Tire size calculator & decoder
Enter a tire size to see its real dimensions — or compare two sizes for the overall-diameter change and speedometer error.
Tire size calculator — what does 235/45R18 mean?
A tire size like 235/45R18 packs four measurements into one code. This calculator turns it into real dimensions — section width, sidewall height, overall diameter and revolutions per mile — and lets you compare two sizes to see the overall-diameter change and how far your speedometer will read off. It's pure geometry, so it works for any size you type.
Reading a tire size, character by character
235 — section width
The tire's width in millimetres, measured sidewall to sidewall.
45 — aspect ratio
The sidewall height as a percentage of the width. Here, 45% of 235 mm ≈ 106 mm of sidewall.
R — construction
R means radial, the standard for modern passenger tires.
18 — rim diameter
The wheel diameter in inches that the tire is built to fit.
Why compare two tire sizes?
When you change tire or wheel size — plus-sizing, going to a taller all-terrain, or fitting what's in stock — the overall diameter changes. That throws off your speedometer and odometer and can affect clearance. A good rule of thumb is to stay within ±3% of the original overall diameter. Enter your current size and a candidate size above to see the percentage change and the speedometer error at 60 mph instantly.
Frequently asked questions
What does 235/45R18 mean?
235 is the tire's section width in millimetres. 45 is the aspect ratio — the sidewall height as a percentage of the width. R means radial construction. 18 is the wheel diameter in inches the tire fits. Together they define the tire's exact dimensions.
How do I compare two tire sizes?
Enter your current size and a second size in the calculator above. It shows the change in overall diameter as a percentage and how much your speedometer will be off, so you can tell at a glance whether the swap is a close match.
How much bigger or smaller can I go safely?
A common guideline is to keep the new tire's overall diameter within ±3% of the original. Larger differences can affect your speedometer reading, clearance and, on some vehicles, ABS and traction-control behaviour. The calculator flags whether you're inside that ±3% window.
How does tire size affect my speedometer?
Your speedometer assumes the factory tire diameter. A larger overall diameter makes the speedometer read slower than you're actually going; a smaller diameter makes it read faster. The calculator estimates the error at 60 mph for any two sizes you compare.
What is the aspect ratio in a tire size?
The aspect ratio is the middle number — the sidewall height expressed as a percentage of the section width. A lower number (like 40) means a shorter, sportier sidewall; a higher number (like 65) means a taller, more cushioned one. For 235/45R18 the sidewall is 45% of 235 mm.
Is this calculator accurate for any tire?
Yes — the results are pure geometry derived from the numbers in the size code, so they're consistent for any valid size. Real tires can vary slightly by brand and model, so treat the figures as precise nominal values for planning and comparison.
Related tools: Tire Finder, Reverse tire search and the Tire Sizes API.