This tire size calculator compares the tire on your car with a size you are thinking about buying. It reads metric, LT and flotation sizes and returns the diameter difference, your true speed at any speedometer reading, revs per mile, and how the sidewall and ride height change.
Below the tool you will find what each input and result means, how the size check works, a worked plus-sizing example and the mistakes that cause rubbing or a wrong speedometer.
Tire Size Calculator
Type the size on your car now and the size you want, in metric (225/45R17) or flotation (33×12.50R15) format, then press Compare Tires.
Dimensions come from the size code for an unloaded tire on its measuring rim. Real tires vary by brand, tread and rim width, and a loaded tire rolls on a slightly smaller radius. Use the tire maker’s published diameter and revs per mile for exact figures.
What a Tire Size Calculator Does
A tire size code such as 225/45R17 is a compact recipe for the tire’s shape. It does not state the overall height, so two sizes are hard to compare by eye. A tire size calculator turns each code into real dimensions and lines them up, so you can see whether a new size is taller, shorter, wider or the same.
The comparison matters because almost everything a size change affects follows from the change in overall diameter: the speedometer, the odometer, the effective gearing and how high the vehicle sits.
Why Compare Before You Buy Tires
Tires are expensive and hard to return once mounted. A size that looks close on paper can still rub a fender liner, confuse a speedometer or sit outside what the tire shop is willing to install. Checking the numbers first tells you which questions to ask and which sizes to drop from the list.
It also helps when your original size is hard to find. Comparing alternatives shows which one keeps the overall diameter closest to stock.
Modern vehicles add another reason. Many use wheel speed for ABS, traction and stability control, transmission shifting and 4WD logic. These systems were set up for the factory size, so a large mismatch can change how they behave. Knowing the exact percentage before you buy tells you whether the vehicle will need its tire size setting updated, which a dealer or a suitable programmer can usually do.
Inputs Used in This Calculator
Stock Tire Size
Use the size printed on the tire placard, usually on the driver door jamb, or on the sidewall if the tires are original. In P225/45R17, P means passenger tire, 225 is the section width in millimeters, 45 is the aspect ratio (the sidewall is 45% of the width), R means radial construction and 17 is the wheel diameter in inches. LT marks a light truck tire. A size with no letter is usually Euro-metric. The load index and speed rating that follow, such as 94V, do not change the dimensions, so the calculator ignores them.
New Tire Size
Enter the size you are considering in the same way. Flotation sizes such as 33×12.50R15 are already in inches: about 33 inches tall, 12.50 inches wide, for a 15 inch wheel. The calculator works out their sidewall from the height and wheel size.
Speedometer Reading
Enter the speed you usually see on the dash, such as 65 mph on the highway. The calculator returns the real road speed at that reading with the new tires, assuming the speedometer was accurate on the stock size.
Show Sizes In
Switch between inches and millimeters for every length on the page. Speeds stay in mph.
How the Calculator Works
For a metric size, every dimension follows from three numbers:
- Sidewall (in) = width (mm) × aspect ratio ÷ 100 ÷ 25.4
- Diameter (in) = wheel diameter + 2 × sidewall
- Circumference (in) = π × diameter
- Revs per mile = 63,360 ÷ circumference, because a mile is 63,360 inches
The percent difference is new diameter ÷ stock diameter − 1. True speed is the speedometer reading multiplied by new diameter ÷ stock diameter, and the ride height change is half of the diameter change, because the axle sits at the tire’s center.
| Step | 225/45R17 | 235/40R18 |
|---|---|---|
| Sidewall | 3.99 in | 3.70 in |
| Diameter | 24.97 in | 25.40 in |
| Circumference | 78.45 in | 79.80 in |
| Revs per mile | 807.6 | 794.0 |
What the Results Tell You
Overall Diameter Difference
The headline is the percent change in height. A positive number means the new tire is taller. The line under it gives the same change in inches or millimeters.
New Tire Diameter
The calculated overall height of the new tire, unloaded and new. Compare it with the maker’s published figure before you buy.
True Speed at Your Reading
A taller tire covers more ground per turn, so the speedometer reads low. At 65 mph shown on a tire that is 1.7% taller, you are really doing about 66.1 mph.
Real Miles per 1,000 Logged
The odometer has the same error as the speedometer. This card shows how many miles you actually drive while the odometer adds 1,000. It matters for service intervals, warranty mileage and fuel economy math.
Revs per Mile
How many times each tire turns per mile, stock and new. It is the figure used to recalibrate speedometers and work out gearing.
Sidewall and Ride Height Change
Sidewall change tells you how much cushion you gain or lose, which affects ride comfort and pothole protection. Ride height change is how much the axle, and everything bolted to it, rises or falls.
The Scale Drawing and the Size Check
The drawing shows both tires side by side on the same ground line, at the same scale. The red dashed line marks the top of the taller tire, so you can see the height gap and how the wheel size and sidewall change.
The size check box sorts the result into three bands. “Close match” means the diameters are within 1.5%. “Inside the 3% guideline” means 1.5% to 3%, the range many tire shops use as a rule of thumb for a substitute size. “Beyond the 3% guideline” means more than 3%, where a speedometer fix and a careful clearance check are usually needed. The 3% figure is a common rule of thumb, not a law or an industry standard. Extra notes appear if the wheel diameter changes or the new tire is much wider.
How to Compare Two Sizes Step by Step
- Type the stock size from the placard or sidewall.
- Type the size you are considering.
- Enter the speedometer reading you drive at most often.
- Choose inches or millimeters.
- Press Compare Tires and read the headline, the drawing and the size check.
- Open the side by side table for width, circumference and the exact revs per mile.
Example: Plus Sizing a Compact Car
Plus sizing means a bigger wheel with a lower-profile tire, keeping the overall diameter close to stock. Each extra inch of wheel is one “plus.” Starting from 205/55R16:
| Fitment | Size | Diameter |
|---|---|---|
| Stock | 205/55R16 | 24.88 in |
| Plus one | 215/45R17 | 24.62 in |
| Plus two | 225/40R18 | 25.09 in |
| Plus three | 225/35R19 | 25.20 in |
All three stay within about 1.3% of stock, so each one lands in the “Close match” band. The trade is the shorter sidewall: sharper steering and a firmer ride, more risk of wheel damage from potholes and usually a higher price per tire.
Trucks often go the other way. Moving from 265/70R17 (31.61 in) to a 35×12.50R17 is about 10.7% taller. At 60 mph shown, the truck is really doing about 66.4 mph, and the axle sits about 1.7 inches higher. That is far beyond the guideline and normally needs a lift or leveling kit, wheels with suitable backspacing and speedometer recalibration.
Who Should Use a Tire Size Calculator
Drivers Changing Wheels
Anyone moving to a bigger or smaller wheel can find the tire that keeps the diameter close to stock.
Truck and SUV Owners Going Bigger
The ride height and true speed figures show what a taller tire will do before you commit to a lift.
Shoppers Replacing a Hard-to-Find Size
If your size is out of stock or discontinued, compare alternatives and pick the closest one that meets the placard’s load requirement.
Common Mistakes and Tips
- Judging by width alone. A 245 is not automatically bigger than a 235. Aspect ratio and wheel size decide height.
- Trusting nicknames. A 285/75R16 is often called a 33, but it calculates to about 32.8 inches.
- Forgetting load capacity. The new tire must carry at least the load on the placard at the pressure you run. When a P-metric tire is used on a light truck, SUV or van, its load capacity is divided by 1.10.
- Mixing sizes on AWD. Most AWD and 4WD makers require all four tires to match in size and be close in tread depth.
- Ignoring tread wear. A tire that wears from 10/32 to 2/32 inch loses about half an inch of height, so a new tire next to worn ones is taller than the numbers suggest.
- Using the code as gospel. Real tires vary by brand and tread, so use the maker’s published diameter for close calls.
Related Car Calculators
- Tire Height Calculator
- Wheel Offset Calculator
- Speedometer Gear Calculator
- RPM Gear Ratio Calculator
- Gear Ratio Speed Calculator
- Fuel Efficiency Calculator
- Car Repair Cost Calculator
Frequently Asked Questions
How do I work out the diameter of a metric tire?
Multiply the width in millimeters by the aspect ratio, divide by 100 and then by 25.4 to get the sidewall in inches. Double it and add the wheel diameter. A 225/45R17 is about 24.97 inches tall.
What is the 3 percent rule for tire sizes?
Many tire shops treat a substitute size within about 3 percent of the original diameter as reasonable. It is a rule of thumb, not a legal standard, and it does not replace checking load capacity, speed rating and clearance.
Will a taller tire make my speedometer wrong?
Yes. The speedometer reads low by roughly the percent the diameter grew. On a tire 3 percent taller, 60 mph on the dash is about 61.8 mph on the road.
Does a bigger tire change my odometer?
Yes. The odometer counts the same rotations as the speedometer, so it logs fewer miles than you drive on a taller tire and more on a shorter one.
How do I read a size like 33×12.50R15?
It is a flotation size in inches: about 33 inches tall, 12.50 inches wide, radial construction, for a 15 inch wheel.
Are P-metric and Euro-metric sizes the same size?
The dimensions calculate the same way, but they follow different load and inflation standards, so the load a tire carries at a given pressure can differ.
How much does a taller tire raise my vehicle?
Half of the diameter increase. A tire 1 inch taller raises the axle and everything on it about half an inch.
Why does my tire measure smaller than the calculator says?
The calculator gives the unloaded design size. The weight of the car flattens the bottom of the tire, worn tread reduces height, and brands differ slightly.