Tires & Detailing · Free tool
Free Tire Size Calculator
Enter your original tire size and the one you are considering. The calculator compares their dimensions and shows what the change does to your speedometer.
- Free to use
- No signup
- Runs in your browser
Original tire
New tire
A size reads width / aspect ratio R wheel diameter. 215/60R16 is 215 mm wide, with a sidewall 60% of that, on a 16-inch wheel.
Speedometer check
Change in overall diameter
+2.2%
Within the 3% guideline for a size change.
Original New
When the dash shows 60 mph, you are doing 61.3 mph.
Width, load rating and wheel offset also decide whether a tire fits. Check with a tire shop before you buy.
| Measurement | Original | New | Change |
|---|---|---|---|
| Sidewall height | 5.08 in | 4.87 in | -0.21 in |
| Overall diameter | 26.16 in | 26.74 in | 0.59 in |
| Circumference | 82.2 in | 84.0 in | 1.8 in |
| Revolutions per mile | 771 | 754 | -17 |
An estimate from the figures you enter, not a quote. Read the disclaimer. Updated .
02Your result
What your tire size comparison means
Two tires can share a wheel size and still differ enough to upset the speedometer or rub the bodywork.
- 01
Change in diameter
How much taller or shorter the new tire stands. Tire makers’ general guideline is to stay within about 3% of the original.
- 02
Speedometer error
A taller tire travels further per turn, so the car goes faster than the dash shows. A shorter tire does the opposite.
- 03
Revolutions per mile
How many times the tire turns in a mile. It is the figure the car uses to work out speed and distance.
What to do next
Set the pressure
A new size does not change the pressure on your door placard. Check what the weather has done to it.
Tire pressure calculatorRead the sidewall
Tread depth, rotation and how to tell when a tire is finished.
Tire guideSee the effect on fuel
Measure your MPG before and after a size change.
MPG calculator
03How it works
The tire size formula
A metric tire size holds three numbers. Everything else is geometry.
Diameter (in) = wheel diameter + 2 × (width × aspect ratio ÷ 100 ÷ 25.4)
The fraction is the sidewall height in inches. 25.4 converts millimeters to inches.
Revolutions per mile = 63,360 ÷ (π × diameter). Actual speed = indicated speed × (new diameter ÷ original diameter)
63,360 is the number of inches in a mile.
What each term means
- width
- The section width of the tire in millimeters: the 215 in 215/60R16.
- aspect
- The aspect ratio: sidewall height as a percentage of the width. The 60 in 215/60R16.
- rim
- The wheel diameter in inches: the 16 in 215/60R16.
- D
- The overall diameter of the inflated tire, in inches.
What the tool assumes
- Dimensions are calculated from the size on the sidewall. Real tires vary slightly between makers.
- A new, unloaded tire. Tread wear and load reduce the rolling diameter a little.
- The speedometer was accurate with the original size.
- Fit depends on width, offset, load rating and clearance too, which the size alone does not show.
04Worked example
Tire size example: 215/60R16 to 225/55R17
Moving from a 215/60R16 to a 225/55R17, a common one-inch wheel upgrade.
- 01
Original sidewall
215 mm × 60% ÷ 25.4
5.08 in
- 02
Original diameter
16 + 2 × 5.08
26.16 in
- 03
New sidewall
225 mm × 55% ÷ 25.4
4.87 in
- 04
New diameter
17 + 2 × 4.87
26.74 in
- 05
Compare the two
26.74 ÷ 26.16 − 1
+2.2%
- 06
Check the speedometer
60 mph × 1.022
61.3 mph
Change in diameter
+2.2%
Inside the 3% guideline. With the dash showing 60 mph the car is doing 61.3 mph, and the tire turns 17 fewer times each mile.
Try your own numbers05Background
How to read a tire size, and why 3% matters
A size such as 215/60R16 is three measurements and a letter. The table shows what each part means.
| Part | In 215/60R16 | What it means |
|---|---|---|
| Width | 215 | The width of the tire in millimeters, sidewall to sidewall. |
| Aspect ratio | 60 | The sidewall height as a percentage of the width. Lower means a shorter sidewall. |
| Construction | R | Radial, the construction used on almost every car tire. |
| Wheel diameter | 16 | The diameter of the wheel the tire fits, in inches. |
When you change size, the overall diameter is what the car notices. BFGoodrich’s guidance is to keep the overall diameter within about 3 percent of the original, because beyond that the speedometer and odometer error becomes significant. Les Schwab gives the same 3 percent figure.
That is why a bigger wheel is normally paired with a lower aspect ratio: the sidewall shrinks as the wheel grows, and the overall diameter stays close to where it started. The recommended pressure does not move either. It is the figure on the vehicle’s Tire and Loading Information label, not the number molded into the tire. See the tire section of the maintenance guide for tread and age limits.
06When to use it
When to use a tire size calculator
Before you pay for tires or wheels that are not the size the car came with.
Changing wheel size
Find the aspect ratio that keeps the diameter close on a larger or smaller wheel.
Choosing winter tires
Check a narrower winter size against your summer size.
Checking the speedometer
See how far the dash reading will be from your true speed.
Comparing two options
Put two candidate sizes side by side before you order.
Mistakes to avoid.
- Changing the wheel size without changing the aspect ratio to match.
- Looking at diameter only and ignoring width, which decides whether the tire rubs.
- Fitting a tire with a lower load index or speed rating than the original.
- Mixing sizes across an axle. Keep both tires on an axle the same.
07Related guides and tools
Keep going from here.
Guides that go with this tool
Related tools
08Questions
Tire Size Calculator: your questions.
How do I calculate tire diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall height in millimeters, divide by 25.4 to convert to inches, double it and add the wheel diameter. A 215/60R16 works out at about 26.2 inches.
Can I use a different tire size on my car?
Often, within limits. A general guideline from tire makers is to keep the overall diameter within about 3 percent of the original, and the new tire must match or exceed the original load index and speed rating and clear the bodywork.
Do bigger tires affect the speedometer?
Yes. A tire with a larger diameter covers more ground per turn, so the car travels faster than the speedometer shows. A smaller tire makes the speedometer read high.
What do the numbers on a tire mean?
In 215/60R16, 215 is the width in millimeters, 60 is the sidewall height as a percentage of the width, R means radial construction and 16 is the wheel diameter in inches.
What does plus sizing mean?
Fitting a larger wheel with a lower-profile tire so that the overall diameter stays about the same. The 215/60R16 to 225/55R17 example on this page is a plus-one fitment.
09Estimates and sources
Read this before you rely on it.
Built and checked by CarTipsMentor Team. Last updated . How we check our work.
Sources
- [1]BFGoodrich, Changing Tire Size on a Car
- [2]Les Schwab, Tire Size Calculator: What Size Tires Should You Get?
- [3]National Highway Traffic Safety Administration, Tires (TireWise)