/32nds minimum tread depth (4/32" guideline and quarter test)
- Modern Tire Dealer (origin of the 3% diameter guideline in the TIA ATS program)
- USTMA: Tire Repair Basics (1/4 inch tread-area rule, plug plus patch)
Results are estimates for planning. Your tire shop and your owner's manual have the final word on what fits your vehicle.