TireSizeCalc

Changing tire size: what changes and how to calculate it

A different tire size changes the speedometer, the ground clearance, the width and the load capacity. This guide shows how to calculate each change before you buy.

Last reviewed · 6 source documents · Written in ASD-STE100 Simplified Technical English

Important. TireSizeCalc does not tell you if a tire fits your vehicle. The wheel width, wheel offset, suspension and body clearance decide this. Ask your vehicle manufacturer or a tire installer.

Some tire manufacturers do not recommend larger sizes. BFGoodrich tells you that a large increase in wheel diameter can cause problems with the anti-lock brakes, the speedometer and the stability system.1

“BFGoodrich Tires does not recommend up-sizing due to safety reasons.”

Quoted from the source 1

The example on this page changes from 265/70R17 to 285/70R17. All numbers are nominal and come from the official sizing formula.2

1. Compare the overall diameters

  1. Find the overall diameter of each size with the calculator. 265/70R17 is 31.61 in. 285/70R17 is 32.71 in.
  2. Subtract the old diameter from the new diameter: 32.71 − 31.61 = 1.10 in.
  3. Divide the difference by the old diameter, then multiply by 100: 3.5%.

2. Calculate the speedometer error

A larger tire goes farther in each turn of the wheel. Its circumference is π × diameter. For this reason, the true speed is the speedometer speed × new diameter ÷ old diameter.

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

With the larger tire, the speedometer shows less than the true speed. At 60 mph on the speedometer, the true speed is 62.1 mph. If the odometer uses the same signal, it also shows fewer miles than you drive.

UN Regulation No. 39 tells vehicle makers that the speedometer must not show less than the true speed.3 This rule applies to vehicle approval in the countries that use UN R39. A larger tire can make the speedometer show less than the true speed.

3. The 3 percent rule of thumb

Many web pages give a limit of 3 percent diameter change. We did not find this limit in a regulation or in a tire manufacturer publication. TireSizeCalc uses 3 percent only to select the near sizes on each size page. It is not a safety limit.

4. Ground clearance

The axle moves up by half of the diameter change. For this example: 1.10 ÷ 2 = 0.55 in. The body rises with the axle. For this reason, the top of the tire moves 0.55 in closer to the fender, and the tire can touch the body or the suspension.

5. Width

285/70R17 is 0.79 in wider than 265/70R17. On the same wheel, the tire is about 0.40 in wider on each side. A new passenger tire can also be up to 7 percent wider than its specified width, or more for some pressure classes.4

6. Load capacity

For passenger cars, FMVSS No. 110 sets a rule for the vehicle manufacturer. The vehicle maximum load on a tire must not be more than the maximum load rating of the tire.5 When you change size, do not decrease the load capacity. A lower load index is a lower load.6

  1. Read the load index on the original tires. Some placards also show it.7
  2. Read the load index of the new tire.
  3. Make sure that the new load index is equal to or more than the original load index. Use the load index chart.

7. Revolutions per mile

265/70R17 turns 638 times each mile. 285/70R17 turns 617 times each mile. This is 63,360 inches in a mile divided by the circumference of the tire.

More guides

Printed from https://tiresizecalc.net/guides/changing-tire-size/. Figures as published on 2026-11-01. The sources are listed below.

Sources

  1. Tire Upgrade (plus sizing). BFGoodrich Tires (Michelin North America (Canada) Inc.). Undated web page. Plus sizing. Retrieved 2026-11-01.
  2. 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 6.1.2.1. Retrieved 2026-11-01.
  3. UN Regulation No. 39 - Uniform provisions concerning the approval of vehicles with regard to the speedometer equipment including its installation (Revision 1). UNECE; Official Journal of the European Union. OJ L 120, 13.5.2010. 2010-05-13. Para 5.3. Retrieved 2026-11-01.
  4. 49 CFR 571.139 - Standard No. 139; New pneumatic radial tires for light vehicles. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations (eCFR). 49 CFR 571.139. eCFR up to date as of 2026-10-15. S6.1.2. Retrieved 2026-11-01.
  5. 49 CFR 571.110 - Standard No. 110; Tire selection and rims for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations, annual edition (GovInfo). 2006-10-01. S4.2.1. Retrieved 2026-11-01.
  6. 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 2.27 and Annex 4. Retrieved 2026-11-01.
  7. 49 CFR 571.110 - Standard No. 110; Tire selection and rims for motor vehicles with a GVWR of 4,536 kilograms (10,000 pounds) or less. National Highway Traffic Safety Administration (NHTSA), Code of Federal Regulations, annual edition (GovInfo). 2006-10-01. S4.3(i). 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.