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Auto & Vehicle
Tire Size Calculator — Diameter, Sidewall & Speedometer Error
Decode the numbers on your sidewall and see what a new size really changes
Every tire sold in the United States carries a code molded into the sidewall — something like 225/45R17 — and most drivers have no idea what those numbers mean until they're staring at a tire shop quote or trying to plus-size their wheels. This tire size calculator turns that code into the dimensions that actually matter: the sidewall height, the overall diameter of the mounted tire, the circumference (how far the tire rolls in one turn), and the revolutions per mile. Then it compares any two sizes side by side so you can see exactly how a new tire changes your ride height, your clearance, and — most importantly — your speedometer reading.
The code reads in three parts. In 225/45R17, the 225 is the section width in millimeters (the widest point of the tire, sidewall to sidewall). The 45 is the aspect ratio — the sidewall height as a percentage of that width. The R means radial construction (almost every modern tire), and the 17 is the wheel (rim) diameter in inches that the tire is built to fit.
The math is direct. Sidewall height = width × aspect ÷ 100 = 225 × 45 ÷ 100 = 101.25 mm, which is 101.25 ÷ 25.4 = 3.99 in. Because a tire has a sidewall on the top and the bottom, overall diameter = rim + 2 × sidewall = 17 + 2 × 3.99 = 24.97 in. Multiply by π for circumference (about 78.4 in) and divide 63,360 inches per mile by that number to get revolutions per mile.
The reason this matters for everyday drivers is the speedometer. Your car's speedometer and odometer are calibrated for the factory tire's diameter. Fit a taller tire and each wheel revolution covers more ground, so you're actually going faster than the dash shows — your odometer also under-counts the miles. Fit a shorter tire and the opposite happens. The speedometer error is simply the percent difference in diameter between the two tires: if the new tire is 3% larger, your true speed is about 3% higher than indicated, and an indicated 60 mph is really about 61.8 mph.
A common rule of thumb among enthusiasts and off-roaders is to keep a replacement or plus-size tire within about 3% of the original diameter so the speedometer stays close, the gearing and ABS behave, and the tire still clears the fenders and suspension. Bigger jumps are done all the time for lifted trucks and off-road builds, but they usually call for a speedometer recalibration. Use this calculator to decode your current tire, test a new size before you buy, and check the diameter difference and speedometer error so there are no surprises in the driveway.
📖 See also: How to Calculate Your Car Payment
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• Sidewall height (mm) = section width × (aspect ÷ 100) • Sidewall (in) = sidewall (mm) ÷ 25.4 • Overall diameter (in) = rim diameter + 2 × sidewall (in) • Circumference (in) = diameter × π • Revolutions per mile = 63,360 ÷ circumference (in) • Speedometer error (%) = ((new diameter − old diameter) ÷ old diameter) × 100
📰 Formula
• Sidewall height (mm) = section width × (aspect ÷ 100) • Sidewall (in) = sidewall (mm) ÷ 25.4 • Overall diameter (in) = rim diameter + 2 × sidewall (in) • Circumference (in) = diameter × π • Revolutions per mile = 63,360 ÷ circumference (in) • Speedometer error (%) = ((new diameter − old diameter) ÷ old diameter) × 100
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Reading the aspect ratio as inches — it's a percentage of the width, not a measurement.
- Forgetting to count both sidewalls (multiply sidewall by 2 before adding to the rim).
- Comparing tires on different rim diameters without checking that the wheel actually fits.
- Assuming any tire within 3% will clear — width and offset also affect fender and suspension clearance.
- Mixing metric width (mm) with the inch rim size in one step — convert sidewall to inches first.
💡 Tips
- Keep a replacement or plus-size tire within about 3% of the stock diameter to avoid speedometer and gearing issues.
- A taller tire makes your speedometer read low (you're going faster than it shows) and your odometer under-count miles.
- When plus-sizing, drop the aspect ratio as you raise the rim size to keep overall diameter roughly the same.
- Check the load index and speed rating too — diameter is only part of a safe fitment.
- Larger overall diameter raises ride height by half the diameter increase (the tire grows top and bottom).
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❓ Frequently asked questions
What do the numbers on a tire mean, like 225/45R17?
225 is the section width in millimeters, 45 is the aspect ratio (sidewall height as a percent of width), R means radial, and 17 is the wheel diameter in inches. So a 225/45R17 has a 225 mm wide tread, a sidewall about 101 mm (4 in) tall, on a 17-inch wheel.
How do I calculate the overall diameter of a tire?
Find the sidewall height (width × aspect ÷ 100, then ÷ 25.4 for inches), double it, and add the rim diameter. For 225/45R17: 225 × 0.45 = 101.25 mm = 3.99 in, so diameter = 17 + 2 × 3.99 = 24.97 in.
How much can I change tire size without recalibrating the speedometer?
A widely used rule of thumb is to stay within about 3% of the original tire's overall diameter. Inside that range the speedometer error is small. Larger changes are common on lifted trucks but usually need a speedometer recalibration.
Does a bigger tire make my speedometer read high or low?
A larger-diameter tire makes your speedometer read low — you're actually going faster than the dash shows — because each revolution covers more ground. A smaller tire makes it read high. The error roughly equals the percent change in diameter.
How do I figure out my speedometer error after changing tires?
Speedometer error (%) = (new diameter − old diameter) ÷ old diameter × 100. If the new tire is 3% larger and your speedometer shows 60 mph, your true speed is about 60 × 1.03 = 61.8 mph.
What is plus-sizing a tire?
Plus-sizing means fitting a larger-diameter wheel with a lower-profile tire while keeping the overall diameter about the same. For example, going from 225/45R17 to a 245/40R18 keeps the rolling diameter close so handling improves without throwing off the speedometer.
Will a taller tire affect my gas mileage reading?
Indirectly, yes. A taller tire makes the odometer under-count miles, so calculated MPG looks lower than reality until you correct the distance. Pair this with a gas mileage or fuel cost calculator and adjust your miles by the diameter difference.
How many revolutions per mile does my tire make?
Revolutions per mile = 63,360 (inches in a mile) ÷ tire circumference in inches, where circumference = diameter × π. A 24.97-inch tire turns about 808 times per mile.
Can I put a wider tire on the same wheel?
Often a little wider, but each wheel width has a range of tire widths it can safely take, and a wider tire also needs fender and suspension clearance. This calculator compares diameter; always confirm width, load index, and clearance for your specific vehicle.