What the numbers on a tire sidewall actually mean
A size code like 265/70R17 encodes three separate measurements, and only one of them is already in inches. The 265 is the section width in millimetres — the widest point of the inflated, unloaded tire, measured sidewall bulge to sidewall bulge, not the width of the tread. The 70 is the aspect ratio: the sidewall height expressed as a percentage of that section width, so this sidewall is 70% of 265 mm, or 185.5 mm. The R means radial construction. The 17 is the rim diameter in inches, because the rim dimension survived unconverted from the imperial era.
That mixed-unit scheme is why you cannot compare two sizes in your head. Going from 265/70R17 to 285/70R17 sounds like a 20 mm change, but the aspect ratio multiplies it: the sidewall grows by 70% of 20 mm on each side, so the overall diameter grows by 28 mm, or 1.10 inches. Meanwhile 245/75R17 has a narrower section and a taller overall diameter than 265/70R17. The only reliable way to compare is to convert both codes to a single number — overall diameter in inches — which is exactly what this calculator does.
Every downstream consequence of a tire change flows from that one number. Your speedometer, odometer, ABS wheel-speed comparison, traction control, transmission shift scheduling and effective final-drive ratio are all calibrated against the number of times the tire turns per mile, and that is fixed by diameter alone. Width affects grip, load capacity, fender clearance and steering feel, but it does not move the speedometer.
The formula, and why each term is there
Overall diameter is the rim plus two sidewalls: D = R + 2 · (W · A / 100) / 25.4. Work from the inside out. W · A / 100 converts the aspect ratio percentage into an actual sidewall height in millimetres. Dividing by 25.4 puts it in inches so it can be added to the rim diameter, which is already in inches. Multiplying by two counts the sidewall at the top of the wheel and the sidewall at the bottom — the tire is a ring, and the wheel sits in the middle of it.
From diameter, everything else follows geometrically. Circumference is π · D. Revolutions per mile is the number of inches in a mile divided by that circumference: 63,360 ÷ (π · D). Ground clearance under the axle changes by half the diameter change, because half the extra diameter goes above the axle centreline and half below it — a 1.10-inch bigger tire lifts the axle 0.55 inches, not 1.10.
Speedometer error is a pure ratio. The instrument counts wheel revolutions and multiplies by an assumed circumference. Fit a tire whose real circumference is 3.49% larger and every revolution covers 3.49% more ground than the instrument assumes, so your true speed is 3.49% higher than the number on the dial: v_true = v_indicated × D₂ / D₁. The same ratio applies to the odometer, and therefore to your fuel-economy readout — worth knowing before you conclude a tire change hurt your mileage. If you want to check that separately, work it through the fuel economy (MPG) calculator using corrected miles.
Worked example: 265/70R17 to 285/70R17 on a half-ton truck
A pickup leaves the factory on 265/70R17 and the owner wants the wider 285/70R17 for a slightly more aggressive stance. Work both sizes by hand.
- Current sidewall. 265 × 70 ÷ 100 = 185.5 mm. Divide by 25.4: 185.5 / 25.4 = 7.30315 in.
- Current diameter. 17 + 2 × 7.30315 = 17 + 14.60630 = 31.60630 in.
- New sidewall. 285 × 70 ÷ 100 = 199.5 mm; 199.5 / 25.4 = 7.85433 in.
- New diameter. 17 + 2 × 7.85433 = 17 + 15.70866 = 32.70866 in.
- Difference. 32.70866 − 31.60630 = 1.10236 in. As a percentage: 1.10236 ÷ 31.60630 × 100 = 3.49%.
- Clearance. Half of 1.10236 = 0.551 in of extra static axle height, and the same 0.551 in less space between the top of the tire and the fender liner.
- Revolutions per mile. Old: 63,360 ÷ (π × 31.60630) = 63,360 ÷ 99.29 = 638.1. New: 63,360 ÷ (π × 32.70866) = 63,360 ÷ 102.76 = 616.6. You lose 21.5 revolutions per mile.
- Speedometer. At an indicated 60 mph you are really doing 60 × 32.70866 ÷ 31.60630 = 62.1 mph. At an indicated 75 you are doing 77.6.
The verdict: this swap is just outside the 3% band that fitment guides treat as the safe limit. It is a very common change and thousands of trucks run it, but you should expect a speedometer that reads about 2 mph slow at highway speed, a 3.5% optimistic odometer, and a small loss of effective gearing that shows up as slightly lazier acceleration and a lower cruising engine speed.
How to read the result
Stay inside ±3% on overall diameter unless you are prepared to deal with the consequences. That is the working tolerance the tire and wheel trade uses, and it exists because vehicle systems that compare wheel speeds — ABS, traction control, electronic stability control, and on four-wheel-drive vehicles the transfer case — assume all four tires turn at the same rate for a given road speed. A diameter mismatch bigger than a few percent, especially between axles, feeds those systems a signal that looks like wheel slip.
Never mix diameters front to rear on a full-time all-wheel-drive vehicle. A centre differential or viscous coupling absorbs the difference by slipping continuously, and on many transfer cases that is a heat failure rather than a compromise. The tolerance there is far tighter than 3% — check the manufacturer's figure, which is often quoted as a maximum circumference difference in millimetres.
Read the clearance number as a static best case. The extra half-inch of axle height is real, but the number that decides whether a tire rubs is the dynamic one: what happens at full steering lock with the suspension compressed. A taller tire arrives at the fender liner sooner, and a wider tire arrives at the upper control arm and the frame rail sooner. Neither this nor any other size calculator can tell you those clearances; you measure them on the vehicle.
Treat revolutions per mile as a comparison, not a spec. The value here is theoretical, from free-rolling circumference. A manufacturer's published revs-per-mile is measured on a loaded tire, which deflects, so the real figure typically runs a couple of percent higher. Comparing two theoretical values against each other is valid; comparing one theoretical value against a printed one is not.
If your change also involves new wheels, the offset matters at least as much as the size. Run it through the wheel offset and backspacing calculator before you order, because a wider tire on a wheel with less offset moves outward at both edges.
Plus-sizing reference: staying near a 205/55R16 baseline
| Size | Sidewall (in) | Overall diameter (in) | Difference | Rev/mile |
|---|---|---|---|---|
| 205/55R16 (baseline) | 4.439 | 24.878 | — | 810.7 |
| 225/45R17 | 3.986 | 24.972 | +0.38% | 807.6 |
| 235/45R17 | 4.163 | 25.327 | +1.80% | 796.3 |
| 235/40R18 | 3.701 | 25.402 | +2.11% | 794.0 |
| 215/50R17 | 4.232 | 25.465 | +2.36% | 792.0 |
| 235/35R19 | 3.238 | 25.476 | +2.41% | 791.6 |
| 245/40R18 | 3.858 | 25.717 | +3.37% | 784.2 |
Every figure is the formula above evaluated at that size: sidewall = width × aspect ÷ 100 ÷ 25.4, diameter = rim + 2 × sidewall, rev/mile = 63,360 ÷ (π × diameter). Note that 245/40R18 breaks the 3% rule while the physically larger-looking 235/35R19 does not — sidewall height, not rim size, drives overall diameter.
Mistakes that make a tire comparison useless
- Comparing section widths instead of diameters. Section width tells you about fender and suspension clearance. It tells you nothing about speedometer error, gearing or ride height. Those come from diameter only.
- Assuming section width is tread width. Section width is measured at the widest bulge of the sidewall on a specified measuring rim. Tread contact width is always narrower, and both change when you mount the tire on a wheel wider or narrower than the measuring rim.
- Forgetting the load index and speed rating. This calculator compares geometry only. A size that fits perfectly can still be illegal or unsafe on your vehicle if its load index is below the placard requirement — critical on trucks, vans and anything that tows.
- Applying the clearance number to the fender. Half the diameter change lifts the axle, and the same half is added to the top of the tire. Both numbers are the same magnitude, but the fender gap does not grow — it shrinks by that amount when you fit a taller tire.
- Ignoring the difference between a placard size and what is on the car. Compare against the size actually fitted if the vehicle has already been changed, and against the door-jamb placard if you want to know how far you have drifted from stock.
- Treating a 3% diameter change as free. It is a 3% change in effective gearing too, which shifts cruising rpm, shift points and the load your engine sees when towing.
Where size comparison sits among the other checks
Size comparison answers one question — how far the new tire deviates from the old one geometrically — and it is the first of several checks, not the whole job. After it, you need the load index and speed rating against the door placard, the wheel width range the tire manufacturer approves for that size, wheel offset and backspacing against your suspension and fenders, and on a lifted or lowered vehicle a physical clearance check at full lock and full compression.
Flotation sizes, printed as 33x12.50R15, are a different scheme: the first number is already the nominal overall diameter in inches and the second is the section width in inches. If a shop quotes you one of each, convert the metric size to inches with this calculator and compare like with like — but expect the flotation number to be a nominal marketing figure that a real tire may miss by a quarter-inch or more.
Once you know the diameter change, the downstream calculators fill in the consequences. A taller tire raises the effective final-drive ratio and changes how the vehicle accelerates and stops; that matters for both the braking stopping distance calculator, where contact patch and grip dominate, and for the road trip fuel cost calculator, where an over-optimistic odometer flatters your mileage. If you are chasing clearance for a heavier build, check the loaded-weight side of the equation with the towing capacity and payload calculator before you commit to a bigger, heavier tire on the same axles.
One last practical note: a bigger tire is a heavier tire, and the added mass is unsprung and at a larger radius. Both the rotational inertia and the wheel-bearing load go up. On a vehicle already near its rated axle load, that is a real cost, not a cosmetic one.
Key terms
- Section width
- The widest point of an inflated, unloaded tire, sidewall to sidewall, measured on the manufacturer's specified measuring rim. It is not tread width.
- Aspect ratio
- Sidewall height as a percentage of section width. A 45-series tire has a sidewall 45% as tall as the tire is wide, so lowering the aspect ratio shortens the sidewall for a given width.
- Plus sizing
- Fitting a larger-diameter wheel with a proportionally shorter sidewall so overall diameter stays roughly constant. Plus one means one inch more rim; plus two means two.
- Revolutions per mile
- How many times the tire turns to cover a mile. It sets speedometer and odometer calibration, and it is the quantity ABS and traction control compare across the four corners.
- Static loaded radius
- The distance from the axle centreline to the ground on a loaded tire — always less than half the free diameter, because the tire deflects. It is what actually determines ride height.