Tire Size Calculator
3% larger diameter.
| 225/45R17 | 245/40R18 | |
|---|---|---|
| Section width | 225 mm (8.86 in) | 245 mm (9.65 in) |
| Sidewall height | 3.99 in | 3.86 in |
| Overall diameter | 24.97 in | 25.72 in |
| Circumference | 78.5 in | 80.8 in |
| Revolutions per mile | 808 | 784 |
| Effect of the change | Value |
|---|---|
| Diameter difference | +0.744 in (+2.98%) |
| Ground clearance half the diameter change | +0.372 in |
| Speedometer reads 60 mph | True speed 61.8 mph |
| Odometer error | Under-reads by 2.98% |
A tyre size like 225/45R17 reads as: 225 mm section width, sidewall height 45% of that width, on a 17-inch rim. Only the middle number is a ratio, which is why changing the width also changes the sidewall height and therefore the overall diameter — 245/45R17 is noticeably taller than 225/45R17, not just wider. Overall diameter is what matters for the speedometer, because it counts wheel revolutions: a larger tyre covers more ground per turn, so the dial under-reads and you are going faster than it says. Keep the change within about 3%, which is the usual guidance and roughly what factory tolerance allows. Beyond that, expect meaningful speedometer and odometer error, and on many vehicles effects on ABS, stability control, and transmission shift points. Also check that the new size physically fits — width and offset determine whether it rubs on suspension or bodywork at full lock, which no calculator can tell you.
Compare two tyre sizes for diameter, width, and sidewall — and see exactly how much speedometer error a size change causes.
How to use this calculator
- Read your current size off the tyre sidewall — it will look like 225/45R17 — and enter the three numbers.
- Enter the size you are considering.
- Set an indicated speed to see the true speed you would be doing at that reading.
Reading a tyre size
| Part | Means | In 225/45R17 |
|---|---|---|
| 225 | Section width in millimetres | 225 mm ≈ 8.86 in |
| 45 | Aspect ratio — sidewall as % of width | 101.25 mm ≈ 3.99 in |
| R | Radial construction | Almost universal today |
| 17 | Wheel diameter in inches | 17 in |
The unit mixture is genuinely odd — millimetres for width, a percentage for the sidewall, and inches for the wheel — and it is a historical accident rather than a design. It also means the middle number is the one that catches people out: it is a ratio, so changing the width changes the sidewall height too.
The formula
Sidewall (in) = Width (mm) × Aspect ÷ 100 ÷ 25.4 · Diameter = Rim + (2 × Sidewall)
True speed = Indicated × (New diameter ÷ Old diameter) · Revs per mile = 63,360 ÷ (π × Diameter)
Worked example — 225/45R17 to 245/40R18
- Original sidewall: 225 × 0.45 = 101.25 mm = 3.99 in
- Original diameter: 17 + (2 × 3.99) = 24.97 in
- New sidewall: 245 × 0.40 = 98 mm = 3.86 in
- New diameter: 18 + (2 × 3.86) = 25.72 in
- Difference: +0.74 in, or +2.98% — just inside tolerance
- At an indicated 60 mph you would actually be doing 61.8 mph
This is a textbook plus-one fitment: a bigger wheel with a shorter sidewall, keeping the overall diameter close enough that the speedometer stays usable.
Why the speedometer cares about diameter
A speedometer does not measure speed directly. It counts wheel revolutions and multiplies by a fixed assumption about how far the car travels per revolution — an assumption baked in at the factory for the original tyre size.
Fit a larger tyre and each revolution covers more ground than the car thinks, so it under-reports: the dial says 70, you are doing 72. Fit a smaller tyre and the reverse happens. The error is exactly the percentage difference in diameter, and it applies to the odometer too — which quietly accumulates. Tyres 3% larger mean 3,000 miles missing from a 100,000-mile total, affecting service intervals, warranty coverage, and what you disclose at resale.
Stay within about 3%
Three percent is the widely used guideline, roughly matching the tolerance vehicle manufacturers design for. Within it, the error is small enough that the speedometer stays within legal accuracy and the vehicle's electronics behave normally. Beyond it, several things degrade at once:
- Speedometer and odometer error becomes large enough to matter legally and practically.
- ABS and stability control take wheel speed as an input and can behave unpredictably when the assumed rolling diameter is wrong.
- Transmission shift points on many automatics reference vehicle speed, so shift quality and efficiency suffer.
- Effective gearing changes. A larger tyre is a taller final drive, dulling acceleration and raising the load on the drivetrain.
- Speed and load ratings may no longer meet the vehicle's requirements — check these separately, since diameter is not the only spec that matters.
What this calculator cannot tell you
Whether the tyre physically fits. Overall diameter is only one dimension; width and wheel offset determine whether the tyre contacts the strut, inner wing, or bodywork — and it may clear when driving straight and rub at full steering lock or under suspension compression. Check a fitment guide for your specific vehicle and test at full lock, loaded, before committing to a set.
On all-wheel-drive vehicles, mismatched rolling diameters between axles make the drivetrain fight itself and can cause expensive damage. Replace all four together, and never mix sizes unless the manufacturer specified a staggered fitment.
How we calculate this
Sidewall height is section width × aspect ratio ÷ 100, converted from millimetres at 25.4 mm to the inch. Overall diameter is the rim diameter plus two sidewalls, since there is one above the wheel and one below. Circumference is π × diameter, and revolutions per mile is 63,360 (inches in a mile) ÷ circumference — which is the mechanism by which tyre size reaches the speedometer. True speed is indicated speed × (new diameter ÷ old diameter). Ground clearance changes by half the diameter difference, because only the bottom half of the tyre sits between the axle and the road.
Sources
Read more
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17, the 225 is section width in millimetres, the 45 is the aspect ratio — sidewall height as a percentage of that width — and the 17 is the wheel diameter in inches. R means radial construction. So this tyre is 225 mm wide with a sidewall 101 mm tall, fitted to a 17-inch wheel, giving an overall diameter of about 25 inches.
How much bigger tire can I fit?
Keep the overall diameter within about 3% of the original. Beyond that the speedometer and odometer become noticeably inaccurate, and on many vehicles ABS, stability control, and transmission shift points are affected because they take wheel speed as an input. Physical clearance is a separate question the arithmetic cannot answer.
How do bigger tires affect the speedometer?
They make it under-read, so you are travelling faster than it shows. The speedometer counts wheel revolutions, and a larger tyre covers more ground per turn. Fit tyres 3% larger and an indicated 70 mph is really about 72. Smaller tyres do the reverse — the dial over-reads and you are going slower than it says.
Does changing tire size affect the odometer?
Yes, by the same percentage as the speedometer, and it compounds over time. Tyres 3% larger mean your odometer under-records by 3% — 3,000 miles missing over 100,000. That matters for service intervals, warranty coverage, and resale disclosure.
What is a plus size fitment?
Going up in wheel diameter while reducing the sidewall to keep the overall diameter roughly the same — 225/45R17 to 245/40R18, for example. Plus sizing improves steering response and appearance, at the cost of ride comfort, more vulnerability to pothole damage, and usually higher tyre prices.
Does a wider tire change the overall diameter?
Yes, unless the aspect ratio changes to compensate. Aspect ratio is a percentage of width, so a wider tyre at the same ratio has a taller sidewall too — 245/45R17 is noticeably taller than 225/45R17, not just wider. This is the single most common mistake when picking a wider tyre.
Can I put different size tires on the front and rear?
Some performance cars come from the factory with a staggered fitment, and that is fine. Fitting different sizes yourself is not — on an all-wheel-drive vehicle, differing rolling diameters make the drivetrain fight itself and can cause expensive damage to the transfer case or differentials. Even on two-wheel drive it can confuse stability control.
Will bigger tires fit under my car?
This calculator cannot tell you. Overall diameter is only part of it — width, wheel offset, and suspension travel determine whether a tyre rubs on the strut, inner wing, or bodywork at full steering lock or under compression. Check a fitment guide for your specific vehicle, and test at full lock before committing.