TireSizeCalc

Tire size calculator

Enter two sizes to compare height, width, sidewall and speedometer change. Every number comes from the official sizing formula.

Size comparison265/70R17
Dimension265/70R17 285/70R17New vs current
Overall diameter
UN R30 §6.1.2.1		31.61 in 802.8 mm32.71 in 830.8 mm+1.10 in
Section width
UN R30 §6.1.2.1		10.43 in 265.0 mm11.22 in 285.0 mm+0.79 in
Sidewall height
UN R30 §6.1.2.1		7.30 in 185.5 mm7.85 in 199.5 mm+0.55 in
Rim diameter
Marked on the tire		17 in17 in0 in
Circum­ference
From diameter		99.29 in102.76 in+3.47 in
Revolutions per mile
From diameter		638617−21
Allowed real diameter
UN R30 §6.1.5		31.17–32.19 in32.24–33.34 in
Nominal, not measuredReal tires vary by brand and load range.
31.61 in
  • 265/70R17 31.61 in
  • 285/70R17 32.71 in
Drawn to scale on one ground line. Charcoal: current size. Blue outline: new size.

Speedometer change

mphTrue speed 62.1 mph (+2.1 mph)

If your speedometer is calibrated for 265/70R17 and you fit 285/70R17, the tire is +3.5% in diameter, so your true speed differs from the reading as shown below. That is more than a 3% change.

Speedometer reading versus true speed
ReadingTrue speedDifference
30 mph31.0 mph+1.0 mph
45 mph46.6 mph+1.6 mph
55 mph56.9 mph+1.9 mph
60 mph62.1 mph+2.1 mph
70 mph72.4 mph+2.4 mph
80 mph82.8 mph+2.8 mph

Most-searched sizes

Ordered by US monthly search volume (keyword data pulled November 2026). Each page shows the size in inches, a to-scale drawing, speedometer change and every size within 3%.

275/55R2031.91″ × 10.83″275/65R1832.07″ × 10.83″235/65R1729.03″ × 9.25″275/60R2032.99″ × 10.83″275/65R2034.07″ × 10.83″275/70R1833.16″ × 10.83″285/70R1732.71″ × 11.22″315/70R1734.36″ × 12.40″265/70R1731.61″ × 10.43″285/60R2033.46″ × 11.22″285/75R1632.83″ × 11.22″305/55R2033.21″ × 12.01″285/55R2032.34″ × 11.22″235/75R1528.88″ × 9.25″285/65R1832.59″ × 11.22″285/75R1733.83″ × 11.22″295/70R1733.26″ × 11.61″315/75R1634.60″ × 12.40″255/75R1732.06″ × 10.04″265/75R1631.65″ × 10.43″285/70R1833.71″ × 11.22″295/55R2032.78″ × 11.61″305/70R1733.81″ × 12.01″265/70R1832.61″ × 10.43″

See every size, grouped by rim.

How to read a tire size

In LT285/70R17 121/118S, LT means light truck, 285 is the section width in millimeters, 70 is the sidewall height as a percentage of that width, R means radial and 17 is the rim diameter in inches. 121/118 is the load index (3,197 lb single, 2,910 lb dual) and S is the speed rating.

The calculator also accepts inch sizes such as 33x12.50R15. The numbers are the nominal diameter and width in inches; the official UN R54 table lists 33x12.50R15LT at 32.52 in tall. See inch tire sizes.

Read the full guide to tire sizes.

Printed from https://tiresizecalc.net/. Figures as published on 2026-11-01. The full working and the official sources are on that page.

Sources

  1. UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Revision 3). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.29/Rev.3. 2007-03-29. Para 2.15 and 6.1.2.1: aspect ratio and outer diameter, D = d + 2H. Retrieved 2026-11-01.
  2. UN Regulation No. 54 - Uniform provisions concerning the approval of pneumatic tyres for commercial vehicles and their trailers (Revision 2). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.53/Rev.2. 2004-04-16. Para 6.1.2.1: the same formula for light truck tires. Retrieved 2026-11-01.
  3. UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Revision 3). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.29/Rev.3. 2007-03-29. Para 6.1.5: allowed outer diameter, Dmin = d + 2H x 0.97, Dmax = d + 2H x 1.04. Retrieved 2026-11-01.
  4. Computed by tiresizecalc.net from the cited formula. Sri Murugan Systems. Circumference = pi x diameter; revolutions per mile = 63,360 / circumference; 1 in = 25.4 mm exactly. Retrieved 2026-11-01.
  5. UN Regulation No. 54 - Uniform provisions concerning the approval of pneumatic tyres for commercial vehicles and their trailers (Revision 2). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.53/Rev.2. 2004-04-16. Annex 4: load index 121 and 118 in kg (converted to lb). Retrieved 2026-11-01.
  6. UN Regulation No. 30 - Uniform provisions concerning the approval of pneumatic tyres for motor vehicles and their trailers (Revision 3). United Nations Economic Commission for Europe (UNECE). E/ECE/324 E/ECE/TRANS/505 Rev.1/Add.29/Rev.3. 2007-03-29. Para 2.29.1: speed symbols. Retrieved 2026-11-01.
  7. OpenRush US keyword index - keyword research pull. OpenRush (third-party SEO data). Order of the most-searched list only (third-party US search volume estimates; values not shown). Retrieved 2026-11-01.

Last reviewed 2026-11-01. The full source list, with dates and file checksums, is in How the numbers are calculated. To report an error, email contactus@tiresizecalc.net.