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Tyre tool

Tyre size calculator and comparison

Decode any tyre size, see its real diameter, and compare an alternative size against the original.

By Tirupati Tyres editors

Type the size printed on your tyre's sidewall to see what every part of it means and how big the tyre really is. Then set a second size against it to check whether a change of wheels or tyres stays within the usual 3% of the original diameter.

Decode a tyre size

Worked example: 205/55 R16 91V

205 mm113 mm205/55 R16632 MM TALL · 16-INCH RIM
Cross-section of a 205/55 R16 tyre, drawn to its own proportions: 205 mm wide with a 113 mm sidewall on a 16-inch rim.
Width
205 mm
Aspect ratio
55 %
Construction
Radial
Rim diameter
16 in
Sidewall height
112.8 mm
Overall diameter
631.9 mm
Circumference
1985 mm
Revs per km
504
Load index 91
615 kg per tyre
Speed rating V
240 km/h

205 mm across the tread, a sidewall 55% of that (112.8 mm), on a 16-inch (406.4 mm) rim: 406.4 + 2 × 112.8 = 631.9 mm tall.

205/55 R16: prices by brand, cars that use it and sizes within 3%

Compare two sizes

Worked example: 185/65 R15 against 195/55 R16

Close match

A close match: the diameter is within 2% (−0.1%) and the width within 20 mm. The tyre still has to suit the rim and carry at least the original load index and speed rating.

Speedometer shows 100
99.9 km/h true
Ride height
−0.3 mm
Diameter change
−0.1%
Measure185/65 R15195/55 R16Change
Diameter, mm621.5620.9−0.6
Width, mm185195+10
Sidewall, mm120.3107.3−13
Rim, inches1516+1
Circumference, mm19521951−1
Revs per km512513+1

With 195/55 R16 fitted, an indicated 100 km/h is really 99.9 km/h: the speedometer over-reads a little more than before. 195/55 R16 needs 16-inch wheels in place of 15-inch ones, so the change includes new rims.

Tyre diameter calculator

A tyre's height is not printed anywhere on it. It follows from three of the numbers in the size. The rim diameter is given in inches, so multiply it by 25.4 for millimetres. The sidewall height is the aspect ratio taken as a percentage of the width: a 205/55 tyre has a sidewall 55% of 205 mm, which is 112.75 mm. There is one sidewall above the rim and one below it, so the overall diameter is the rim plus two sidewalls: 406.4 + 2 × 112.75 = 631.9 mm for 205/55 R16.

Multiply the diameter by π for the circumference (1,985 mm here) and divide a kilometre by that for the revolutions the wheel makes per kilometre (504). These are nominal figures for a new tyre carrying no weight. On the car the loaded radius is a few millimetres smaller, and tread depth and the brand's own moulds move the real height a little either way.

Tyre size comparison

The comparison answers the usual upsizing question: will a different size behave like the one the car left the factory on? The working rule is to keep the overall diameter within 3% of the original. Inside that band the speedometer, the odometer and the ABS and stability-control sensors, which all count wheel revolutions, stay close to how they were set up.

A taller tyre covers more road per revolution, so the speedometer reads low: at an indicated 100 km/h you are going faster. Ground clearance changes by half the diameter difference. Width is a separate question. A tyre more than about 20 mm wider may need a wider rim and can rub the wheel arch or the suspension at full steering lock. Whatever the numbers say, the replacement must carry at least the original load index and speed rating, and the safest choice is a size the carmaker lists for your variant.

How to read a tyre size

Take 205/55 R16 91V, a marking found on many mid-size cars and compact SUVs:

205
Section width in millimetres, sidewall to sidewall, measured on the rim the tyre is designed for.
55
Aspect ratio: the sidewall's height as a percentage of the width. A lower number means a shorter, stiffer sidewall.
R
Radial construction, the standard for car tyres. ZR in the same place marks a radial rated above 240 km/h.
16
Rim diameter in inches.
91
Load index: 615 kg per tyre. See the load index and speed rating chart.
V
Speed symbol: up to 240 km/h.

A P in front (P205/55 R16) is the American way of writing a passenger size and reads the same. LT marks a light-truck tyre, a heavier construction that is not a straight swap for a car tyre. Elsewhere on the sidewall the DOT code ends in four digits that give the week and year the tyre was made; the tyre age check reads them.

Diameter of every size we track

Size Width Sidewall Rim Diameter Circumference Revs per km
145/70 R12 145 mm 101.5 mm 12 in 507.8 mm 1595 mm 627
145/80 R12 145 mm 116.0 mm 12 in 536.8 mm 1686 mm 593
145/80 R13 145 mm 116.0 mm 13 in 562.2 mm 1766 mm 566
155/80 R13 155 mm 124.0 mm 13 in 578.2 mm 1816 mm 551
175/70 R13 175 mm 122.5 mm 13 in 575.2 mm 1807 mm 553
165/65 R14 165 mm 107.3 mm 14 in 570.1 mm 1791 mm 558
165/70 R14 165 mm 115.5 mm 14 in 586.6 mm 1843 mm 543
165/80 R14 165 mm 132.0 mm 14 in 619.6 mm 1947 mm 514
175/65 R14 175 mm 113.8 mm 14 in 583.1 mm 1832 mm 546
175/70 R14 175 mm 122.5 mm 14 in 600.6 mm 1887 mm 530
185/65 R14 185 mm 120.3 mm 14 in 596.1 mm 1873 mm 534
185/70 R14 185 mm 129.5 mm 14 in 614.6 mm 1931 mm 518
175/60 R15 175 mm 105.0 mm 15 in 591.0 mm 1857 mm 539
175/65 R15 175 mm 113.8 mm 15 in 608.5 mm 1912 mm 523
185/60 R15 185 mm 111.0 mm 15 in 603.0 mm 1894 mm 528
185/65 R15 185 mm 120.3 mm 15 in 621.5 mm 1952 mm 512
185/70 R15 185 mm 129.5 mm 15 in 640.0 mm 2011 mm 497
195/65 R15 195 mm 126.8 mm 15 in 634.5 mm 1993 mm 502
195/80 R15 195 mm 156.0 mm 15 in 693.0 mm 2177 mm 459
205/65 R15 205 mm 133.3 mm 15 in 647.5 mm 2034 mm 492
215/75 R15 215 mm 161.3 mm 15 in 703.5 mm 2210 mm 452
235/75 R15 235 mm 176.3 mm 15 in 733.5 mm 2304 mm 434
185/55 R16 185 mm 101.8 mm 16 in 609.9 mm 1916 mm 522
185/60 R16 185 mm 111.0 mm 16 in 628.4 mm 1974 mm 507
195/55 R16 195 mm 107.3 mm 16 in 620.9 mm 1951 mm 513
195/60 R16 195 mm 117.0 mm 16 in 640.4 mm 2012 mm 497
205/55 R16 205 mm 112.8 mm 16 in 631.9 mm 1985 mm 504
205/60 R16 205 mm 123.0 mm 16 in 652.4 mm 2050 mm 488
205/65 R16 205 mm 133.3 mm 16 in 672.9 mm 2114 mm 473
215/60 R16 215 mm 129.0 mm 16 in 664.4 mm 2087 mm 479
215/65 R16 215 mm 139.8 mm 16 in 685.9 mm 2155 mm 464
215/70 R16 215 mm 150.5 mm 16 in 707.4 mm 2222 mm 450
235/70 R16 235 mm 164.5 mm 16 in 735.4 mm 2310 mm 433
205/55 R17 205 mm 112.8 mm 17 in 657.3 mm 2065 mm 484
215/55 R17 215 mm 118.3 mm 17 in 668.3 mm 2100 mm 476
215/60 R17 215 mm 129.0 mm 17 in 689.8 mm 2167 mm 461
225/45 R17 225 mm 101.3 mm 17 in 634.3 mm 1993 mm 502
225/60 R17 225 mm 135.0 mm 17 in 701.8 mm 2205 mm 454
225/65 R17 225 mm 146.3 mm 17 in 724.3 mm 2275 mm 439
235/65 R17 235 mm 152.8 mm 17 in 737.3 mm 2316 mm 432
245/65 R17 245 mm 159.3 mm 17 in 750.3 mm 2357 mm 424
265/65 R17 265 mm 172.3 mm 17 in 776.3 mm 2439 mm 410
215/55 R18 215 mm 118.3 mm 18 in 693.7 mm 2179 mm 459
225/50 R18 225 mm 112.5 mm 18 in 682.2 mm 2143 mm 467
225/55 R18 225 mm 123.8 mm 18 in 704.7 mm 2214 mm 452
235/45 R18 235 mm 105.8 mm 18 in 668.7 mm 2101 mm 476
235/55 R18 235 mm 129.3 mm 18 in 715.7 mm 2248 mm 445
235/60 R18 235 mm 141.0 mm 18 in 739.2 mm 2322 mm 431
245/60 R18 245 mm 147.0 mm 18 in 751.2 mm 2360 mm 424
255/60 R18 255 mm 153.0 mm 18 in 763.2 mm 2398 mm 417
255/65 R18 255 mm 165.8 mm 18 in 788.7 mm 2478 mm 404
265/60 R18 265 mm 159.0 mm 18 in 775.2 mm 2435 mm 411
265/65 R18 265 mm 172.3 mm 18 in 801.7 mm 2519 mm 397
225/55 R19 225 mm 123.8 mm 19 in 730.1 mm 2294 mm 436
235/55 R19 235 mm 129.3 mm 19 in 741.1 mm 2328 mm 430
245/55 R19 245 mm 134.8 mm 19 in 752.1 mm 2363 mm 423
255/60 R19 255 mm 153.0 mm 19 in 788.6 mm 2477 mm 404
265/55 R19 265 mm 145.8 mm 19 in 774.1 mm 2432 mm 411
245/50 R20 245 mm 122.5 mm 20 in 753.0 mm 2366 mm 423
255/45 R20 255 mm 114.8 mm 20 in 737.5 mm 2317 mm 432
265/55 R20 265 mm 145.8 mm 20 in 799.5 mm 2512 mm 398

Nominal unloaded figures. Each size links to its own page with the cars that use it, the sizes within 3% of its diameter and indicative prices by brand.

Sources

  1. European Tyre and Rim Technical Organisation (ETRTO): tyre and rim standards (checked 25 Oct 2026)
  2. UNECE: UN Regulation No. 30, tyres for passenger cars and their trailers (addenda to the 1958 Agreement, Regulations 21 to 40) (checked 25 Oct 2026)
  3. NHTSA: tyre safety, sidewall markings, the tyre identification number and tyre ageing (checked 25 Oct 2026)
  4. US eCFR 49 CFR 571.139: new pneumatic radial tyres for light vehicles (dimensions, load ratings and labelling) (checked 25 Oct 2026)