Tire Size Comparison Calculator
Compare two tire sizes: overall diameter, width and sidewall change, speedometer error, ride height, gearing and fitment warnings.
About this tool
Tyre sizes look like a code because they are one. In metric form, 225/45R17 means a 225 mm section width, a sidewall that is 45% of that width, and a 17 inch rim. From those three numbers the whole geometry follows: sidewall height, overall diameter, circumference and the number of revolutions per mile. This calculator does that arithmetic for two sizes at once and, more usefully, reports what changes when you switch between them.
The part most calculators leave out is the direction of the speedometer error. A speedometer is calibrated to the rolling radius of the tyre the car was built with. Fit a larger tyre and each revolution covers more ground, but the instrument still counts revolutions against the old circumference — so the car is genuinely travelling faster than the needle says. Fit a smaller tyre and the reverse happens. Both the size and the sign of that error are reported here, because getting the sign backwards is worse than not reporting it at all.
Alongside that you get the two figures that people who change tyres actually need: ride height, which is half the change in overall diameter, and the change in engine speed at a given road speed, which is what makes a taller tyre feel as though the car has lost its edge. Fitment flags are raised against widely used rules of thumb — a diameter change beyond about three percent, a rim diameter that needs new wheels, a section width that grows far enough to threaten clearance.
Why only some size changes raise a warning
The fitment flags are not symmetrical, and that is deliberate. A tyre that grows 20 mm wider is flagged; a narrower one is not. A load index or speed rating that drops is flagged; one that rises, or that simply matches, is not.
Each flag exists to catch a particular way of getting it wrong, and each has a direction. Width is checked because extra section width is what reaches for the strut, the liner and the arch. Going narrower has consequences of its own, but it does not create a clearance problem, so no clearance warning is raised.
Load index and speed rating are compared only when both sizes state them, and only when the new figure is lower. Dropping a load index lowers the maximum weight each tyre may carry; raising it does not. The speed ladder is a published ISO and ETRTO table, so that comparison is a lookup and not a calculation.
Two checks work in both directions. A diameter change is flagged past about three percent either way, because both a taller and a shorter tyre shift the speedometer and the odometer. A change of rim diameter is flagged outright, since it means new wheels are needed rather than just new tyres.
How the speed rating is read, and why ZR never trips it
Speed letters are matched against a fixed ladder running from L at 120 km/h to Y at 300 km/h, and a warning appears only if the new letter sits lower. ZR is above the ladder and returns no value, so it can never trigger one.
The ratings are a published ladder rather than a computed figure, so the comparison is two codes looked up in the same list. Both sizes have to carry a rating: a size written without one has nothing to compare against, and the check is skipped instead of being assumed.
ZR means above 240 km/h with no single defined ceiling, which is why it is not on the ladder. A pair such as 225/45ZR17 and 245/40ZR17 therefore produces no speed warning, and it should not — neither side has a number in that table to compare.
Dimensions and service descriptions are worked out separately. The geometry comes from the numbers in the size code, while the load index and the speed letter come from the character pairs after the rim diameter. Keeping them apart is what stops a capacity rating from being read as a length.
Why the P and LT prefixes change nothing in the numbers
P225/45R17 and 225/45R17 produce identical geometry here. The prefix is read and kept, but it describes the tyre service type — passenger car, light truck, trailer — and not a dimension, so it moves no diameter, width or sidewall figure.
A size code carries two kinds of information. The width, the aspect ratio and the rim diameter are measurements, and the whole geometry follows from them. Letters such as P, LT, ST, T and C are a service classification: they say what kind of vehicle the tyre was designed for.
The tool parses those letters so that a code copied off a sidewall is accepted rather than rejected, and then leaves the geometry untouched. Two sizes that differ only in the prefix compare as equal, because in every measurement that matters here they are equal.
The parts of the code that do decide whether a swap is acceptable are handled as separate checks: the load index and speed rating as service figures, the rim diameter, section width and overall diameter as dimensions. Blending the two would mean reading a capacity rating as a length.
How far the speedometer drifts after a tire change, and which way
Going from 205/55R16 to 205/60R16 grows the diameter by 3.24%: at an indicated 100 km/h the car is really doing 103.2. To travel a true 100 the needle has to sit at 96.9. The direction is that the speedometer reads low and the car goes faster.
A speedometer multiplies wheel rotations by circumference, and circumference follows diameter. A larger diameter covers more ground per rotation, so actual speed exceeds the reading. This is the easiest sign to get backwards, which is why the page lists indicated, actual and error separately.
One ordinary step up in aspect ratio already crosses the 3% tolerance: 205/55R16 to 205/60R16 is +3.24%, reading 100 at a true 103.2; to 215/60R16 is +5.14%, a true 105.1; 225/45R17 to 225/50R17 is +3.55%.
The odometer drifts with it. Showing 1,000 km means about 1,032 km were actually driven. Service intervals, warranty mileage and used-car mileage are all counted off that dial, so this is not merely a display problem.
Shrink the diameter instead and every direction flips: the reading runs high, the car is slower than shown, and the odometer accumulates too fast. Both cases reduce to one sentence — once the circumference changes, the gauge is no longer accurate.
Frequently asked questions
How do I work out the overall diameter of a tyre?
Take the section width in millimetres, multiply by the aspect ratio as a percentage to get the sidewall height, then add two sidewalls to the rim diameter converted to millimetres. A 225/45R17 therefore has a 101.25 mm sidewall and an overall diameter of 431.8 + 202.5 = 634.3 mm, which is about 24.97 inches.
Will a bigger tyre make my speedometer read high or low?
Low. A larger rolling radius means the car covers more ground per revolution while the speedometer still assumes the original circumference, so the true speed is higher than the indicated speed. The error is simply the percentage change in diameter, and it applies to the odometer too — a larger tyre makes the odometer under-count the distance actually travelled.
What is the difference between 225/45R17 and 33x12.50R15?
They are two ways of writing the same three measurements. The metric form gives width, aspect ratio and rim, from which the diameter is derived. The inch flotation form — used on light trucks and off-road tyres — gives the diameter and width directly. Both are supported here, and the inside-flotation form also reveals the aspect ratio it implies, so the two can be lined up in one table.
How much tyre size difference is acceptable?
The usual rule of thumb is to keep the overall diameter within three percent of the original. Beyond that the speedometer drift becomes noticeable, and anti-lock braking and stability systems that rely on wheel speed can be affected. Width matters for a different reason: the wheel arch, strut and inner liner decide how much extra section width will clear, and that depends on the wheel offset rather than the tyre alone.
Does a larger tyre change acceleration and fuel consumption?
It changes the effective final drive ratio. A taller tyre turns fewer revolutions per mile, so at any given road speed the engine revs lower. That usually helps cruising economy slightly and hurts acceleration, because the engine is working against a numerically lower overall ratio. The change is proportional to the change in diameter, and it is shown here as the engine speed at the same road speed.
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