Tire Size Calculator
Put two sizes side by side in 225/45R17 notation. Sidewall, diameter, circumference and revolutions per mile for each — plus the difference between them and the speedometer error the swap introduces.
Two sizes, side by side
The first number on the sidewall. In 225/45R17 it is the 225.
The 45 — sidewall height as a percentage of the width, not a fixed height.
The 17, after the R. This one is in inches — the notation mixes both systems.
The size you are considering. Widening at the same aspect ratio also makes the tire taller.
Drop this to claw back the height a wider tire or a bigger wheel adds.
An inch of wheel adds an inch of diameter unless the sidewall gives it back.
245/40R18 is 25.72 in across against 24.97 in for 225/45R17 — +2.98%, which is inside the 3% rule of thumb.
- Diameter change
- +0.74 in
- As a percentage
- +2.98%
- Within the 3% rule
- Yes
- Speedometer
- Reads low by 2.89%
- Odometer over 100,000 miles
- 2,893 miles unrecorded
| Measurement | 225/45R17 | 245/40R18 |
|---|---|---|
| Sidewall height | 101.25 mm | 98 mm |
| Overall diameter | 24.97 in | 25.72 in |
| Circumference | 78.45 in | 80.79 in |
| Revolutions per mile | 807.6 | 784.3 |
| Section width | 8.86 in | 9.65 in |
| Speedometer reads | You are actually doing | Difference |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 mph |
| 60 mph | 61.8 mph | +1.8 mph |
| 70 mph | 72.1 mph | +2.1 mph |
- Inside about 3% of the original diameter, the speedometer, odometer, ABS and traction control stay close enough to their calibration to behave.
- Diameter is necessary, not sufficient. Whether the wheel clears the strut, the arch liner at full lock and the brake caliper depends on offset and backspacing, which a tire size does not contain.
- Load index and speed rating must be matched or exceeded. They are stamped after the size — the 94Y in 225/45R17 94Y — and are a safety requirement, not a preference.
Three numbers, two unit systems, one diameter
A P-metric size is not three measurements of the same kind. The first is a width in millimetres, the second is a percentage of that width, and the third is a diameter in inches. Everything below follows from converting them into one unit.
sidewall = W × (A ÷ 100) • D = R + 2 × (sidewall ÷ 25.4) • rev/mile = 63,360 ÷ (π × D)- W
- Section width in millimetres — the 225
- A
- Aspect ratio in percent — the 45
- R
- Rim diameter in inches — the 17
- D
- Overall diameter in inches, rim plus two sidewalls
The sidewall counts twice because the tire has one above the wheel and one below it. The inch is exactly 25.4 mm by international definition, so that conversion introduces no error of its own. There are 63,360 inches in a mile — 5,280 feet times twelve — which is where revolutions per mile comes from.
The speedometer correction is a ratio and nothing else. The car counts revolutions and multiplies by the circumference it was calibrated with, so true speed = indicated speed × (new diameter ÷ old diameter). Circumference cancels: π appears on both sides, and only the diameters matter.
225/45R17 to 245/40R18, step by step
The original: 225 × 0.45 = 101.25 mm of sidewall, which is 3.99 inches. Two of them plus a 17-inch rim gives an overall diameter of 24.97 inches, a circumference of 78.45 inches, and 807.6 revolutions per mile.
The replacement: 245 × 0.40 = 98 mm, or 3.86 inches. Two of those plus an 18-inch rim gives 25.72 inches, a circumference of 80.79 inches, and 784.2 revolutions per mile. An inch more wheel, 20 mm more tread, and five points less sidewall nets out at 0.74 inches taller.
As a percentage: 0.74 ÷ 24.97 = 2.98%, which slips inside the 3% rule with almost nothing to spare. The speedometer now reads low by that ratio — at an indicated 60 mph the car is doing 61.8 mph, at 30 it is doing 30.9, and at an indicated 70 it is doing 72.1.
What actually breaks past three percent
Three percent is a rule of thumb, not a law and not a manufacturer’s specification. What makes it a sensible line is that a single number — wheel speed — is consumed by far more systems than the dial in front of you.
- Speedometer. Reads low by the same ratio the tire is oversized. At 3% that is under 2 mph at highway speed; at 10% it is 7 mph, which is the difference between a warning and a citation.
- Odometer. Under-counts by the same ratio, and unlike the speedometer the error accumulates. See below.
- ABS and traction control. Both work by comparing wheel speeds — against each other to detect a locking or spinning wheel, and against a modelled ground speed. A uniform offset shifts the reference for every intervention threshold.
- Stability control. Compares the yaw rate the car is producing against the yaw rate the steering angle and road speed say it should be. A wrong road speed is a wrong target.
- Transmission shift points. On vehicles that schedule shifts by road speed rather than by output shaft speed alone, every shift moves with the tire size.
- Clearance. A taller or wider tire can foul the wheel arch, the arch liner or the strut at full lock and full suspension compression — a combination you may only find once, on a bumpy road, mid-turn.
One thing worth knowing before you assume the factory reading was ever right: most speedometers over-read from new, deliberately. UNECE Regulation No. 39, which applies across Europe and much of the world, requires that the indicated speed never be lower than the true speed, which manufacturers satisfy by building in a margin. A slightly taller tire can therefore bring an indicated reading closer to the truth rather than further from it — but it does that by cancelling one error with another, and it does nothing for the odometer.
Two percent, over a hundred thousand miles
A speedometer error is instantaneous — it is wrong now, and it is wrong by the same amount later. An odometer error integrates. Fit tires 2.98% larger, as in the example above, and the odometer records only 97.1% of what you drive: after 100,000 real miles it shows about 97,107, with 2,893 miles missing.
That matters in three places. A mileage-limited warranty expires nearly three thousand miles early in real terms, because the car reaches the odometer figure later than it reaches the actual one. Service intervals stretch by the same proportion, so an oil change nominally every 10,000 miles happens every 10,300. And a car advertised at 97,000 miles has really covered 100,000, which matters to a buyer and, in some jurisdictions, to the accuracy of the disclosure the seller is signing.
None of this is fraud and all of it is invisible. It is a good reason to note the date and the odometer reading whenever the tire size changes.
What this calculator assumes
- Nominal dimensions from the size code. Real tires vary by manufacturer and model by a few millimetres at the same nominal size, and the moulded dimensions on a spec sheet are measured on one specified rim width.
- A free-standing, unloaded tire. A loaded tire has a smaller rolling radius than its geometric one, and a tire at speed grows slightly. Both effects apply to whichever tire is fitted, so they largely cancel in a comparison.
- The car’s speedometer was calibrated for the original size and nothing else has been altered — no gearing change, no recalibration, no different-sized wheels front to rear.
- Diameter only. Nothing here checks fitment: offset, backspacing, suspension travel and caliper clearance decide whether a wheel and tire physically go on the car, and none of them are in a tire size.
- Load index and speed rating are not modelled. They are stamped after the size — the 94Y in 225/45R17 94Y — and must be matched or exceeded whatever the geometry says.
Tire size FAQ
What does 225/45R17 actually mean?
225 is the section width in millimetres — the widest point of the tire, tread shoulder to tread shoulder. 45 is the aspect ratio: the sidewall height as a percentage of that width, so 45% of 225 mm is 101.25 mm of sidewall. R means radial construction. 17 is the wheel diameter in inches. Two of the three numbers are metric and one is imperial, which is why the size on the sidewall tells almost nobody how tall the tire is.
Will bigger tires make my speedometer read wrong?
Yes, and predictably. The car counts wheel revolutions and multiplies by the circumference it was calibrated with. A tire 3% larger covers 3% more ground per revolution, so the speedometer reads about 3% low — at an indicated 60 mph you are doing nearer 62. A smaller tire does the reverse and makes the speedometer read high.
How far from the original diameter can I go?
The widely used rule of thumb is 3% of the original overall diameter, and this page flags anything outside it. Past that the speedometer and odometer are visibly wrong, and the same wheel-speed signal feeds ABS, traction control and stability control, all of which compare wheel speeds against each other and against a calibrated ground speed. On vehicles that shift by road speed, the transmission's shift points move too.
Does a wider tire also mean a taller tire?
At the same aspect ratio, yes — and this catches people out. The ratio is a percentage of the width, so 45% of 245 mm is 110.25 mm against 101.25 mm for a 225. Going from 225/45R17 to 245/45R17 adds 0.71 inches of diameter without touching the wheel. That is why plus-sizing charts drop the aspect ratio every time the width or the rim goes up.
Does the odometer error matter for warranty or resale?
It compounds, which is the part people miss. An odometer under-reading by 2% records 98,000 miles for every 100,000 driven. A powertrain warranty expiring at 100,000 miles has quietly been used up 2,000 miles early, and a car advertised at 98,000 miles has really done 100,000. Keep a note of when the tires changed and by how much.
Can the speedometer be recalibrated for the new size?
Often, and on many vehicles it is a dealer or scan-tool setting rather than hardware — some manufacturers list tire size as a configurable parameter precisely because they sell several wheel options. Aftermarket recalibration modules exist for vehicles that do not expose it. Recalibrating fixes the speedometer and odometer; it does not change what the suspension and bodywork will clear.
Sources and review notes
- NHTSA — Tires, for what each marking on the sidewall means and the load and speed information stamped alongside the size
- NIST Special Publication 811 — the inch as exactly 25.4 mm, the conversion every figure on this page passes through
- UNECE vehicle regulations — Regulation No. 39, which requires an indicated speed never lower than the true speed
- ETRTO — the European body that publishes the dimensional standards tire sizes are built to
The geometry here is exact and has nothing behind it to go stale. The 3% figure is an industry rule of thumb rather than a published standard, and it is stated as one throughout — check your vehicle’s own documentation and the placard on the driver’s door jamb before changing size.