Tire Size Calculator

A tire size calculator turns a sidewall code like 225/45R17 into real dimensions: overall diameter, sidewall height, circumference and revolutions per mile. That 225/45R17 works out to a 24.97-inch diameter and about 808 revs per mile. Compare two sizes and it also shows the speedometer error before you buy.

This page is the size-and-geometry tool; for what a size change does downstream, reach for a focused sibling. Use the Speed, Distance & Time Calculator to turn a speedometer offset into trip time, and the Gas Mileage Calculator when a diameter change has skewed your odometer and MPG. Pick the size here; work out what it changes there.

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, measured from sidewall edge to sidewall edge.
• 45: aspect ratio, the sidewall height as a percentage of the width. A lower number means a shorter, "lower-profile" sidewall.
• R: radial construction, the standard for modern passenger tires.
• 17: wheel (rim) diameter in inches that the tire is built to fit.

That reading matches NHTSA's tire safety guide, which also explains the optional letter in front: P for a passenger-vehicle tire and LT for a light-truck tire. You may also see a load index and speed rating after the size (for example 91V); they are explained further down and 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.

Worked calculation: 245/40R18 step by step

Here is the full chain of arithmetic the calculator runs for a single size, using 245/40R18 so you can follow every number:

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 outside a realistic range (width 100 to 500 mm, aspect ratio 20 to 100, rim 8 to 30 inches), the calculator explains what to enter instead of showing 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% (2.98% before rounding). That sits right at the edge of the common ±3% rule of thumb: expect the speedometer to read about 3% low (60 indicated ≈ 61.8 true), check that your vehicle maker lists the 18-inch size, 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 read noticeably low (60 indicated is about 62.1 true) 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. A jump like this usually also needs a clearance check, and a load range at least equal to the original.

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 metric calculations; real tires vary by brand and wear.

Alternative tire sizes for 225/45R17 (compare sizes)

Looking for a size that fits the same wheels or stays close to a 225/45R17? These candidates are sorted by overall diameter, with the difference from stock and the true speed at an indicated 60 mph. Staying inside ±3% keeps the speedometer offset small, but it does not by itself make a size safe: NHTSA advises the original size or another size the vehicle maker recommends.

Nominal metric values vs. a 225/45R17 baseline; confirm load rating, wheel width and clearance before fitting. Enter any pair in compare mode above for an exact result.

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, true speed is higher or lower than the reading by the same percentage as the change in overall diameter, which is exactly what the compare mode reports.

Tire diameter change vs. true speed (speedometer error chart)

Read your true speed and odometer drift straight off the diameter change. A positive change means a taller tire (speedometer reads low, so you are going faster than it shows); a negative change means a shorter tire.

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%. Within ±3% the speedometer and gearing change only slightly; clearance, wheel width and load index still need their own check.

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, which 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 load capacity and rated speed; match or exceed what your vehicle maker specifies.

Limitations and assumptions

• It uses the nominal metric formula, not a brand's measured diameter, which can differ slightly.
• 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.
• The speedometer figure is the geometric offset from the size change only; the speedometer itself may also have a small error, so compare with a GPS reading if precision matters.
• It does not read flotation sizes such as 33x12.50R17.
• 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 worse 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 (with a leading "P," as in P225/45R17, which NHTSA describes as a tire for passenger vehicles) or the same size without a letter (225/45R17). The geometry is identical; the letter signals the tire category and load assumptions. Light-truck tires carry an LT prefix (for example LT265/70R17) and a load range letter for their load-carrying capability and inflation limits, 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 only accepts the metric width / aspect / rim format, so compare a flotation tire by setting its stated diameter against the metric result: 285/70R17 works out to 32.71 inches, about 0.9% shorter than a nominal 33-inch tire.

Load index and speed rating: the code after the size

Most tires show a service description right after the size, such as 91V on a 225/45R17. NHTSA's tire safety guide explains both parts. The load index (the two- or three-digit number) is a measure of how much weight each tire can support. The speed rating letter is the speed at which the tire is designed to be driven for extended periods:

NHTSA adds that tires able to exceed 149 mph sometimes use ZR in the size, and those rated above 186 mph always do. Neither number changes the dimensions this calculator works out, but when you change sizes the new tire should still meet the load and speed ratings your vehicle maker specifies on the door-jamb placard or in the owner's manual.

Tips for picking a replacement size

• Start from the door-jamb placard, not the old tire. NHTSA points drivers to the Tire and Loading Information label on the driver's door edge or post, or the owner's manual, for the correct size; a previous owner may have already fitted something off-spec.
• Compare against the original in compare mode. The closer the diameter difference is to 0%, the less the speedometer, ABS and stability systems are affected; the common rule of thumb is to stay inside ±3%.
• 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. It is handy for a quick glance, but it cannot show your exact before-and-after comparison or the precise speedometer offset 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

• NHTSA, Tire Safety: Everything Rides On It (DOT HS 809 361): how to read P and LT sizes, load index, speed ratings Q to Y, and the advice to buy the original size or another size the vehicle maker recommends.
• NHTSA, Tires: buying, maintenance and labeling (TireWise).
• 49 CFR 571.139 (FMVSS No. 139): definitions of section width and light truck (LT) tire, and the required sidewall markings.
• NIST, Handbook 44 (2026), Appendix C: 1 inch = 25.4 mm and 1 mile = 1,609.344 m, both exact.