Tyre Size Calculator

Written byOleh Soloninupdated 25 September 2026

Original width, mm

Original profile, %

Original rim, inches

New width, mm

New profile, %

New rim, inches

Result

Verdict
Within 3% on diameter — the calculator's yardstick; the permitted sizes are the maker's list
Difference in diameter, %0.38%
Original diameter, mm631.9
New diameter, mm634.3
Change in ride height, mm1.2
Calculated speed at an indicated 100100.4
New circumference, mm1,993
Sidewall height112.8 mm on the original and 101.3 mm on the new tyre
New tyre in inches25 × 8.9 R17
Similar sizes within 3% of the diameter
195/65 R15 634.5 mm +0.41 % 225/55 R15 628.5 mm −0.54 % 205/60 R15 627.0 mm −0.78 % 215/60 R15 639.0 mm +1.12 % 235/55 R15 639.5 mm +1.20 % 225/50 R16 631.4 mm −0.08 % 195/60 R16 640.4 mm +1.35 % 235/50 R16 641.4 mm +1.50 % 215/50 R16 621.4 mm −1.66 % 195/55 R16 620.9 mm −1.74 % 225/45 R17 634.3 mm +0.38 % 205/50 R17 636.8 mm +0.78 % 195/50 R17 626.8 mm −0.81 % 215/45 R17 625.3 mm −1.04 % 235/45 R17 643.3 mm +1.80 % 195/45 R18 632.7 mm +0.13 % 215/40 R18 629.2 mm −0.43 % 225/40 R18 637.2 mm +0.84 % 205/45 R18 641.7 mm +1.55 % 235/35 R18 621.7 mm −1.61 %

Three numbers on the sidewall and what they mean

Compares two tyre sizes by overall diameter, shows the change in ride height and the speedometer error, and says whether the swap stays inside the usual three percent tolerance.

A marking such as 205/55 R16 reads: 205 mm wide, a sidewall 55% of that width, R for radial construction, and a 16-inch rim. The overall diameter follows from those numbers: the rim in millimetres plus two sidewalls.

A speedometer counts wheel revolutions, not speed. A taller tyre covers more ground per turn, so your real speed is higher than the dial shows — by the same percentage as the diameter grew. Which sizes a car may wear is the carmaker's list; the calculator shows the geometry.

How to use

Enter the original tyre size from the sidewall or the door-pillar placard: width, profile and rim diameter.
Enter the size you want to fit.
Read the difference in diameter, the change in ride height and your real speed when the speedometer shows 100 km/h.
Below that is a table of similar sizes on rims one inch smaller and one or two inches larger, with each one's deviation from the original.

Reading a tyre size

In 205/55 R16 the first number is the section width in millimetres, the second the sidewall height as a percentage of that width, R the radial construction and 16 the rim diameter in inches. The sidewall is 205 × 0.55 = 112.75 mm, the rim 16 × 25.4 = 406.4 mm, and the overall diameter is the rim plus two sidewalls: 406.4 + 2 × 112.75 = 631.9 mm.

Off-road tyres are sized in inches: 31×10.5 R15 is 31 inches overall (787.4 mm), 10.5 inches wide (266.7 mm) on a 15-inch rim. The calculator shows the new tyre in that form too.

Every row comes from the calculator on this page.
Size	Overall diameter, mm	Circumference, mm	In inches
175/70 R13	575.2	1,807	22.6 × 6.9 R13
185/65 R15	621.5	1,952	24.5 × 7.3 R15
195/65 R15	634.5	1,993	25 × 7.7 R15
205/55 R16	631.9	1,985	24.9 × 8.1 R16
205/60 R16	652.4	2,050	25.7 × 8.1 R16
215/60 R16	664.4	2,087	26.2 × 8.5 R16
225/45 R17	634.3	1,993	25 × 8.9 R17
215/55 R17	668.3	2,100	26.3 × 8.5 R17

Similar sizes

Sizes with almost the same overall diameter let you change rim size without changing the wheel's overall size: 205/55 R16, 195/65 R15, 225/45 R17 and 195/45 R18 are all within half a percent of one another. The calculator's table finds such sizes for any tyre from the geometry alone.

A matching diameter does not make a size right for the car: rim width, the arch and the suspension limit how wide a tyre can go. The carmaker lists approved sizes, and the tyre maker lists approved rim widths.

What this calculator shows

The overall diameter of both tyres and the gap between them as a percentage
Your real speed when the speedometer reads 100
The change in ride height — half the difference in diameter

What to keep in mind

Diameter comes from the geometry of the marking; a particular tyre model can differ by a few millimetres.
The 3% threshold is the calculator's yardstick for comparison, not a legal limit: the approved sizes are in the car's documentation.

FAQs

Can I fit a different tyre size?

The geometry is only part of the answer. The size must also clear the arch and suit the rim: the carmaker lists approved sizes, and the tyre maker lists approved rim widths.

How does tyre size affect the speedometer?

The instrument counts wheel revolutions and multiplies by the factory circumference. A larger tyre covers more ground per turn, so your real speed is higher than shown and the odometer under-counts.

How much does ride height change?

By half the difference in diameter: 10 mm more overall diameter is 5 mm more ground clearance. The other half goes upward into the arch.

What does the R in a tyre size mean?

Radial construction of the carcass, not radius. The number after it is the rim diameter in inches: R16 is a radial tyre for a 16-inch rim.

What happens if I fit 215/60 R16 instead of 205/55 R16?

The diameter grows from 631.9 to 664.4 mm, 5.14% more. At an indicated 100 km/h you are doing 105.1, and the car sits 16.3 mm higher.

What profile goes on a rim one inch larger?

A lower one, to keep the diameter. 205/55 R16 and 225/45 R17 are 0.38% apart, and 225/40 R18 is 0.84% out: the rim grew, the sidewall shrank, the overall size barely moved.

Worked example

Going up an inch of rim

Input: 205/55 R16 → 225/45 R17

Output: 631.9 mm against 634.3 mm, 0.38% apart

Note: The classic pairing: the rim gains an inch, 25.4 mm, while each sidewall loses 11.5 mm — 23 mm between them — so the overall diameter grows by just 2.4 mm. If the speedometer read true on the original tyres, an indicated 100 becomes a real 100.4 km/h.

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Result

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