Tire Size Comparison Calculator
Compare two tire sizes side by side. Enter the width, aspect ratio and rim diameter of your current and proposed tires to see the change in overall diameter, circumference and speedometer accuracy.
Compare Two Tire Sizes
Tire Size Comparison Formula
Where:
- \(W\) — section width of the tire (mm), the first number in the size code
- \(A\) — aspect ratio (%), the second number in the size code
- \(D\) — rim diameter (inches), the last number in the size code
- \(h\) — sidewall height (mm)
- \(OD\) — overall diameter of the inflated tire (mm)
- \(C\) — rolling circumference (mm)
- \(\Delta\%\) — percentage difference between the two overall diameters
The sidewall height is the aspect ratio expressed as a percentage of the section width. The overall diameter is the rim diameter (converted from inches to millimeters) plus two sidewall heights, one on each side of the wheel.
Tire Size Comparison – Calculation Example
Example: comparing a current 225/60R16 tire with a proposed 245/60R17 tire.
Sidewall heights:
\[ h_1 = \frac{225 \times 60}{100} = 135 \text{ mm} \qquad h_2 = \frac{245 \times 60}{100} = 147 \text{ mm} \]
Overall diameters:
\[ OD_1 = 25.4 \times 16 + 2 \times 135 = 406.4 + 270 = 676.4 \text{ mm} \]
\[ OD_2 = 25.4 \times 17 + 2 \times 147 = 431.8 + 294 = 725.8 \text{ mm} \]
Size difference:
\[ \Delta = 725.8 - 676.4 = 49.4 \text{ mm} \qquad \Delta\% = \frac{49.4}{676.4} \times 100 \approx 7.30\% \]
Result: the proposed tire is 49.4 mm (7.30%) larger in diameter — a difference large enough to noticeably affect speedometer readings and gearing.
How to Use the Calculator
- Read the size code off your current tire sidewall, for example 225/60R16.
- Enter 225 in the "Section width" field under Current Tire, 60 in "Aspect ratio", and 16 in "Rim diameter".
- Enter the three numbers of the tire you are considering in the same three fields under Proposed Tire.
- Read the results: sidewall height, overall diameter and circumference for both tires, plus the diameter change in mm and as a percentage.
- Check the "Size difference" value — a result within ±3% of zero is generally considered an acceptable match for the original equipment size.
What Tire Size Comparison Means
A tire size code such as 225/60R16 contains everything needed to compute the tire's physical dimensions. The first number (225) is the section width in millimeters — the widest point of the tire from sidewall to sidewall. The second number (60) is the aspect ratio: the sidewall height as a percentage of the width, so the sidewall is 60% of 225 mm, or 135 mm. The final number (16) is the rim diameter in inches. An R simply means radial construction.
When you change wheel or tire sizes — for example moving to larger rims — the goal is usually to keep the overall diameter close to the original. That is because the overall diameter determines how far the tire travels in one revolution, which in turn drives your speedometer, odometer, gearing and even ABS and traction-control calibration. A larger-diameter tire travels farther per revolution, so your speedometer will read slower than your actual speed; a smaller tire has the opposite effect.
The percentage difference shown by this calculator tells you how far the proposed size deviates from the original. As a rule of thumb, a deviation within about 3% is considered safe and keeps speedometer error minor. Beyond that, the error becomes noticeable: with the default example of a 7.30% increase, a speedometer showing 60 mph corresponds to a true speed of roughly 64 mph.
Tips, Assumptions and Limitations
- Assumptions: the calculations use the nominal size code values. Real mounted tires vary slightly by manufacturer, tread depth and inflation pressure, so measured diameters can differ by a few millimeters.
- Width and clearance: the calculator compares diameters, but a wider tire (245 vs 225 mm) also needs extra clearance inside the wheel well and may require a different rim width. Check your vehicle's specifications before buying.
- Speedometer error: the percentage difference approximates your speedometer and odometer error. A positive value means the speedometer under-reads; a negative value means it over-reads.
- Speed rating and load index: when changing sizes, make sure the new tire meets or exceeds the load index and speed rating of the original equipment tire.
- Mixed sizes: never mix tire sizes or diameters on the same axle, and on all-wheel-drive vehicles all four tires should have essentially identical diameters to avoid drivetrain stress.
If you are planning other home or vehicle-related projects, you may also find our gravel calculator, mulch calculator and wallpaper calculator useful for estimating materials. And if you need to substitute one size for another in the kitchen, the baking pan size calculator works on the same principle.
Frequently Asked Questions
How do I calculate the overall tire diameter in inches instead of millimeters?
Convert the sidewall height to inches first: divide the section width by 25.4, then multiply by the aspect ratio as a decimal. For a 225/60R16: \( 225 / 25.4 \times 0.60 \approx 5.31 \) inches per sidewall. Then \( OD = D + 2h = 16 + 2 \times 5.31 \approx 26.6 \) inches. You can also just divide the calculator's millimeter result by 25.4.
What is the maximum acceptable difference when switching tire sizes?
A common guideline is staying within ±3% of the original overall diameter, which keeps speedometer and odometer error minor. Some tire professionals aim for ±2% or tighter on modern vehicles, because ABS, traction control and gear shifts are calibrated to the original rolling circumference. A larger deviation is not automatically unsafe, but expect a noticeable speedometer error and check that the tire still clears the wheel well at full suspension travel and steering lock.
How much will my speedometer be off with the new tire size?
The speedometer error equals the percentage difference in diameter. If the new tire is 5% larger, your true speed is about 5% higher than shown: a 60 mph reading means roughly 63 mph actual, and your odometer will also undercount miles by 5%. A smaller tire works the opposite way — the speedo over-reads and the odometer overcounts. Multiply the displayed speed by \( 1 + \Delta\%/100 \) to estimate true speed.
What is plus sizing and how do I choose an equivalent tire?
Plus sizing means increasing the rim diameter while reducing the aspect ratio to keep the overall diameter nearly constant — for example moving from 225/60R16 to 225/55R17 ("plus one"). To find the match yourself, keep the sidewall height similar: each extra inch of rim needs about 12.7 mm less total sidewall height, so drop the aspect ratio accordingly. Always confirm the result with this calculator and check the rim width range printed on the tire.
Why does my measured tire diameter differ from the calculated value?
The size code gives nominal dimensions; real tires vary by brand, model, tread depth and pressure. A worn tire can be 5–10 mm smaller in diameter than new, and load-range or extra-load versions may run slightly larger. Manufacturers' published revs-per-mile figures are the most accurate reference. Treat calculated values as close estimates — differences of a few millimeters are normal and do not meaningfully change the comparison.
What are the most common mistakes when comparing tire sizes?
The classic errors are forgetting that the rim diameter is in inches while the width is in millimeters, and counting only one sidewall instead of two in the overall diameter. Others include comparing section widths while ignoring clearance, mixing tire sizes on an all-wheel-drive vehicle, and dropping to a lower load index or speed rating on the new size. Also remember that a wider tire may need a wider rim — check the manufacturer's approved rim width range.
Do wider or larger tires affect fuel economy and handling?
Generally, yes. Wider tires increase rolling resistance and aerodynamic drag, which typically costs a small amount of fuel economy, while shorter sidewalls (plus sizing) sharpen steering response but stiffen the ride. A larger overall diameter effectively lengthens gearing, slightly reducing engine rpm at cruising speed. If you stay within about 3% of the original diameter, these effects are minor and mostly come down to the tire model and compound rather than size alone.