Tire Size Calculator: How to Read & Compare Tire Measurements

How to Read a Tire Size

Every tire carries a standardized code on its sidewall. Once you know what each number means, sizing decisions become straightforward. Take the common size 225/65R17 as an example.

Some tires also include a load index and speed rating after the wheel diameter, like 225/65R17 102H. The 102 is the load index (882 lbs per tire) and H is the speed rating (130 mph max).

All the Calculations Derived

From the three numbers in a tire size, you can calculate every meaningful dimension. Here's the math for 225/65R17:

Sidewall Height

Sidewall height (mm) = section width × (aspect ratio ÷ 100)
= 225 × 0.65 = 146.25mm

Overall Diameter

Overall diameter (mm) = (sidewall height × 2) + (wheel diameter × 25.4)
= (146.25 × 2) + (17 × 25.4)
= 292.5 + 431.8 = 724.3mm (28.5 inches)

Circumference

Circumference (mm) = π × overall diameter
= 3.14159 × 724.3 = 2,274.8mm (89.6 inches)

Revolutions Per Mile

Revolutions per mile = 63,360 inches per mile ÷ circumference in inches
= 63,360 ÷ 89.6 = 707 rev/mile

These numbers matter because changing any of them cascades into speedometer error, odometer drift, and powertrain stress. Use our Tire Size Calculator to compare two sizes side by side instantly.

5 Things That Change When You Alter Tire Size

Ranked by safety impact:

1. Speedometer Accuracy

Your vehicle's speedometer and odometer are calibrated to a specific tire circumference. A larger tire covers more ground per revolution, so your speedometer reads lowerthan your actual speed. According to NHTSA tire safety guidelines, a 3% increase in overall diameter produces roughly a 3% low speedometer reading — that's going 61.8 mph when your dash says 60. The same error accumulates in your odometer, affecting service intervals and fuel economy calculations.

2. Fuel Economy

Wider tires have larger contact patches and higher rolling resistance. The Tire Industry Association notes that rolling resistance accounts for 5–15% of a vehicle's total fuel consumption. Switching from a 225/65R17 to a 265/65R17 (40mm wider) can reduce fuel economy by 1–3 mpg on a midsize SUV. For the average driver at 15,000 miles per year, that's $75–$225 in extra fuel annually at current prices. Check our gas mileage calculator to model the real cost.

3. Ground Clearance and Rubbing

A taller tire increases ground clearance, which is great for off-road use but can cause the tire to rub against wheel wells, suspension components, or body panels during full steering lock or suspension compression. According to the rubber and tire market association data, the most common aftermarket fitment complaint is tire rub on SUVs and trucks running oversized all-terrain tires without a lift kit. Always verify physical clearance before purchasing.

4. Effective Gear Ratios

Your vehicle's differential gearing was tuned for a specific tire diameter. A taller tire effectively lowers your gear ratio — the engine spins fewer RPMs at highway speed. This can reduce towing power at low speeds and move your engine's power band out of its optimal range. Trucks used for towing with significantly larger tires often need a re-gear (changing the ring and pinion ratio) to restore performance. See our auto loan guide for how these modifications affect vehicle financing and resale value.

5. Load Capacity

Every tire has a load index rating. Switching to a tire with a lower load index — even one that physically fits — reduces how much weight the tire can safely carry. The Tire Industry Association reports that overloaded tiresare a leading cause of tire failure. If you're towing or hauling, always match or exceed the original load index. The door placard inside your driver's door lists the minimum required tire load rating for your vehicle.

Plus-Sizing Explained

Plus-sizing is the practice of increasing wheel (rim) diameter by one inch (plus-one) or two inches (plus-two) while reducing the tire's aspect ratio to keep the overall diameter nearly identical to the factory spec. The goal: a wider, lower-profile tire on a larger rim without changing speedometer accuracy or clearance.

The trade-offs are real. A lower sidewall means less cushioning between your wheel and the road. Ride quality gets harsher, and low-profile tires are more vulnerable to pothole damage. The wider contact patch improves cornering grip and steering response. Whether that trade is worth it depends entirely on how you use the vehicle.

Common Vehicle Tire Sizes and Metric Equivalents

Tire Safety by the Numbers

The stakes are higher than most drivers realize. NHTSA data shows approximately 11,000 tire-related crashes per year in the United States, resulting in roughly 600 fatalities. The leading contributors are underinflation, overloading, and improper tire fitment. Separately, the global rubber and tire market was valued at over $270 billion in 2024 (Statista), reflecting just how central tires are to the transportation economy.

The Tire Industry Association recommends checking tire pressure monthly — not just by appearance. A tire can lose 25% of its pressure and still look normal to the eye. Use our tire pressure calculator to find the correct PSI for your load and conditions, and our tire reading guide for interpreting the full sidewall markings.

How to Use Our Tire Size Calculator

Our tool takes two tire sizes and computes every relevant dimension for both, then shows you the percentage difference. You can compare:

• Overall diameter (inches and mm)
• Section width
• Sidewall height
• Circumference
• Revolutions per mile
• Speedometer error percentage

Enter your current tire size in the first field and the replacement size in the second. The calculator flags any diameter difference over 3% with a warning — that's the threshold where speedometer error and mechanical stress become concerns worth evaluating seriously.

Frequently Asked Questions

How do I read tire size numbers?

A tire size like 225/65R17means: 225mm wide, 65% aspect ratio (sidewall height is 65% of the width), R indicates radial construction, and 17 is the wheel diameter in inches. The sidewall height equals 225 × 0.65 = 146.25mm. Overall diameter is wheel diameter converted to mm plus two sidewalls.

What happens to my speedometer if I change tire size?

A larger tire has a greater circumference, so each wheel revolution covers more ground. Your speedometer reads low — it thinks you're going slower than you are. A 3% larger tire makes your speedometer read roughly 3% low. At an indicated 60 mph, you're actually traveling about 61.8 mph. NHTSA considers this a safety concern.

What is plus-sizing a tire?

Plus-sizing means increasing wheel diameter by one inch (plus-one) or two inches (plus-two) while reducing the tire's aspect ratio to keep the overall diameter the same. A 225/65R17 can be plus-sized to a 235/55R18. The benefit is a wider contact patch and improved handling; the trade-off is a harsher ride.

Does changing tire size affect fuel economy?

Yes. Wider tires increase rolling resistance and aerodynamic drag, reducing mpg. The Tire Industry Association estimates each 10% increase in rolling resistance reduces fuel economy by roughly 1–2%. A significantly larger tire can cost you 1–3 mpg depending on vehicle type, driving speed, and road conditions.

How many revolutions per mile does a tire make?

Revolutions per mile equals 63,360 (inches per mile) divided by the tire's circumference in inches. A 225/65R17 has a circumference of about 83.6 inches, giving roughly 757 revolutions per mile. A 265/70R17circumference is about 92.9 inches — only 682 revolutions per mile.

Can I put a wider tire on my existing wheels?

Generally yes, within limits. A rim can safely mount tires within roughly 10mm to 20mm of its designed width. Going too wide risks the tire unseating from the bead. Going too narrow causes the tire sidewall to bulge dangerously. Always check your vehicle's door placard and owner's manual for approved tire widths.