How to use the tire size calculator
- Choose your units: inches and miles per hour, or millimeters and kilometers per hour.
- Type your current size as tire A, exactly as printed on the sidewall, such as 265/70R17. Type the size you are considering as tire B, such as 275/65R18. Inch sizes like 33x12.50R15 work too.
- Enter the speed your speedometer shows. The speedometer is assumed to be calibrated for tire A.
- Read the headline change in overall diameter, the side-by-side table, the speedometer error and the actual speed at several readings.
Formula
Sidewall height = section width x aspect ratio / 100
Overall diameter = rim diameter + 2 x sidewall height (in the same unit)
Circumference = pi x overall diameter; revolutions per mile = 63,360 inches / circumference in inches
Diameter change % = (diameter B / diameter A - 1) x 100
Actual speed = indicated speed x diameter B / diameter A
Millimeters become inches by dividing by 25.4. Revolutions per kilometer use 1,000,000 mm divided by the circumference in millimeters. A flotation size such as 33x12.50R15 gives the overall diameter (33 in) and section width (12.5 in) directly.
Worked example: 265/70R17 versus 275/65R18
Tire A, 265/70R17: the sidewall is 265 x 0.70 = 185.5 mm, or 7.30 in. The overall diameter is 17 + 2 x 7.30315 = 31.61 in, the circumference is 99.29 in and the tire turns 638.1 revolutions per mile.
Tire B, 275/65R18: the sidewall is 275 x 0.65 = 178.75 mm, or 7.04 in. The overall diameter is 18 + 2 x 7.0374 = 32.07 in, the circumference is 100.77 in and it turns 628.8 revolutions per mile. Tire B is 0.47 in taller, a change of +1.48%, and the axle sits about 0.23 in higher.
Speedometer: when the speedometer reads 65 mph, the actual speed is 65 x 32.0748 / 31.6063 = 65.96 mph, so it reads about 0.96 mph low. In metric units, a reading of 100 km/h means an actual 101.48 km/h. The odometer is off by the same 1.48%, so 100 indicated miles are really 101.48 miles.
How to read a tire size
In 265/70R17, the first number is the section width in millimeters, 70 is the aspect ratio (the sidewall height as a percent of the width), R means radial construction and 17 is the wheel diameter in inches. A letter before the width, such as P or LT, marks the tire's intended service type. After the size you may see a load index and a speed rating, such as 112S. This calculator reads those but does not use them, because they do not change the dimensions.
Older and truck-style sizes list the overall diameter first, as in 33x12.50R15: 33 inches tall, 12.5 inches wide, on a 15-inch wheel.
Why a small change in diameter matters
A taller tire covers more ground per revolution. The speedometer counts revolutions, so it understates your speed, and the odometer understates distance. A shorter tire does the opposite. The effect is the same percentage as the diameter change, which is why the table shows both. A commonly used rule of thumb is to stay within about 3% of the original diameter. It is not a standard, so check your vehicle manual or a tire shop about clearance, gearing, braking and stability control, and note that a changed diameter also changes the effective gear ratio.
Wider tires on a different wheel size can leave the overall diameter almost unchanged. That is the usual goal of "plus sizing": a larger rim with a lower-profile tire. Use the diameter change line to check that you stayed close.
Where the numbers can differ from real life
Actual tires vary by brand and model. The size code gives a nominal dimension, while the real width depends on the wheel width, and new tires have more tread than worn ones, which makes the real diameter slightly larger. Inflation and load also change the rolling circumference a little. If you need accurate speed, compare the speedometer with a GPS reading. For fuel costs after a change, the fuel cost calculator and the MPG calculator help you track the effect.
Assumptions and limits
- Dimensions come directly from the size code. Real tires vary by brand, wheel width, tread depth, inflation and load.
- The speedometer is assumed to be calibrated for tire A. If it was never accurate, the result shows the change from that starting point, not the true speed.
- Speed and distance scale with overall diameter. Slip, flex and wear are ignored.
- The size ranges accepted are 105 to 500 mm width, 15 to 95 aspect ratio and 10 to 30 inch wheels for metric sizes, and 15 to 60 inch diameter for inch sizes.
- Fit, clearance, load rating, speed rating, gearing and safety systems are not checked. Confirm with your vehicle manual or a tire professional.
- The 3% figure is a commonly used rule of thumb, not a legal limit or a manufacturer specification.
Frequently asked questions
What do the numbers on a tire mean?
In 265/70R17, 265 is the width in millimeters, 70 is the sidewall height as a percent of that width, R marks radial construction and 17 is the wheel diameter in inches. The sidewall height here is 185.5 mm.
How much does the speedometer change with a bigger tire?
It changes by the same percentage as the overall diameter. Going from a 31.61 inch tire to a 32.07 inch tire is 1.48% taller, so a 65 mph reading is about 65.96 mph in reality.
What is revolutions per mile?
It is how many times the tire turns to cover one mile, which is 63,360 inches divided by the circumference. A 31.61 inch tire turns about 638 times. Tire makers list it, and it is used to set speedometers and gear calculations.
Can I compare a metric size with an inch size?
Yes. Enter 265/75R16 as tire A and 33x12.50R15 as tire B. The calculator converts both to inches and finds that the second is 4.27% taller, which is above the usual 3% rule of thumb.
Is a different tire size safe to use?
The calculator only compares dimensions. Safety depends on load index, speed rating, rim width, clearance, brakes and electronic stability systems, so follow the vehicle placard and ask a tire professional before changing sizes.