Tire Size Calculator

What does a tire size calculator tell you?

A tire size calculator turns the code on the sidewall into real dimensions, then compares two sizes so you can see whether a new tire will fit and how far it will throw off your speedometer. The two numbers that matter most are the change in overall diameter and the speedometer error that follows from it. Everything else (sidewall, width, circumference, revolutions per mile) explains why.

Your vehicle’s original size is printed on the “Tire and Loading Information” placard. Federal Motor Vehicle Safety Standard 110 requires that placard to list the tire size designation of the tires fitted at first sale, including the spare, and to sit on the driver’s side B-pillar (the door jamb) wherever possible. The same size is molded into each tire’s sidewall, as FMVSS 139 requires for light-vehicle radial tires.

Key terms on this page. Section width: the tire’s widest point, in millimeters, sidewall to sidewall. Aspect ratio: sidewall height as a percentage of section width. Overall diameter: the tire’s height from ground to top, unloaded. Revolutions per mile: how many times the tire turns in one mile; your speedometer and odometer count these. Plus sizing: a bigger wheel with a lower-profile tire that keeps the same overall diameter. Service description: the load index and speed symbol after the size, such as 91W. TPMS: the tire pressure monitoring system required by FMVSS 138.

These are the most common US questions the calculator answers:

How do you use the tire size calculator?

Enter the size on your car now, enter the size you’re considering, and read the two tiles at the top: diameter change and speedometer error.

1. Find your original size on the driver’s door-jamb placard or the sidewall, such as P225/45R17, LT265/75R16 or 33x12.50R15. Load and speed codes after the size (like 91W) are ignored by the calculator, but write them down for the checks below.
2. Type the new size in the second box. The table fills in diameter, sidewall, section width, circumference and revolutions per mile for both tires, plus the difference.
3. Check the diameter tile. “Within ±3%” means the size is in the range most fitment guides accept; “Outside ±3%” means you should check clearance and expect a noticeable speedometer error.
4. Read the true-speed table. It shows what you’re really doing when the dashboard reads 30, 45, 60, 70 and 80 mph on the new tires.
5. Confirm on the road. After fitting, have a passenger run the GPS speedometer at a steady 60 mph and enter both readings in the speedometer calibration calculator to measure the real error.

Changing wheels as well? The wheel offset calculator shows how far a new wheel and tire will poke out or tuck in, in mm and inches.

How is tire size calculated? (Formula and worked examples)

Every number in the table comes from three values in the size code: width, aspect ratio and wheel diameter. Sidewall = width × aspect ÷ 100; diameter = 2 × sidewall + wheel.

Diameter (in) = 2 × (width mm × aspect ÷ 100) ÷ 25.4 + wheel (in)

The steps, for a metric size like P225/45R17:

For flotation sizes the work is shorter, because the first number already is the diameter. A 33x12.50R15 is 33.0 in tall and 12.50 in wide, with a sidewall of (33 − 15) ÷ 2 = 9.0 in.

Worked example: P225/45R17 → P235/45R17. The wider tire has a taller sidewall too, because the 45% is now taken of 235 mm: 105.75 mm. Overall diameter becomes 643.3 mm (25.33 in), so the change is +1.42%. Revolutions per mile drop from 808 to 796.

Worked example: P265/70R17 → 285/70R17, a popular “33-inch look” on half-ton pickups. Diameter grows from 31.61 in to 32.71 in, +3.49%, just outside the ±3% band. Ground clearance at the axle rises by half the diameter gain: (32.71 − 31.61) ÷ 2 = 0.55 in.

Width and aspect ratio interact. Because the sidewall is a percentage of width, changing only the first number also changes height. From a 225/45R17, our calculations give:

The calculator shows nominal geometry from the size code. A real tire’s measured height varies by brand, tread depth and inflation, and a worn tire is shorter than a new one, so treat results within a few tenths of a percent as equal.

How do bigger tires affect your speedometer and odometer?

Your speedometer counts wheel revolutions and multiplies by the circumference it was set for at the factory. Fit a taller tire and each turn covers more road, so the speedometer and odometer read low; fit a shorter one and they read high.

• True speed = indicated speed × (new diameter ÷ original diameter)
• Speedometer error = (original diameter ÷ new diameter − 1) × 100%

So for P225/45R17 → P235/45R17, true speed at an indicated 60 mph is 60 × 25.33 ÷ 24.97 = 60.9 mph. True speed at each dashboard reading, for common US size changes (calculated with this tool):

The odometer drifts by the same percentage, which matters for warranty, lease and maintenance intervals and for a mileage log. Starting from a stock P265/70R17 pickup tire:

Your speedometer may not have been exact on the original tires either, so these tables show the change, not the final error. To know the final error, measure: compare the dash with a GPS speedometer at a steady speed on a straight, open road. GPS.gov reports typical smartphone accuracy of about 4.9 m (16 ft) under open sky; why the two disagree is explained in GPS vs car speedometer, and the GPS trip odometer lets you check distance against mile markers.

What is plus sizing, and which sizes keep the same diameter?

Plus sizing means fitting a wheel one or more inches larger while lowering the tire’s aspect ratio, so the overall diameter, and your speedometer, stay within about 1–3%. Plus-one is one inch of wheel, plus-two is two inches, and so on. The sidewall gets shorter and the wheel gets bigger by the same amount.

Three common US starting points, computed with the calculator:

The usual mistake is keeping the old aspect ratio on a bigger wheel. A 215/55R17 on a car that came with 205/55R16 is 5.76% taller, and a 225/50R18 is 7.96% taller: the speedometer then reads 56.7 and 55.6 mph at a true 60. Plus sizing also trades sidewall for looks and steering response; a shorter sidewall cushions potholes less, and the wider tire must fit your new wheel’s approved rim-width range.

What do the load index and speed rating mean?

The service description after the size, such as 91W, is two codes: 91 is the load index (615 kg, or 1,356 lb, per tire) and W is the speed symbol (168 mph, 270 km/h). A replacement should match or exceed both. The U.S. Tire Manufacturers Association says replacement tires should “be the same size, load index and speed rating designation” recommended by the vehicle or tire maker, and should “not be of a smaller size or load-carrying capacity than what was originally specified.”

Speed symbols you’ll see on US passenger and light-truck tires, from Hankook’s US tire guide:

Load index examples from the same table: 91 = 615 kg (1,356 lb), 95 = 690 kg (1,521 lb), 100 = 800 kg (1,764 lb) per tire. Multiply by four for the set’s rated capacity at the tire’s maximum pressure, but your real limit is the placard’s gross weight. FMVSS 139 defines load rating as “the maximum load that a tire is rated to carry for a given inflation pressure”, so a tire run below the placard pressure carries less.

A speed rating is a laboratory rating, not a recommendation. For comparison, the federal FMVSS 139 high-speed test runs a light-vehicle tire at 85% of its maximum load for three 30-minute stages at 140, 150 and 160 km/h (87, 93 and 99 mph). Every legal US passenger tire passes that; the letter tells you how far beyond it the maker rates the tire.

Does changing tire size affect TPMS, ABS and stability control?

Usually only slightly, if you stay within about 3% of the original diameter. TPMS still has to warn within 20 minutes when a tire is 25% below the placard pressure, so after a size change reset or relearn it and keep inflating to the placard (or the tire maker’s load-table) pressure.

FMVSS 138 requires the TPMS telltale on cars, SUVs, pickups and vans with a gross vehicle weight rating of 10,000 lb or less (excluding dual-wheel axles) to light within 20 minutes of a tire reaching 25% below the manufacturer’s recommended cold pressure, or a minimum activation pressure if that is higher. On a 35 psi placard, that’s about 26 psi.

Two kinds of system react differently to a new size:

• Direct TPMS has a pressure sensor inside each wheel. Swapping tires on the same wheels keeps the sensors; moving to new wheels means new or transferred sensors and a relearn. If a bigger or different-load tire needs a different pressure, the warning threshold won’t know unless the vehicle can be reprogrammed.
• Indirect TPMS compares wheel speeds from the ABS sensors, because a low tire is slightly smaller and spins faster. New tires change those speeds, so most owner’s manuals call for a reset after any tire change; mixing sizes across axles can cause false warnings.

ABS, traction and stability control use the same wheel-speed signals. They tolerate the small differences from wear, but staggered or mismatched sizes the vehicle wasn’t built for can confuse them. Keep all four the same size unless the maker fits a staggered setup, and don’t mix a new tire with three badly worn ones on all-wheel-drive vehicles without checking the owner’s manual.

Is there a legal limit on speedometer error in the US?

For private cars there is no current federal accuracy standard; for commercial trucks and buses there is: ±5 mph at 50 mph.

That 5 mph at 50 mph limit is a 10% band. A truck that went from P265/70R17 to LT315/70R17 (+8.72%) would show 46.0 mph at a true 50 mph, inside the federal limit but close to its edge; a 35x12.50R17 (45.2 mph) would fall outside it. For everyone else, a “speedometer was wrong” argument rarely helps with a ticket, so recalibrate after any change beyond a couple of percent. Many modern vehicles store the tire size in a module a dealer or tuning tool can update; older cable-driven speedometers use a swappable driven gear.

Can I use a different tire size? (225/65 vs 225/60, 235 vs 225 and more)

Usually yes when the new size stays within about 3% of the original diameter, fits your wheel and carries an equal or higher load index and speed rating. That 3% figure is a trade rule of thumb, not a law: tire retailer Les Schwab tells drivers to stay “within 3 percent” of the original height and to match or beat the load and speed ratings, and the U.S. Tire Manufacturers Association advises the size on your placard first. These are the swaps people ask about most, computed from nominal sizes with this calculator:

Three patterns fall out of the table:

• A 5-point jump in aspect ratio (55 → 60, 60 → 65) adds about 3.2–3.3% to the diameter on common 16–18 in sizes, right at the edge of the rule. It is not a like-for-like replacement, and your speedometer will read about 2 mph low at 60 mph.
• One width step (10 mm) at the same aspect ratio adds about 1.3–1.8% because the sidewall grows with the width. It is the gentlest upsize, as long as the tire’s approved rim-width range includes your wheel.
• A bigger number is not always a wider-looking tire. In 225/50 versus 225/60 the width is the same 225 mm; the 60 only has a taller sidewall (60% of 225 mm versus 50%). So “is a 50 or 60 tire wider?” has the answer: neither, the 60 is taller.

A 17-inch tire needs a 17-inch wheel: the last number must match the rim exactly, so “17s instead of 16s” means new wheels and a lower aspect ratio (plus sizing, above). Whatever you pick, mixing sizes on one axle is a no: keep both tires on an axle the same size, and on AWD cars follow the maker’s rule for matching all four.

How do you choose a replacement tire size safely?

Diameter is only one of five checks: load index, speed rating, wheel width, clearance and, on trucks, the LT designation all have to pass too.

1. Load capacity. Match or beat the original load index. P-metric and Euro-metric tires with the same numbers can carry different loads at the same pressure, so compare the index, not the size.
2. Speed rating. Match or exceed the original symbol (the letter after the load index).
3. Wheel width. A wider tire needs a wheel inside its approved rim-width range. One width step (225 to 235 mm) is usually fine on the same wheel; two or three may not be.
4. Clearance. Turn to full lock and load the suspension (passengers in the back, a loaded bed) to check rubbing on fenders, liners and control arms. Remember only half the diameter gain lifts the axle.
5. LT vs P-metric. If the placard shows LT tires, keep LT. Light-truck and passenger tires use different load and inflation tables, and swapping LT for P-metric on a truck is a common mistake.

Sizes sold as “33s, 35s and 37s” vary more than the name suggests:

And the everyday sizes on US roads, for quick reference:

Speeds on this page are in mph first because US speedometers are; for metric figures on an import or a trip abroad, the mph to km/h converter and the speed converter do the maths. Want the corrected speed in your line of sight? The HUD speedometer mirrors your phone’s GPS speed onto the windshield. Never type sizes or check a GPS reading while driving; pull over or let a passenger do it.