Tire Size Calculator
Compare two P-metric tire sizes: overall diameter, sidewall height, circumference, revolutions per mile, and the speedometer error a size change introduces.
Tire Size Calculator: with the default inputs, overall diameter change is 3.18%.
The first number: tread width in millimetres.
The second number: sidewall height as a percentage of the width.
The number after the R: rim diameter in inches.
Stay within ±3% of the original — that is the industry rule of thumb for a safe substitution.
- Original overall diameter
- 25.86 in
- New overall diameter
- 26.68 in
- Original sidewall height
- 112.5 mm
- New sidewall height
- 110.3 mm
- New circumference
- 83.82 in
- Original revolutions per mile
- 780
- New revolutions per mile
- 756
- True speed at that reading
- 61.9 mphBigger tires make the speedometer read low; smaller tires make it read high.
- Speedometer error
- Reads 3.18% low — at an indicated 60 mph you are actually doing 61.9 mphHow far out the dash reads once the new tires are fitted.
- Ground clearance change
- 0.41 inHalf the diameter change — the axle sits this much higher or lower.
- Odometer reading over a true 100 miles
- 96.9Miles the odometer will show after 100 real miles.
Assumptions
- P-metric passenger tire notation only. LT (light truck), flotation sizes such as 33×12.50R15 and older numeric sizes use different conventions and are not handled.
- Overall diameter is computed geometrically from the nominal size: rim diameter + 2 × (section width × aspect ratio). Real moulded diameters vary by a few millimetres between manufacturers of the same nominal size.
- Revolutions per mile are from the unloaded diameter. Manufacturer figures are measured under load and run roughly 2–3% higher.
- The speedometer is assumed accurate on the original tire. Most factory speedometers are deliberately calibrated to read slightly high, so a small increase in diameter can bring the reading closer to true rather than further from it.
- Load index, speed rating and approved rim width are not checked and must be verified against the vehicle placard.
| 225/50R17 | 245/45R18 | Difference | |
|---|---|---|---|
| Sidewall height | 112.5 mm | 110.3 mm | -2.3 mm |
| Overall diameter | 25.86 in | 26.68 in | 0.82 in (3.18%) |
| Circumference | 81.24 in | 83.82 in | 2.59 in |
| Revolutions per mile | 780 | 756 | -24 |
| Section width | 225 mm | 245 mm | 20 mm |
| Speedometer reads (mph) | True speed (mph) | Out by (mph) |
|---|---|---|
| 25 | 25.8 | 0.8 |
| 35 | 36.1 | 1.1 |
| 45 | 46.4 | 1.4 |
| 55 | 56.8 | 1.8 |
| 65 | 67.1 | 2.1 |
| 75 | 77.4 | 2.4 |
How this is worked out
The formula
A P-metric size such as 225/50R17 reads: 225 — section width in millimetres 50 — aspect ratio: sidewall height as a percentage of section width R — radial construction 17 — wheel (rim) diameter in inches Sidewall height (mm) = section width × aspect ratio ÷ 100 Overall diameter (in) = rim diameter + 2 × sidewall height ÷ 25.4 Circumference (in) = π × overall diameter Revolutions per mile = 63,360 ÷ circumference Diameter change = (new diameter ÷ old diameter − 1) × 100% True speed = indicated speed × (new diameter ÷ old diameter) Ground clearance = half the diameter change
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Original — section width
- The first number: tread width in millimetres.from 100 to 400 · whole numbers only · defaults to 225
- Original — aspect ratio
- The second number: sidewall height as a percentage of the width.a percentage · from 20 to 90 · whole numbers only · defaults to 50
- Original — wheel diameter
- The number after the R: rim diameter in inches.from 8 to 30 · defaults to 17
- New — section width
- A number.from 100 to 400 · whole numbers only · defaults to 245
- New — aspect ratio
- A number.a percentage · from 20 to 90 · whole numbers only · defaults to 45
- New — wheel diameter
- A number.from 8 to 30 · defaults to 18
- Speedometer reading(under More options)
- The speed shown on the dash. The calculator returns your true speed on the new tires.in mph, km/h · from 0 to 200 · defaults to 60
What you get back
- Overall diameter changemain answer
- Stay within ±3% of the original — that is the industry rule of thumb for a safe substitution.
- Original overall diameter
- New overall diameter
- Original sidewall height
- New sidewall height
- New circumference
- Original revolutions per mile
- New revolutions per mile
- True speed at that reading
- Bigger tires make the speedometer read low; smaller tires make it read high.
- Speedometer error
- How far out the dash reads once the new tires are fitted.
- Ground clearance change
- Half the diameter change — the axle sits this much higher or lower.
- Odometer reading over a true 100 miles
- Miles the odometer will show after 100 real miles.
What this assumes
- P-metric passenger tire notation only. LT (light truck), flotation sizes such as 33×12.50R15 and older numeric sizes use different conventions and are not handled.
- Overall diameter is computed geometrically from the nominal size: rim diameter + 2 × (section width × aspect ratio). Real moulded diameters vary by a few millimetres between manufacturers of the same nominal size.
- Revolutions per mile are from the unloaded diameter. Manufacturer figures are measured under load and run roughly 2–3% higher.
- The speedometer is assumed accurate on the original tire. Most factory speedometers are deliberately calibrated to read slightly high, so a small increase in diameter can bring the reading closer to true rather than further from it.
- Load index, speed rating and approved rim width are not checked and must be verified against the vehicle placard.
About this calculator
A tire size is three measurements in three different units, which is why the notation confuses people. 225/50R17 means a tread 225 millimetres wide, a sidewall half that tall — 112.5 mm — and a 17 inch wheel. The number that actually matters for fitment is none of those three on its own: it is the overall diameter, rim plus two sidewalls, and this calculator compares it between any two sizes.
The ±3% rule
Everything on a modern car is calibrated to the original tire's rolling circumference: the speedometer, the odometer, ABS, traction and stability control, and on all-wheel-drive cars the centre differential. Change the overall diameter and all of them are wrong by the same percentage. The industry works to ±3% as the acceptable substitution window, and this page's headline output is exactly that figure. Inside it, nothing needs recalibrating. Outside it, your speedometer is meaningfully wrong, your odometer under- or over-counts every mile, and on an AWD car a mismatch between axles can cook a differential.
Plus sizing
Going up a wheel size while dropping the aspect ratio — 225/50R17 to 245/45R18, say — is called plus sizing. Done right the overall diameter barely moves, so nothing needs recalibrating, but the shorter sidewall flexes less and the steering feels sharper. The costs are real: a harsher ride, more road noise, more susceptibility to pothole damage, and usually a heavier wheel, which hurts both ride and acceleration. Under an aspect ratio of about 35 there is very little rubber left between the rim and the road.
Reading the speedometer output
Fit a larger tire and the car travels further per wheel revolution than the speedometer expects, so the dash reads low — you are going faster than it says. A smaller tire does the opposite. The table at the bottom shows the error at real road speeds, which is the practical way to think about it: a 2% error is under 1.5 mph at 70, while a 6% error is over 4 mph and will eventually cost you a ticket.
What this does not check
Load index and speed rating — the numbers and letter after the size, such as 94V — are not modelled here and must at least match the vehicle placard in the door jamb. Rim width matters too: every tire size has a range of rim widths it is approved for, and mounting outside that range distorts the tread's contact patch. Revolutions per mile here are geometric, from the unloaded diameter; manufacturers publish loaded figures, which run a couple of percent higher because the tire flattens where it meets the road. And this is the P-metric system only. LT (light truck), flotation sizes such as 33×12.50R15, and older numeric sizes use different conventions.
Frequently asked questions
▸What do the numbers on a tire mean?
In 225/50R17: 225 is the tread width in millimetres, 50 is the aspect ratio (the sidewall is 50% of 225 mm, so 112.5 mm), R means radial construction, and 17 is the wheel diameter in inches. The overall diameter is the rim plus two sidewalls.
▸How much bigger can my tires be?
The usual limit is ±3% of the original overall diameter. Beyond that the speedometer, odometer, ABS and stability control are all calibrated to the wrong rolling circumference, and on all-wheel drive a mismatch can damage the drivetrain.
▸Do bigger tires make the speedometer read fast or slow?
Slow. A larger tire covers more ground per revolution than the speedometer assumes, so your true speed is higher than the dash shows. Smaller tires make it read high.
▸What is plus sizing?
Fitting a larger wheel with a lower-profile tire so the overall diameter stays roughly the same — 225/50R17 to 245/45R18, for example. Steering response improves; ride comfort, road noise and pothole resistance all get worse.
▸Does a wider tire have a taller sidewall?
Yes, at the same aspect ratio, because the aspect ratio is a percentage of width. A 245/45 has a 110.25 mm sidewall while a 225/45 has 101.25 mm — which is why widening a tire also raises the overall diameter unless you drop the aspect ratio too.
Put this calculator on your own site
A working tire size, free for any site, with no ads and no sign-up. It resizes to fit wherever you paste it and updates itself as this page improves.
Paste this anywhere. It works on any site, carries no ads, never expires, and always shows the current version.
Tire Size Calculator by CalculateItNow
The page's own title. The clearest description of what the link leads to.
The credit line sits outside the widget on purpose, so it is a real link on your page rather than one buried in a frame. Please keep it — it is what pays for CalculateItNow staying free and ad-free. The script only resizes the widget to fit its contents; drop it and the widget still works.
Browse every calculator widget·How to add it to WordPress, Squarespace or Wix
Related calculators
The questions people ask next to a tire size.
Calculate your car's real fuel economy from miles driven and gallons used — or from two odometer readings at fill-up — in US mpg, UK mpg, L/100 km and km/L.
Convert mph, km/h, m/s, ft/s and knots instantly, and see the equivalent running pace in minutes per mile and per kilometer, with speed-limit and wind tables.
Gear ratio from tooth counts, with output RPM, torque multiplication and mechanical advantage, and an optional second stage for compound gear trains.
Estimate a trip's gas cost from distance, your MPG or L/100 km and the price per gallon or liter — with fuel needed, cost per mile and the split per passenger.
Enter any one of radius, diameter, circumference or area and get the other three — with unit conversion and the π formulas worked step by step.