Tire Size Calculator

A tire stamped 225/45R17 works out to an overall diameter of about 24.97 inches, a sidewall of roughly 3.99 inches, a circumference near 78.5 inches, and around 808 revolutions per mile. Swap it for a 245/40R18 and the diameter grows to about 25.72 inches - a 3.0% increase that makes your speedometer read low: at an indicated 60 mph you would actually be doing about 61.8 mph. This tire size calculator turns those cryptic sidewall numbers into real dimensions, and lets you compare two sizes side by side so you can see the speedometer error before you buy.

How to read a tire size

The code on the sidewall packs four facts into a short string. Take 225/45R17:

• 225 - section width in millimeters (the widest point of the tire).
• 45 - aspect ratio: the sidewall height as a percentage of the width. A lower number means a shorter, "lower-profile" sidewall.
• R - radial construction (almost all modern passenger tires).
• 17 - wheel (rim) diameter in inches that the tire is built to fit.

You may also see a load index and speed rating after the size (for example 91V), but those do not affect the dimensions this calculator works out.

The formula

The math mixes millimeters and inches because tire sizing does, so the calculator converts as it goes:

The sidewall is doubled because the tire sits above and below the wheel, so two sidewalls add to the wheel's own diameter. One mile is exactly 1,609,344 mm, which is why revolutions per mile is just that figure divided by the tire's circumference in millimeters.

How to use this calculator

1. Pick a mode: use Single tire to get the dimensions of one size, or Compare two to check a replacement against your original.
2. Enter the width in millimeters (the first number in the size).
3. Enter the aspect ratio as a percentage (the second number).
4. Enter the rim diameter in inches (the number after the R).
5. Read the results: overall diameter is the headline number; the cards show sidewall, circumference and revs per mile.
6. In compare mode, also read the speedometer error and the diameter difference, plus a side-by-side spec card for both tires.

Everything updates instantly as you type. If a field is blank or zero, the calculator shows a prompt instead of an incorrect number.

Who this calculator is for

• Shoppers comparing sizes who want to know if a deal on a slightly different size will fit and read accurately.
• Plus-sizing enthusiasts fitting bigger wheels with shorter sidewalls while keeping the overall diameter close.
• Truck and off-road owners going to a taller tire and wanting to know the speedometer offset and odometer impact.
• DIY mechanics double-checking that a staggered or winter set matches the original rolling diameter.
• Anyone curious about what the numbers on their sidewall actually mean.

Worked example 1: the default comparison

Going from 225/45R17 to 245/40R18 raises the wheel by an inch but trims the sidewall. The original diameter is 24.97 in and the new one is 25.72 in, a difference of 0.74 in or +3.0%. That sits right at the edge of the usual ±3% tolerance, so it is a borderline-acceptable plus-size: expect the speedometer to read about 3% low (60 indicated ≈ 61.8 true), and confirm there is enough clearance for the wider, taller tire.

Worked example 2: only the aspect ratio changes

Suppose you keep your 17-inch wheels and 225 mm width but jump from a 45 to a 50 aspect ratio: 225/45R17 → 225/50R17. The diameter rises from 24.97 in to 25.86 in, about +3.5%. Even though the wheel and width are identical, the taller sidewall pushes the diameter past 3% - enough to make the speedometer noticeably optimistic and worth a recalibration if you keep the size.

Worked example 3: a wider truck tire

On a pickup running 265/70R17 (about 31.61 in), stepping up to a wider 285/70R17 raises the diameter to about 32.71 in, a +3.5% change because the same 70% aspect ratio is now a percentage of a wider tire. At an indicated 60 mph you would really be doing about 62.1 mph, and the taller tire effectively lowers your final drive ratio slightly, softening acceleration. Many lift and leveling kits are sized around exactly this kind of diameter jump.

Common tire sizes and their diameters

This reference shows how overall diameter changes across popular sizes. Use it to sanity-check a result or spot a close substitute:

Values are nominal P-metric calculations; real tires vary by brand and wear.

Why speedometer error happens

Your car does not measure road speed directly - it counts how fast the wheels spin and multiplies by the factory tire's circumference. Fit a larger tire and each rotation covers more ground than the car assumes, so the needle reads lower than your true speed and the odometer undercounts miles. A smaller tire does the opposite. Because the relationship is purely geometric, the speedometer error equals the percentage change in overall diameter - which is exactly what the compare mode reports.

Plus sizing without breaking your speedometer

The trick to fitting bigger wheels is to remove sidewall as you add rim. Roughly, each extra inch of wheel diameter (25.4 mm) needs about 12.7 mm less sidewall on each side to hold the overall diameter steady. In practice you drop the aspect ratio - and sometimes nudge the width - until compare mode shows a diameter difference near 0%. Stay within ±3% and your speedometer, gearing and clearance should be largely unaffected.

Key tire terms

• Section width: the tire's width in millimeters at its widest point (the first number).
• Aspect ratio: sidewall height as a percentage of width (the second number). Lower = shorter sidewall.
• Sidewall height: the actual height of the rubber between rim and tread, in mm or inches.
• Overall diameter: the full height of the mounted tire - what determines rolling distance.
• Rolling circumference: the distance covered in one rotation; drives revs per mile and speedometer reading.
• Plus sizing: larger wheel plus shorter sidewall to keep diameter roughly constant.
• Load index / speed rating: the trailing code (e.g. 91V) for maximum load and speed - match or exceed your originals.

Limitations and assumptions

• It uses the nominal P-metric formula, not a brand's measured diameter, which can differ by several millimeters.
• Tread wear reduces diameter over a tire's life; a new tire and a worn one of the same size differ slightly.
• It does not check load index, speed rating, wheel width, offset/backspacing, or physical clearance.
• Speedometer error is the geometric offset; some vehicles deliberately build in a small factory bias as well.
• Results are a planning estimate - always verify a specific tire's spec sheet and fitment before buying.

How tire size affects fuel economy and gearing

Changing your tire diameter quietly changes two things you might not expect: your effective gearing and your measured fuel economy. A taller tire travels farther per rotation, so at any given engine RPM the car covers more ground - it behaves like a slightly taller (numerically lower) final drive ratio. That can lower cruising RPM and, in some cases, marginally improve highway economy, but it also softens acceleration and makes the engine work harder from a stop. A shorter tire does the reverse: quicker off-the-line feel, higher cruising RPM, and usually worse highway economy.

There is also a measurement trap. Because a larger tire makes the odometer undercount miles, your calculated miles per gallon will look better than reality until you correct for the diameter change - you drove farther than the odometer credited you for the same fuel. If you have permanently changed sizes, apply the diameter offset from this tool before you trust any MPG figure. Run the corrected mileage through the Gas Mileage Calculator, estimate a road trip with the Fuel Cost Calculator, and if the speedometer offset matters for a long drive, the Speed, Distance & Time Calculator shows how a few percent of error compounds over hundreds of miles.

Metric, P-metric and LT tire codes

Most passenger sizes you will type into this calculator are P-metric (sometimes shown with a leading "P," as in P225/45R17) or plain metric (just 225/45R17, common on European cars). The geometry is identical; the leading letter only signals the tire's standards family and load assumptions. Light-truck tires carry an LT prefix (for example LT265/70R17) and are built with stiffer construction for heavier loads, but their dimensions still follow the same width / aspect / rim formula. You will also meet flotation sizing on trucks and off-road rigs, written as overall diameter first - 33x12.50R17 means a 33-inch tall, 12.5-inch wide tire on a 17-inch wheel. Flotation sizes already state the diameter, so there is no aspect ratio to convert; this calculator is built for the metric width / aspect / rim format, and you can compare a flotation tire by entering its known diameter as the reference.

Tips for picking a replacement size

• Start from the door-jamb placard, not the old tire. The sticker in the driver's door frame lists the factory size and pressures; a previous owner may have already fitted something off-spec.
• Compare against the original in compare mode. Aim for a diameter difference inside ±3% (ideally under 1% for a daily driver) to keep the speedometer, ABS and stability systems honest.
• Match a wider tire to a wheel that can take it. Each rim width has a recommended tire-width range; mounting a tire too wide for the wheel distorts the tread and the real-world width.
• Keep all four close. On all-wheel-drive vehicles especially, mismatched rolling diameters across axles can stress the drivetrain - replace in matched sets and watch tread depth.
• Re-check clearance at full lock and full compression. A size that clears while parked can still rub when you turn or hit a bump, particularly with wider or taller fitments.

Tire size calculator vs. a tire size chart

A printed tire size chart lists a fixed set of common sizes and their diameters - handy for a quick glance, but it cannot show your exact before-and-after comparison or the precise speedometer error for an in-between size. This calculator does the same geometry on demand for any width, aspect ratio and rim you enter, and the compare mode puts the two sizes next to each other with the diameter difference and speed offset spelled out. Use the reference table on this page as the chart-style sanity check, and the calculator above for the actual decision.

Sources and standards

• National Highway Traffic Safety Administration (NHTSA) - tire safety, sizing and load information.
• Conversion constants: 1 inch = 25.4 mm; 1 mile = 1,609,344 mm (geometric, brand-independent).