Tire Size Calculator

What Is Tire Size?

A tire size is a manufacturing specification written as a compressed sentence. Read left to right, 225/45R17 gives a width, a proportion, and a rim — and only the first of those three is an absolute measurement. The other two mean nothing on their own, so two tires whose codes start with the same number can stand inches apart.

How to Read 225/45R17

Section width comes first, in millimeters, measured at the widest point of the mounted, inflated, unloaded tire. The second number is not a height — it is a percentage. A 45 means the sidewall stands 45 percent of 225 mm, which is 101.25 mm, or 3.99 inches. The R marks radial construction, and the final number is the rim diameter the tire is built to seal against, quoted in inches rather than millimeters.

The middle number is an aspect ratio in the ordinary sense, one dimension expressed as a percentage of another, and the same idea governs screens, prints and photographs in the Aspect Ratio Calculator.

Two sidewalls stack, one above the rim and one below, so the overall diameter is 17 + 2 × 3.99 = 24.97 inches. That figure — not the width, not the rim — is what a size change has to protect. On this tire the rim alone accounts for 68.1 percent of the total height; on a 265/70R17 truck tire the same 17-inch rim is only 53.8 percent of it.

The Letters Around the Numbers

The three numbers are the only part this calculator reads. The rest of the sidewall still decides whether a tire is legal on your car. A P before the size means P-metric, a passenger-car construction; LT means light truck, built for higher inflation pressures and heavier loads; no prefix at all usually means Euro-metric. After the size sits a service description such as 91V — a load index and a speed symbol — and XL or Reinforced marks a higher-load version of identical dimensions.

• P225/45R17 — P-metric, passenger-car construction.
• LT265/70R17 — light truck construction, with its own load range and pressure rating.
• R — radial. Some high-speed tires print ZR in its place.
• 91V — load index 91 and speed symbol V, both entirely separate from the size.
• XL or Reinforced — same three dimensions, higher load capacity at a higher inflation pressure.

Federal rules for light-vehicle radial tires require the size designation, the maximum permissible inflation pressure, the maximum load rating, the word radial and the DOT symbol to be marked on the tire, so none of this is information you have to hunt for — it is molded into the rubber.

Section Width Is Not Tread Width

Section width is measured across the widest part of the tire, which is the bulge of the sidewall, not the contact patch. Tread width is narrower, and how much narrower depends on the tread pattern and on the wheel: the same tire mounted on a wider rim stretches flatter and measures wider. Every size is approved for a range of rim widths, and mounting outside that range distorts the profile the code describes.

The consequence is easy to miss. Moving from 225 to 245 millimeters at a fixed 45 aspect ratio adds 20 mm of width — and 9 mm of sidewall at each end, because the sidewall is a percentage of a number that just grew. That is why 245/45R17 stands 25.68 inches against 24.97 for the 225, a 2.84 percent increase produced by a change most people describe as purely a width change.

Inch Sizes and Flotation Sizes

Off-road, trailer and some light-truck tires use a different convention entirely: 31×10.50R15 states overall diameter, then section width, then rim, all in inches. Nothing needs decoding — and nothing is precise, because those figures are nominal names rather than measurements, and the tire is usually a little under. This calculator reads the metric code only, so a flotation size has to be matched to a metric equivalent before it can be compared.

The same looseness infects metric marketing names. Tires sold as 33s are a family, not a size, and the family members are not the same height — the truck table below puts numbers on the gap.

How Do You Calculate Tire Size?

Every output on this page descends from one geometric fact: a tire's overall diameter is its rim plus two sidewalls. Circumference, revolutions per mile and speedometer error all follow from that single number and the constant π.

The Tire Size Formula

The 25.4 is exact: an inch is defined as 25.4 millimeters, so that conversion contributes no rounding of its own. The 63,360 is simply the number of inches in a mile, 5,280 feet times 12. The circumference step is the standard circle relationship C = πd, so the distance a tire covers per revolution is fixed entirely by its diameter and not at all by its width.

Sidewall height (mm) = section width (mm) × aspect ratio ÷ 100
Sidewall height (in) = sidewall height (mm) ÷ 25.4
Overall diameter (in) = rim diameter (in) + 2 × sidewall height (in)

Circumference (in) = π × overall diameter (in)
Revolutions per mile = 63,360 ÷ circumference (in)
Diameter change (%) = (new diameter − old diameter) ÷ old diameter × 100

Notice that the sidewall is doubled while the rim is not. A sidewall 10 percent taller does not produce a tire 10 percent taller, because the rim contributes a fixed amount that no aspect ratio can touch. That asymmetry is the reason plus-sizing works at all.

Step by Step

• Read the current size off the sidewall or the driver's door placard, in width / aspect / rim order.
• Multiply the width by the aspect ratio and divide by 100 — that is the sidewall height in millimeters.
• Divide by 25.4 to express that height in inches.
• Double it and add the rim diameter. That is the overall diameter.
• Repeat both steps for the candidate size.
• Subtract, divide by the original diameter, multiply by 100. That percentage is the diameter change and the speedometer error at once.

The calculator runs both sizes together, so the comparison arrives already made. The sequence is still worth knowing, because it shows exactly why the aspect ratio has to come down whenever the rim goes up.

Worked Example: 225/45R17 to 235/40R18

Start with the original: 225 × 45 ÷ 100 = 101.25 mm of sidewall, which is 3.99 inches; two of those on a 17-inch rim give 24.97 inches overall. Now the candidate: 235 × 40 ÷ 100 = 94.00 mm, or 3.70 inches, on an 18-inch rim — 25.40 inches overall.

Calculator output — 225/45R17 to 235/40R18

+1.72% diameter · 24.97″ → 25.40″ · indicated 60 mph = actual 61.0 mph · 79.8″ around, 794 revs/mile

The rim grew a full inch; the tire grew 0.43 inches, or 1.72 percent. That is plus-sizing in one line — the sidewall gives back most of what the wheel takes. The new tire turns 794 times per mile against the original's 808, which is the same fact stated backwards, and the speedometer now under-reads by 1.72 percent.

The Speedometer Error Formula

A speedometer does not measure speed. It counts rotations — at a wheel, at the transmission output shaft, or off the ABS tone ring — and multiplies by the distance the factory tire covered in one turn. Fit a taller tire and every rotation covers more ground than the stored constant assumes, so the display falls behind reality. Fit a shorter one and the display runs ahead of it.

True speed = indicated speed × (new diameter ÷ old diameter)
True distance = odometer distance × (new diameter ÷ old diameter)

225/45R17 → 235/40R18, at an indicated 60 mph:
60 × (25.40 ÷ 24.97) = 61.0 mph

This is also why the error is a fixed percentage rather than a fixed number of miles per hour. The same +1.72 percent tire that turns an indicated 60 into 61.0 turns an indicated 30 into 30.5 and an indicated 80 into 81.4 — the gap widens with speed, which is precisely where it matters.

Tire Size Chart

Every figure in the tables below came out of this calculator. Diameters are nominal, computed from the size code, which makes them directly comparable with one another — the one job a chart has to do.

Passenger Car Sizes by Overall Diameter

Sorted by height rather than by name, the pattern behind factory wheel options becomes visible: sizes cluster, because carmakers hold diameter nearly constant across every wheel a model is offered with.

195/65R15 and 225/45R17 sit 0.01 inches apart — two rim sizes and 30 millimeters of width between them, and effectively identical rolling circumference. That coincidence is the entire logic of a wheel upgrade.

Truck and SUV Sizes, and What a 33-Inch Tire Measures

Truck sizes carry tall aspect ratios, so one step of width or one step of aspect ratio moves the diameter much further than the same step does on a car. The table also settles a recurring argument about nominal names.

285/70R17 is sold everywhere as a 33, and it computes to 32.71 inches. 275/70R18 is also sold as a 33, and it computes to 33.16. Run one against the other and the calculator returns +1.37 percent — 0.45 inches of real difference hiding behind one marketing number. Sizing a lift, a fender flare, or a speedometer correction off the nickname rather than the arithmetic is how clearance problems start.

Plus-Size Chart: Ladders That Hold Diameter

A plus-one adds an inch of rim and subtracts a step of aspect ratio, usually adding a width step as well. Done properly the diameter barely moves, and every row here is a real pairing run through the calculator.

Only the last row trips the warning, and it trips it by 0.12 of a percentage point. Plus-two is where the arithmetic starts fighting back: there is no aspect ratio between 35 and 40 to fine-tune with, so the available steps get coarse exactly when precision matters most.

Change One Number at a Time

Isolating each of the three numbers shows how unequal they are. Every row starts from 225/45R17 at 24.97 inches and changes one thing.

One inch of rim is worth 4.00 percent here, five points of aspect ratio are worth 3.55 percent, and a full 20 millimeters of width buys only 2.84 percent. That ranking explains the whole plus-sizing recipe: a rim increase has to be paid for with an aspect-ratio decrease, and width is the safest of the three to move on its own.

Speedometer and Odometer Error Chart

The diameter change is the speedometer error, and the odometer error is its mirror image. The final column is what the dashboard would show after 1,000 miles of genuine travel on the new size.

Read the bottom row before assuming taller is the risky direction. A shorter tire makes the odometer over-count against real distance — by 3.68 percent here, not the 3.55 percent the diameter moved — so 1,000 miles driven register as 1,037, mileage a lease or a warranty will happily charge for.

Tire Size Examples Worked in Full

Four swaps that come up constantly, each run through the calculator and then read back in full.

205/55R16 to 225/45R17: the Upgrade That Changes Nothing

A staple factory option jump on compact sedans. The 205/55R16 stands 24.88 inches and the 225/45R17 stands 24.97 — 0.09 inches, or 0.38 percent, of difference.

Calculator output

+0.38% diameter · 24.88″ → 24.97″ · indicated 60 mph = actual 60.2 mph · 78.5″ around, 808 revs/mile

Nothing needs recalibrating; over 1,000 true miles the odometer would show 996. What does change is the feel. The tire is 20 mm wider and its sidewall is 11.5 mm shorter — 101.25 mm against 112.75 — so the steering sharpens and the ride firms up, while the drivetrain never notices the swap happened.

195/65R15 to 205/60R16: a Plus-One That Runs Tall

The same plus-one recipe on a smaller wheel, and it does not hold diameter nearly as well. The 195/65R15 is 24.98 inches; the 205/60R16 is 25.69 — up 2.82 percent, which spends most of the ±3 percent budget on a single step.

Calculator output

+2.82% diameter · 24.98″ → 25.69″ · indicated 60 mph = actual 61.7 mph · 80.7″ around, 785 revs/mile

It passes, but barely, and it shows why small wheels are unforgiving: on a 15-inch rim, one inch of rim is a larger fraction of the whole tire, so each step costs more percentage than the identical step on a 19-inch wheel. Curiously, going further helps — 215/50R17 from the same original lands at +1.94 percent, so the plus-two sits closer to stock than the plus-one does.

265/70R17 to 285/70R17: the 33-Inch Step

The most common truck upsize keeps the aspect ratio and adds 20 mm of width, which on a 70-series tire means 14 mm of extra sidewall at each end: 185.5 mm becomes 199.5. The tire goes from 31.61 inches to 32.71 — up 3.49 percent, and flagged.

Calculator output

+3.49% diameter · 31.61″ → 32.71″ · indicated 60 mph = actual 62.1 mph · 102.8″ around, 617 revs/mile — over the ±3% rule

The new tire turns 617 times per mile against the original's 638. At an indicated 70 the truck is genuinely doing 72.4, and 12,000 miles of real driving register as about 11,600 — roughly 400 miles that never appear on the dash. This is the band where recalibration, either through the vehicle's own tire-size setting or a dealer tool, stops being optional.

255/40R19 to 255/35R20: a Wheel Change With No Diameter Change

Width held constant, rim up one inch, aspect ratio down five points. The outcome is as clean as this arithmetic gets: 27.03 inches before, 27.03 inches after.

Calculator output

−0.01% diameter · 27.03″ → 27.03″ · indicated 60 mph = actual 60.0 mph · 84.9″ around, 746 revs/mile

Speedometer, odometer, ABS reference speeds and effective gearing all carry on as though nothing happened. Everything that does change is invisible to this calculator: five points less sidewall over a 20-inch wheel means less rubber between rim and pothole, a harsher ride, a heavier wheel to accelerate and stop, and a more expensive tire to replace.

How to Read Your Result

The headline is a single percentage, and it is doing four jobs at once. That one number is the diameter change, the speedometer error, the odometer error, and the effective gearing change — one ratio between the two diameters, applied forward to speed and inverted for distance and engine speed, so the last two land a fraction away from the headline rather than exactly on it.

The ±3% Band and Where It Comes From

The calculator flags any swap whose diameter change exceeds 3.00 percent in either direction. That threshold is a fitment convention used across the tire trade, not a federal or international standard. What makes it a defensible line is that around 3 percent, speedometer drift, clearance margins and the wheel-speed assumptions inside ABS and stability control all start to matter together rather than one at a time.

Because the cut is strict, the verdict can look arbitrary near the edge: 245/40R18 from a 225/45R17 comes in at +2.98 percent and passes, while 245/35R19 comes in at +3.12 percent and does not. The honest reading is that anything above roughly 2.5 percent is a conversation with a tire shop rather than a pass or a fail.

Speedometer, Odometer, and the Miles You Are Billed For

A taller tire makes the speedometer read low — the display says 60 while the car does 61. That is the direction that costs money in tickets, because the driver is moving faster than the instrument admits. A shorter tire reads high, which is safer legally and worse financially: the odometer over-counts, so lease allowances, warranty limits and service intervals all arrive early.

Factory speedometers already lean one way deliberately. The UN regulation that governs speedometer and odometer approval requires that the indicated speed never fall below the true speed, with the permitted error running only in the over-reading direction. Most cars therefore leave the factory reading slightly fast, which means a modestly taller tire tends to cancel part of that built-in optimism rather than push an already-low reading lower.

The odometer error runs the other way, and close to but not exactly the same size: the shortfall is measured against the longer true distance, so it works out as the diameter change divided by one plus itself. A +3.49 percent tire under-counts by 3.37 percent, which turns 12,000 miles actually driven into about 11,600 on the dash — some 400 miles of allowance produced by geometry rather than restraint, and 400 miles of maintenance interval quietly stretched along with it.

Effective Gearing and Cruising RPM

Diameter is the last gear in the driveline. A taller tire lengthens the overall ratio: the engine turns more slowly for a given road speed, acceleration softens, and highway cruising quietens. A shorter tire does the opposite: it shortens the overall ratio, and the car feels quicker without a single change under the hood.

The scale is easy to state. An engine holding 2,500 rpm on a 225/45R17 drops to about 2,458 rpm on the +1.72 percent 235/40R18, and to 2,416 rpm on the +3.49 percent truck upsize. Push the comparison to the extreme — 225/45R17 to 265/70R17, a 26.56 percent jump — and 2,500 rpm becomes roughly 1,975. That collapse in engine speed is exactly why a serious lift is normally accompanied by a differential re-gear rather than by a tire alone.

Circumference and Revolutions per Mile

Circumference is the ground a tire covers in one turn, π times the diameter. Revolutions per mile is the same fact inverted: 63,360 inches in a mile divided by that circumference. A 225/45R17 turns 808 times per mile; a 265/70R17 turns 638. The taller tire does a fifth less work to cover the same road.

Those numbers travel further than curiosity. Revolutions per mile is the figure that speedometer recalibration tools and GPS-based correction devices are fed, it is how axle-ratio choices get evaluated against a tire change, and it is the specification tire manufacturers publish when they compare sizes — which makes it the cleanest single number to carry into a shop.

The diameter change is an ordinary percentage change between two quantities, and to see the same arithmetic applied to anything else, use the Percentage Increase Decrease Calculator.

When the Calculator Refuses to Answer

Four checks run before any arithmetic happens, and each returns a specific instruction in place of a number. Width must fall between 100 and 400 millimeters, so a 22.5 typed in centimeters is rejected rather than quietly turned into a wrong diameter. Aspect ratio must fall between 20 and 90 as a whole percent, so 0.45 is refused. Rim diameter must fall between 10 and 26 inches. And all six fields must hold a positive number.

• "Enter both tire sizes (width / aspect / rim)" — a field is blank, zero or negative.
• "Width is in millimeters (e.g., 225)" — a width outside 100–400 mm.
• "Aspect ratio is a percent (e.g., 45)" — an aspect ratio outside 20–90.
• "Rim diameter is in inches (e.g., 17)" — a rim outside 10–26 inches.

The ranges are wide enough for everything from a 145-section city-car tire to a 26-inch wheel, and tight enough that a decimal entered in the wrong unit gets caught instead of producing a plausible but wrong answer.

Limits: What the Sidewall Math Cannot Tell You

The calculation itself is exact. What it describes is narrower than most people assume, and the gap between the two is where the expensive mistakes live.

Nominal Geometry Is Not a Fitment Check

A diameter inside ±3 percent says a tire will roll at close to the right speed. It says nothing about whether that tire will clear the strut at full steering lock, clear the fender liner at full suspension compression, or suit the width of the wheel it is going onto. Wheel offset, brake caliper size and the vehicle's own bump-stop travel are all outside this arithmetic, and any one of them can veto a size the percentage approves.

The authoritative starting point for what a vehicle was built around is the placard on the driver's door jamb. Federal rules require that label to state the original tire size designation and the manufacturer's recommended cold inflation pressure for front, rear and spare — which is why it outranks any chart, this one included.

Nominal Diameter Is Not Measured Diameter

Two tires wearing the same size code rarely measure the same. Tread depth, tread pattern, mold design and the rim width used all move the real figure, and manufacturers publish measured diameters and revolutions per mile that differ from the geometric ones. Under load the gap grows: a tire carrying a car deflects, so the rolling radius is smaller than half the free-standing diameter and the true revolutions per mile run above the calculated value.

For comparing two sizes this barely matters, because both are computed identically and most of the error cancels in the ratio. For predicting an exact speedometer correction it matters more, and the tire maker's published specification for the specific model is the better source than any size-code arithmetic.

Load Index and Speed Rating Are Not in the Three Numbers

This calculator reads width, aspect ratio and rim. It never sees the service description, and a size that sails through the diameter test can still be the wrong tire. A load index below what the vehicle requires is a safety failure no matter how well the diameter matches, and a lower speed symbol changes the tire's heat tolerance at sustained speed, which also changes the pressure it wants.

The rule practitioners use is to meet or beat the original on both. Going wider is where this gets missed most often, because a wider tire is not automatically a stronger one — load capacity is a construction property, not a consequence of width.

AWD, ABS, and Stability Control

Mismatched diameters across axles are a mechanical problem rather than a calibration one. A center differential or clutch pack forced to absorb a permanent speed difference between front and rear runs hot and wears out, and the tolerance many manufacturers publish for that difference is measured in thirty-seconds of tread depth, not in percent. The ±3 percent band covers replacing all four tires with a different size; it is far too loose to describe a difference between axles on the same vehicle.

Electronic systems have their own stake in the answer. ABS, traction control and stability control all derive wheel speed from rotation counts, and cruise control, the speed limiter and every distance-based service reminder inherit the same percentage error. Changing tire size changes what all of them believe about the car, which is why the recalibration question belongs in the same conversation as the fitment question.