How it works
A metric tire size such as 225/45R17 packs three measurements into one code. The first number is the section width in millimeters. The second is the aspect ratio: the sidewall height as a percentage of the width. The “R” means radial construction, and the last number is the wheel (rim) diameter in inches.
Because the width is metric and the rim is in inches, the calculator converts millimeters to inches by dividing by 25.4 (exactly 25.4 mm per inch).
Sidewall height (in) = width × aspect ÷ 100 ÷ 25.4
Overall diameter (in) = 2 × sidewall + rim diameter
Circumference = π × diameter Revolutions per mile = 63,360 ÷ circumference
63,360 is the number of inches in a mile. The sidewall is counted twice because there is rubber above and below the wheel.
Percent difference and speedometer error
Your speedometer counts wheel rotations and assumes the original tire’s circumference. A taller tire covers more ground per turn, so the car goes faster than the dial shows.
Difference % = (new diameter ÷ old diameter − 1) × 100
Actual speed = indicated speed × new diameter ÷ old diameter
The odometer is off by the same percentage, so mileage logs, fuel economy readings and warranty mileage drift by that amount too.
Methodology notes
- These are nominal, unloaded dimensions calculated from the size code. Actual tires vary by brand and tread depth, and a loaded tire’s rolling radius is slightly smaller.
- Results are for P-metric and Euro-metric sizes. Flotation sizes such as 33x12.50R15 already state the diameter in inches.
- Check fitment, load index and speed rating with a tire professional before changing sizes.
Worked examples
Swapping 225/45R17 for 225/50R17 at 60 mph
The original tire has a 3.99-inch sidewall and a 24.97-inch diameter; the new one is 4.43 and 25.86 inches. That is 3.55% taller. Revolutions per mile fall from 807.6 to 779.9. When the speedometer shows 60 mph you are really doing 62.1 mph, so it reads 2.1 mph slow.
Lifted truck: 265/70R17 to 285/75R17 at 65 mph
Diameter grows from 31.61 to 33.83 inches (2.22 inches, or 7.04%), and the tread is 0.79 inches wider. At an indicated 65 mph the truck is actually traveling 69.6 mph. Many owners recalibrate the speedometer after a change this large.
Plus-one wheel upgrade: 205/55R16 to 225/45R17 at 70 mph
Moving to a 17-inch wheel with a lower-profile tire keeps the overall size close: 24.88 versus 24.97 inches, a difference of 0.38%. At 70 mph indicated the actual speed is 70.3 mph, which is within normal tolerance.
How much tire size difference is safe?
A common industry rule of thumb is to stay within about 3% of the original overall diameter; Tire Rack, for example, says staying within a 3% diameter change is desirable for cars and vans. Check the tire placard on the driver’s door jamb and your owner’s manual for the original size. Inside that range the speedometer error is small, the tire is likely to clear the suspension and body, and systems such as anti-lock brakes, traction control and the transmission’s shift logic, which rely on wheel-speed sensors, behave as designed. Larger changes are possible, especially on trucks, but they may need a lift, trimming, regearing or a speedometer recalibration. Always confirm the new tire’s load index meets or exceeds the figure on your door placard.
Effects of going bigger or smaller
- Taller tires raise ground clearance and lower engine speed at cruise, but they slow acceleration, can reduce braking performance and usually cost fuel economy, especially if they are heavier or more aggressive.
- Shorter tires sharpen acceleration and make the speedometer read high, which is safer for tickets but makes the odometer add miles faster.
- Wider tires can add grip in the dry but may hydroplane sooner, ride louder and rub at full lock.
- Lower-profile sidewalls sharpen steering response but ride firmer and leave wheels more exposed to pothole damage.
Reading the rest of the sidewall
After the size you will usually see a load index and speed rating, such as 94V. The number maps to the maximum load each tire can carry, and the letter is the maximum sustained speed. You will also find the DOT code, whose last four digits give the week and year of manufacture, and the UTQG ratings for treadwear, traction and temperature. Because the odometer error affects recorded miles, a tire change can also change how quickly you reach service intervals, which shifts the per-mile figures in our ownership cost calculator and the MPG your trip computer reports.
Frequently asked questions
What do the numbers on a tire mean?
In 225/45R17, 225 is the width in millimeters, 45 is the sidewall height as a percentage of the width, R means radial, and 17 is the wheel diameter in inches.
How do I calculate tire diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall in millimeters, divide by 25.4 to convert to inches, double it, and add the rim diameter. A 225/45R17 is about 24.97 inches tall.
How much will bigger tires change my speedometer?
By the same percentage as the diameter changes. A tire 3.5% taller makes the car travel about 62.1 mph when the speedometer shows 60.
What is the maximum tire size difference I should use?
A widely used industry rule of thumb, cited by retailers such as Tire Rack, is to stay within about 3% of the original diameter on cars and vans. Check your door placard and owner’s manual, plus clearance and load rating.
Do bigger tires hurt gas mileage?
Usually. Larger, heavier and more aggressive tires add rotating mass and rolling resistance. Also, because the odometer under-reads, a calculated MPG will look worse than it really is unless you correct for the size change.
What are revolutions per mile?
The number of times the tire turns to cover one mile: 63,360 inches divided by the circumference in inches. Taller tires have fewer revolutions per mile.
Can I put a different size tire on the same wheel?
Often, as long as the rim diameter matches and the wheel width is within the range the tire maker approves for that tire width. A tire shop can confirm the approved rim width range.