Enrichment · Passion Projects · Vehicles

Will the new tires
fool the speedometer?

Type the size on the tires now and the size you are considering. The calculator decodes both, compares their heights against the ±3 percent rule of thumb, and shows what the speedometer will read at real speeds.

Compare the sizesWhat you get

The tire size calculator

The worked example is an SUV moving to a winter set on 18-inch wheels. Nothing you type leaves this browser.

Loading the calculator. If nothing appears, turn on JavaScript.

What you get

Enrichment · Passion ProjectsFree

What you finish with
Both sizes decoded, the diameter difference in percent, revolutions per mile and a speedometer table. It prints as a summary for the tire shop and downloads as a spreadsheet.
Who it is for
Drivers and families buying tires, automotive technology students, and math classes working with percent change.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The Passion Projects space in Caveat Enrichment.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Copy the current size from the sidewall or the door-jamb placard.
  2. Type the size you are considering.
  3. Read the diameter difference.
  4. Check the speedometer table.
  5. Confirm the load index, speed rating and fit with the vehicle’s manual.

Compare the sizes

How the sizes are compared

Decode the code, then compare heights.

The worked example: 225/65R17 to 235/60R18Sidewall5.76 in225 mm × 65% ÷ 25.4Diameter28.52 in17 + 2 × sidewallNew diameter29.10 in18 + 2 × 5.55Difference+2.1%60 shown = 61.2 trueThe worked example: 225/65R17 to 235/60R18Sidewall5.76 in225 mm × 65% ÷ 25.4Diameter28.52 in17 + 2 × sidewallNew diameter29.10 in18 + 2 × 5.55Difference+2.1%60 shown = 61.2 true
The size code is a recipe: width in millimetres, sidewall as a percent of width, rim in inches.

Reading the code

In 225/65R17, 225 is the section width in millimetres, 65 is the sidewall height as a percent of that width, R means radial, and 17 is the wheel diameter in inches. The load index and speed rating that follow, such as 102H, matter for safety but not for size.

Why height matters

The speedometer counts wheel turns. A taller tire covers more ground per turn, so the car goes faster than the speedometer says and the odometer under-counts. Tire shops commonly keep a replacement within about 3 percent of the original diameter; the vehicle maker’s list of approved sizes is the final word.

Sources

  1. National Highway Traffic Safety Administration. Tires: understanding the sidewall and tire safety (nhtsa.gov).
  2. U.S. Department of Transportation, eCFR. 49 CFR 575.104, Uniform Tire Quality Grading Standards, and tire information placard rules (ecfr.gov).
  3. Tire and Rim Association. Year Book: tire size designation (us-tra.org).
  4. National Institute of Standards and Technology. Handbook 44: the inch, the mile and the millimetre (nist.gov).

The worked example is made up. Nominal sizes are calculated; measured tires differ slightly by brand and inflation. Your entries stay in this browser. Support: team@caveatacademic.com.