Tyre size calculator
Compare two car tyre sizes — metric, P- and LT-metric, or flotation like 33x12.50R15 — for diameter, width, sidewall and revs per mile, see the speedometer error at each speed and the ride-height change, draw them to scale, check rim widths, and work out wheel offset, backspacing, poke and inner clearance.
Added
Works with
- Vehicle log Cars and EVslog fuel, service and tyre changes — a new size changes the odometer reading by the speedometer error shown here
How to use it
Type the size on the car now and the one you’re thinking of. The page works out both tyres' dimensions and what changes: how much taller or shorter, how wrong the speedometer will be, how much the car rises, and whether the new tyre suits the rim. Below that, the wheel: change the width and offset and see how far the rim and tyre move out toward the arch and in toward the suspension.
For bicycle tyre pressures, see bike tyre pressure; this page is for cars, vans and trucks.
How to use it
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Enter the two sizes. All of these work:
225/45R17,225/45ZR17 94W,P215/65R15,LT265/75R16,215/65R16C,33x12.50R15LT,35x12.50-20. Sizes with no aspect ratio (155R13,7.50R16) are worked out from the old series and flagged. -
Comparison gives diameter, section width, sidewall, circumference and revs per mile and per km for each, and the differences. Effective gearing changes by the inverse of the diameter.
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To scale draws the two tyres side by side and in cross-section at the same scale.
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Speedometer is what the car really does when the speedometer, set up for tyre A, shows a speed with tyre B fitted. Switch between mph and km/h.
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Rim widths gives the usual approved range for each tyre and the measuring rim it’s specified on.
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Wheel offset: give the current and new wheels' width in inches and offset (ET) in mm. The table shows backspacing and front spacing for each and how far the outer edge (poke) and inner edge move, for the rim and for the tyre (its width estimated on that rim).
The sums
A metric size is section width in mm / sidewall as a percentage of width, then the rim diameter in inches: 225/45R17 has a 101.25 mm sidewall and is 17 × 25.4 + 2 × 101.25 = 634.3 mm tall. A flotation size gives the overall diameter and width in inches directly. Revs per mile is 1,609,344 mm ÷ (π × diameter). These are nominal: under load a tyre rolls 2–3 % less far per turn, and the same size varies by a few millimetres between makers, so use the maker’s revs per mile when you have it.
The speedometer counts wheel turns. With a tyre that is 2 % taller, each turn covers 2 % more road, so you are going 2 % faster than it shows. Speedometers are allowed to read high, never low: in the UK and EU up to 10 % + 4 km/h over.
Rim ranges are a rule of thumb from ETRTO and Tire and Rim Association practice: the measuring rim is about 72 % of the section width for 70- and 75-series tyres, rising to about 88 % for 40 series and lower, and the approved range runs from half an inch (or one inch) under to one inch over. A tyre gets about 5 mm wider for each half inch of rim. The tyre maker’s table is the authority.
Offset (ET) is the distance from the rim’s centreline to the hub face; backspacing is from the hub face to the inner lip, taking the overall width as the rim width plus one inch. A wider wheel at the same offset grows equally both ways; less offset moves the whole wheel outward.
To see what a tyre change does to engine speed in each gear, use the engine calculator. The OBD-II tool reads the speed the car’s computer reports, which you can compare with a GPS.
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