What the numbers on the sidewall mean
A size like 225/45R17 is three separate facts written together. The 225 is the section width in millimetres, measured at the widest point of the sidewall rather than across the tread. The 45 is the aspect ratio: the sidewall height as a percentage of that width, so 45 percent of 225 mm gives a sidewall of 101.25 mm. The 17 is the rim diameter in inches, and it is inches even though everything to the left of it is metric, which is the single most confusing thing about the format.
To get the overall diameter you convert the sidewall to inches and add two of them to the rim, because the tire has a sidewall above the wheel and another below it:
Overall diameter (in) = rim + 2 × (width_mm × aspect ÷ 100) ÷ 25.4
For 225/45R17 that is 17 + 2 × 101.25 ÷ 25.4 = 17 + 7.972 = 24.97 inches. For 245/40R18 it is 18 + 2 × 98 ÷ 25.4 = 18 + 7.717 = 25.72 inches. Those two are a common "plus one" pair, and the point of the pairing is that the taller rim is offset by a shorter sidewall so the overall diameter barely moves.
Circumference is π times the diameter, and revolutions per mile follows from there. A mile is 63,360 inches, so:
Revolutions per mile = 63,360 ÷ (π × diameter_in)
A 24.97 inch tire has a circumference of 78.45 inches and turns 807.6 times per mile. That figure is the one the vehicle actually cares about, because a speedometer counts wheel revolutions and multiplies by an assumed distance per revolution.
Which way the speedometer error goes
The vehicle counts turns of the wheel. It was told, at the factory, how far one turn covers. Fit a taller tire and each turn covers more ground than the vehicle believes, so you travel further than it reports: the speedometer reads low and the odometer under-counts. Fit a shorter tire and the opposite happens — the speedometer reads high and the odometer racks up miles you did not drive.
The arithmetic is a straight ratio. True speed equals indicated speed multiplied by new diameter divided by old diameter. Going from 24.97 to 25.72 inches is a factor of 1.0300, so an indicated 60 mph is a true 61.8 mph and an indicated 70 is a true 72.1. Over 1,000 real miles the odometer would show 971.
| New tire vs original | Speedometer | Odometer | Indicated 60 mph is really |
|---|---|---|---|
| 3% taller | Reads low | Under-counts | 61.8 mph |
| 1% taller | Reads slightly low | Under-counts slightly | 60.6 mph |
| Same diameter | Unchanged | Unchanged | 60.0 mph |
| 1% shorter | Reads slightly high | Over-counts slightly | 59.4 mph |
| 3% shorter | Reads high | Over-counts | 58.2 mph |
Worth knowing separately: many speedometers are deliberately calibrated to read a little high from new, so a modest increase in diameter can move a car from reading three percent fast to reading approximately true. That is a happy accident rather than a design goal, and it does not make the odometer correct.
Plus sizing, and why the diameter is supposed to stay put
Going up a rim size while dropping the aspect ratio is called plus sizing, and the convention is to hold overall diameter within roughly three percent of the original. Three percent is not a legal figure or an engineering limit; it is the band inside which the speedometer stays close enough to honest, the gearing feels the same, and the wheel-speed signals stay in the range the vehicle's electronics were calibrated against. Tire retailers use it as their substitution rule of thumb.
What plus sizing costs you is sidewall. A 45-series sidewall on a 17 inch rim is just under four inches of rubber between the road and the wheel rim; a 40-series on an 18 is about 3.9 but with less deflection because the shorter sidewall is stiffer. Steering response sharpens, impact harshness increases, and the wheel becomes considerably easier to damage on a pothole. That is the actual trade, and no calculator can tell you whether you want it.
Changing overall diameter changes what the speedometer and odometer report, and the wheel-speed signal those systems produce is the same signal ABS, traction and stability control work from. A different diameter or a wider section can also foul the liner, the strut or the suspension arms at full lock or full compression, and on a four-wheel-drive vehicle a mismatch between axles loads the driveline. Check clearance at lock and at full droop before committing to a size.
Flotation sizes and where this stops being definitional
Light-truck flotation sizing — 33x12.50R15 and the like — states the nominal overall diameter directly in inches, followed by section width in inches and rim diameter. This page reads that format too, and for those sizes the diameter shown is simply the number on the sidewall rather than something computed.
Everything above is definitional arithmetic and it is exact given the size. What it is not is a measurement of the tire in your driveway. The published section width applies at a specified measuring rim width, and the same size mounted on a narrower or wider wheel comes out narrower or wider than the label. Loaded rolling radius is smaller than half the free diameter because the tire deflects under the car, so a real vehicle covers slightly less ground per revolution than these figures suggest. Manufacturers publish measured overall diameter and revolutions-per-mile figures for each specific tire, and those measured numbers commonly differ from the calculated ones by one or two percent in either direction. If a fitment is genuinely marginal, the manufacturer's spec sheet for that exact tire is the document to go and read.
Questions people ask
How much tire size difference is acceptable?
The working convention across tire retail is within about three percent of the original overall diameter, and this calculator flags the comparison against that band. The reasoning is practical rather than regulatory: three percent keeps the speedometer error under about two mph at highway speed, keeps the effective gearing close enough that acceleration and cruising rpm barely move, and keeps the wheel-speed signal inside the range the vehicle's ABS and stability systems were calibrated against. It is not a legal limit and it is not a fitment check. A size that is within three percent on diameter can still be far too wide to clear the suspension, and a size outside three percent can be perfectly fine on a vehicle where the manufacturer offered it as a factory option. On all-wheel-drive vehicles the tolerance between axles is much tighter than three percent — mismatched rolling diameters front to rear put a permanent speed difference through a centre differential or coupling that was not designed to absorb it.
Will bigger tires make my speedometer read wrong?
Yes, in a predictable direction. A larger overall diameter covers more ground per wheel revolution than the vehicle was calibrated for, so the speedometer reads lower than your true speed and the odometer counts fewer miles than you actually drove. The error is proportional: a two percent larger tire gives a two percent error at every speed, so an indicated 60 is a true 61.2. Smaller tires reverse it, reading high and over-counting miles. On many vehicles the calibration constant can be corrected in the body or powertrain module with a scan tool, and on some it is a dealer-level or aftermarket-programmer job; on older vehicles with a mechanical drive it is a physical gear change in the transmission. Whether it is worth correcting depends on how large the error is and whether you care about the odometer, which is what warranty periods, service intervals and resale value are all measured against.
What does the 45 in 225/45R17 mean?
It is the aspect ratio: the sidewall height expressed as a percentage of the section width, not as any absolute measurement. A 225/45 has a sidewall of 45 percent of 225 mm, which is 101.25 mm or just under four inches. The same 45 on a 275 section width would be 123.75 mm, so identical aspect ratios do not mean identical sidewalls. Lower numbers mean shorter, stiffer sidewalls — sharper steering response, more impact harshness, more exposure to wheel damage from potholes. Higher numbers mean more rubber absorbing the road. The R that follows means radial construction, which is what almost everything on the road has used for decades, and any letters before it are speed-rating designations that occasionally migrate to that position.
Do wider tires change the overall diameter?
Only through the aspect ratio, which is why the arithmetic catches people out. Going from 225 to 245 at the same aspect ratio and rim makes the tire taller as well as wider, because 45 percent of 245 is more sidewall than 45 percent of 225 — that pair differs by 0.71 inches in overall diameter. To go wider while keeping the diameter, the aspect ratio has to come down: 245/40R18 sits within three quarters of an inch of 225/45R17 precisely because both the rim went up and the aspect ratio came down. Section width also has a fitment consequence that diameter does not. Extra width has to go somewhere, usually inboard toward the strut and the suspension arms, and that is often what limits a fitment rather than height. The published width also assumes a specified measuring rim, so the same size on a wider wheel measures wider than the label.