Tire Size Calculator
Quick Answer: Enter your current tire size and the size you are considering, for example 205/55R16 versus 225/45R17. The calculator compares diameter, sidewall, circumference and revs per mile, and shows the speedometer error. Keep the diameter change within 3 percent of stock for a safe fit.
Compare Two Tire Sizes
From the door jamb placard, e.g. 205/55R16.
The size you are considering, e.g. 225/45R17 or 33x12.5R15.
Within the usual 3% rule. This size is a close match for your original tire.
| Measurement | 205/55R16 | 225/45R17 | Difference |
|---|---|---|---|
| Overall diameter | |||
| Section width | |||
| Sidewall height | |||
| Rim diameter | |||
| Circumference | |||
| Revs per mile | |||
| Ground clearance change | -- | ||
Speedometer: what you see vs your real speed
When your speedometer reads 60 mph, your actual speed is 60.2 mph.
| Speedometer | 20 mph | 30 mph | 40 mph | 50 mph | 60 mph | 70 mph | 80 mph |
|---|---|---|---|---|---|---|---|
| Actual speed |
Your odometer is off by the same percentage as the speedometer.
Plus sizing ideas within 3% of your current tire
Enter a valid current tire size to see suggestions.
All math runs in your browser. Nothing you enter is sent or saved anywhere. Calculated dimensions assume every tire is made to the exact same specs. Real tires vary slightly by manufacturer, and revs per mile under load run about 3% higher than the unloaded number shown here.
How to Read a Tire Size
Every tire carries its size on the sidewall in a code that looks confusing until you know the parts. Take 205/55R16, one of the most common sizes on compact cars. The first number, 205, is the section width in millimeters, measured from one sidewall to the other at the widest point. The second number, 55, is the aspect ratio. It means the sidewall height is 55 percent of the width, so this tire has a sidewall about 113 millimeters tall. The letter R means radial construction, which almost every passenger car tire uses today. The last number, 16, is the rim diameter in inches, so this tire only fits a 16 inch wheel.
Letters before the numbers add more detail. P means passenger car, the most common prefix in the US. LT means light truck, for pickups and larger SUVs that carry heavy loads and tow trailers. ST means special trailer, and T means temporary, for spare tires not meant for everyday driving. After the rim size you may see a load index and a speed rating, for example 91V: 91 means the tire can carry 1,356 pounds, and V means a maximum speed of 149 mph. Some trucks use flotation sizes instead: 33x12.5R15 means 33 inches tall, 12.5 inches wide, radial, on a 15 inch rim. The calculator accepts both formats.
How Tire Size Is Calculated
The whole comparison comes from three numbers and two simple formulas. First, the sidewall height is the section width times the aspect ratio. For a 205/55R16 tire, that is 205 times 0.55, which is 112.75 millimeters, or 4.44 inches. Second, the overall diameter is the rim diameter plus twice the sidewall height, once for the top of the tire and once for the bottom. That is 16 plus 2 times 4.44, which is 24.88 inches. The circumference is the diameter times pi, about 78.16 inches, and the revolutions per mile are 63,360 inches in a mile divided by the circumference, about 811 turns per mile.
Try the worked example tire shops use: a 285/75R16 tire. The sidewall is 285 times 0.75, which is 213.75 millimeters, or 8.42 inches. Double it and add the 16 inch rim, and the overall diameter is 32.83 inches. That is why a 285/75R16 is often sold as a 33 inch tire. One warning belongs with every calculated number: the math assumes every tire is made to the exact same specs, but real tires vary slightly by manufacturer, and revs per mile under load run about 3 percent higher than the unloaded number, because the tire squashes where it meets the road.
The 3 Percent Rule
The 3 percent rule is the line most tire shops and car makers use. A replacement tire whose overall diameter is within 3 percent of the original size is generally accepted as safe. On a 25 inch tire, 3 percent is about 0.75 inches. Inside that band, the speedometer error stays small, the tire usually clears the wheel well, and the ABS, traction control and stability control systems keep working as designed, because they expect the wheels to turn at close to the factory rate.
Go past 3 percent and the problems stack up. The speedometer and odometer both read wrong by the same percentage, so you can be speeding without knowing it. A taller tire can rub the fender liners or suspension parts at full steering lock or when the suspension compresses over a bump, and a much wider tire can rub even if the diameter is right. On all wheel drive cars, a big difference between the front and rear tire diameters can stress the drivetrain, which is why those cars need all four tires matched closely. The calculator flags any change over 3 percent.
How Tire Size Affects Your Speedometer
A speedometer does not measure speed directly. It counts how fast the wheels turn and multiplies by the circumference of the factory tire. Change the tire, and the same number of turns covers a different distance, so the reading goes wrong. A taller tire travels farther per turn, so the car goes faster than the speedometer says. A shorter tire travels less per turn, so the car goes slower than the speedometer says. The error is the same percentage at every speed: if the error is 3 percent, the real speed is 3 percent higher or lower at 20 mph and at 80 mph alike.
Use the slider to see it for your sizes. With the default comparison, 205/55R16 versus 225/45R17, the diameter change is only about 0.4 percent, so 60 mph on the dial is about 60.2 mph in reality. Change to a tire 3 percent taller and 60 mph on the dial becomes 61.8 mph, which is enough to matter in a speed camera zone. Change to a tire 5 percent taller and 60 mph becomes 63 mph.
What Is Plus Sizing?
Plus sizing is the usual way to fit bigger wheels without changing the overall diameter much. Plus one means a rim one inch larger than stock, plus two means two inches larger, and the tire gets a lower aspect ratio to compensate. Going from a 205/55R16 to a 225/45R17 is a classic plus one: the rim grows by an inch, the sidewall gets shorter, and the overall diameter stays within half a percent. The calculator lists plus sizing ideas for your current tire, showing sizes on rims from one inch smaller to two inches larger that keep the diameter within 3 percent, closest first.
The appeal is real. A wider tire with a shorter sidewall usually gives sharper steering response and more grip in dry corners, and larger wheels fill the wheel arches better. The costs are real too. A shorter sidewall means a firmer ride, because there is less rubber to absorb bumps, and the tire is more likely to be damaged by a pothole. Larger wheels and tires cost more to buy and replace, and a wider tire often adds rolling resistance, so fuel economy drops a little.
What Else Changes With Tire Size
Tire size touches more than the speedometer. Ground clearance moves by half the diameter change, so a tire 1 inch taller lifts the car by half an inch, and a tire 1 inch shorter drops it by half an inch. Fuel economy usually drops a little with wider or taller tires, because a wider tire has more rolling resistance and a taller tire changes the effective gearing. The effect is usually 1 to 2 MPG, but over 12,000 miles a year that is real money.
Finally, remember the practical checks that no calculator can do for you. Check clearance at full steering lock in both directions and with the suspension compressed, not just with the car sitting still. Check that the tire does not touch the brake lines, the suspension arms or the fender liners. Check the load index on the new tire: it must carry at least as much weight as the factory tire, especially on trucks and SUVs that tow.
Frequently Asked Questions
How do I read a tire size like 205/55R16?
205 is the section width in millimeters, 55 is the aspect ratio, meaning the sidewall height is 55 percent of the width, R means radial construction, and 16 is the rim diameter in inches. A P at the start means passenger car, and LT means light truck.
What is the 3 percent rule for tire size?
Most tire shops and car makers accept a replacement tire whose overall diameter is within 3 percent of the original size. Inside that band the speedometer error stays small and the tire usually clears the wheel well. Beyond 3 percent, expect a bigger speedometer error, possible rubbing, and effects on ABS and stability control.
How does a bigger tire affect my speedometer?
A taller tire travels farther per turn, so the car goes faster than the speedometer says. A tire 3 percent taller makes the speedometer read about 3 percent low, so 60 mph on the dial is about 61.8 mph in reality. A shorter tire does the opposite and makes the speedometer read high.
Can I put a different tire size on the same rims?
Yes, within limits. The rim diameter in the size must match your wheels, so a 16 inch tire only fits 16 inch rims. Width and aspect ratio can change as long as the overall diameter stays within about 3 percent of stock and the tire clears the suspension and fenders at full lock.
What is plus sizing?
Plus sizing means fitting a larger rim with a lower profile tire so the overall diameter stays about the same. Plus one is one inch larger, plus two is two inches larger. A wider tire and a shorter sidewall usually sharpen handling, but the ride gets firmer and the wheels cost more.
Does changing tire size affect my odometer?
Yes, by the same percentage as the speedometer. A tire 3 percent taller makes the odometer read about 3 percent low, so it records fewer miles than you actually drive. A shorter tire makes it read high. That also affects calculated MPG if you use odometer miles.
Will a wider tire hurt my MPG?
Usually a little. A wider tire has more rolling resistance and more aerodynamic drag, so fuel economy often drops by a small amount. A taller tire can also change the effective gearing. The effect is usually 1 to 2 MPG at most, but it adds up over a year of driving.
Where do I find my stock tire size?
Check the placard on the driver side door jamb, which lists the factory size and the correct tire pressure, and check the sidewall of the tires already on the car. The owner manual lists it too. Use the door jamb size as the original size in this calculator, not a size a previous owner fitted.
Reviewed by the AutoFixr Editorial Team. Tire dimensions are calculated from the size code and assume every tire is made to the exact same specs. Real tires vary slightly by manufacturer. Always confirm clearance and load rating on your car before buying, and follow your owner manual.