Tire Size Calculator
This tire size calculator turns a tire size such as 225/65R17 into real dimensions: overall diameter, sidewall height, width, circumference, and revolutions per mile. Enter a second size to compare them, and the tire size calculator shows the diameter difference, the change in ride height, and what your speedometer will read when you switch.
Results
The Tire Size Calculator is part of our Tires & Wheels collection. Browse more tires & wheels to size, convert, and check your work alongside this tool.
What the tire size calculator does
A metric tire size gives the width in millimeters, the sidewall height as a percentage of that width, and the wheel diameter in inches. The tire size calculator works out the sidewall in millimeters and inches and adds two sidewalls to the wheel for the overall diameter. With a second size, it compares the two, since a taller tire makes the speedometer read low and changes the gearing.
How to read tire size and calculate diameter
For a comparison, the actual speed at an indicated speed is Indicated × New diameter ÷ Old diameter. Real tires vary a little with brand, tread depth, and load.
Tire size calculator example
Switching from 225/65R17 to 235/60R18:
The new tire is 2.06% taller, the truck sits about 0.29 in higher, and at an indicated 60 mph you are doing about 61.2 mph.
How to use the tire size comparison
Read the size from the tire sidewall: in 225/65R17, 225 is the width, 65 is the aspect ratio, and 17 is the rim. Enter those three numbers. To compare, enter the new size in the optional fields; leave them at zero to see a single tire. Press Calculate.
Reading a tire size difference
Most shops recommend staying within about 3% of the original diameter. Beyond that, the speedometer, odometer, ABS, stability control, and automatic transmission shift points can all be affected. A taller tire also lowers the effective gearing, which can reduce acceleration. Check clearance at full steering lock and suspension compression before fitting a larger size.
When to use a tire diameter calculator
- Upsizing wheels while keeping the same diameter
- Fitting taller tires for off-road use
- Correcting a speedometer after a tire change
- Comparing winter and summer tire sizes
- Converting metric sizes to inches
Common tire size calculator mistakes
- Reading the aspect as millimeters. It is a percentage of the width.
- Mixing wheel and tire diameter. The rim number is the wheel.
- Ignoring load index. The new tire must carry the vehicle weight.
- Skipping clearance checks. Turn and compress before buying.
Tips for a tire size comparison
- Keep within 3% of the original diameter.
- Match load and speed ratings.
- Recalibrate the speedometer after a big change.
Speedometer calculator tire size check
The speedometer counts wheel turns, so it assumes the original diameter. With a 2.06% taller tire, every reading is 2.06% low, and the odometer under-counts the same amount. To check on the road, time a measured distance and work out the true speed with the speed distance time calculator. Many cars can be recalibrated by a dealer or a programmer.
Tire size converter: plus sizing
Plus sizing fits a larger wheel with a lower profile tire so the diameter stays the same. Going from 17 to 18 in wheels, an aspect ratio about 5 points lower usually keeps you within 1 to 2%. The tire circumference, π times the diameter, is what the speedometer really reads, and the circle calculator converts between the two. Grip and braking also change with tire size and compound; the stopping distance calculator shows how much distance a change in grip adds.
Tire size chart: diameter and sidewall height
| Size | Overall diameter | Sidewall height | Revs per mile |
|---|---|---|---|
| 205/55R16 | 24.88 in | 4.44 in | 811 |
| 225/65R17 | 28.52 in | 5.76 in | 707 |
| 235/60R18 | 29.10 in | 5.55 in | 693 |
| 245/45R19 | 27.68 in | 4.34 in | 729 |
| 265/70R17 | 31.61 in | 7.30 in | 638 |
| 275/55R20 | 31.91 in | 5.95 in | 632 |
| 285/75R16 | 32.83 in | 8.42 in | 614 |
Tire Size Calculator FAQ
How do you read a tire size?
In 225/65R17, 225 mm is the width, 65 means the sidewall is 65% of the width, R means radial, and 17 is the wheel diameter in inches.
How tall is a 225/65R17 tire?
About 28.52 in (724 mm) overall.
How do bigger tires affect the speedometer?
A taller tire covers more ground per turn, so you go faster than the speedometer shows. A 2.06% taller tire reads 60 at about 61.2 mph.
How much can tire diameter change?
Stay within about 3% of the original to avoid speedometer and safety system problems.