Tire Size Calculator
Calculate tire sidewall height, overall diameter, circumference, revolutions per mile, and wheel RPM from a metric tire size (width, aspect ratio, rim diameter) — with an optional second size to compare and get the resulting speedometer error.
Input
The first number in a tire size, e.g. the 225 in 225/50R17.
Sidewall height as a % of width, e.g. the 50 in 225/50R17.
Wheel diameter in inches, e.g. the 17 in 225/50R17.
Optional. Displayed speed to compute wheel RPM at — and, with a comparison size below, actual speed.
Width of the size you're comparing against.
Aspect ratio of the size you're comparing against.
Rim diameter of the size you're comparing against.
Output
| Metric | Value |
|---|---|
| No data yet | |
Guides
Every metric tire carries its dimensions in its sidewall code — a size like 225/50R17 encodes tread width, sidewall height, and rim diameter, but none of the numbers you actually need: overall diameter, circumference, or how many times the wheel turns per mile. This calculator works those out, and lets you compare two sizes side by side to see whether a different tire will throw off your speedometer.
How to use it
- Enter the three numbers from your tire size — width in mm, aspect ratio %, and rim diameter in inches. For 225/50R17 that's 225, 50, and 17.
- Optionally set a speed to compute wheel RPM at (defaults to 60 mph).
- To compare against a different size, open Compare to another size and enter its three numbers. You'll get the diameter difference, the speed your car is actually doing when the odometer reads your entered speed, and a fitment verdict.
The results table updates as you type. Use the copy or download button to export it as CSV.
How tire size codes work
In 225/50R17:
- 225 is the section width in millimetres, measured sidewall to sidewall.
- 50 is the aspect ratio — the sidewall height as a percentage of the width. A 50 aspect ratio on a 225 tire means 112.5 mm of sidewall.
- R means radial construction.
- 17 is the rim (wheel) diameter in inches.
The formulas
Sidewall height = Width × (Aspect Ratio ÷ 100).
Overall diameter = (Rim diameter × 25.4) + (2 × Sidewall height) — the rim is converted to millimetres, and sidewall is counted twice because there's one above and one below the wheel.
Circumference = Overall diameter × π.
Revolutions per mile = 1,609,344 mm ÷ Circumference in mm.
Wheel RPM = Revolutions per mile × (Speed in mph ÷ 60).
Speedometer error — your speedometer is calibrated for the factory tire's diameter, so fitting a larger tire makes you travel further per revolution than the gauge assumes. Actual speed = Indicated speed × (New diameter ÷ Original diameter).
What's the 3% rule?
The widely used rule of thumb is to keep a replacement tire's overall diameter within about 3% of the original. Beyond that, the speedometer and odometer drift noticeably, and ABS, traction control, and transmission shift points — all of which infer wheel speed from rotation — start working from wrong numbers. The comparison verdict in this tool flags anything outside that band.
What is plus sizing?
Plus sizing means fitting a larger-diameter wheel while shrinking the sidewall to keep overall diameter roughly the same — 225/50R17 → 235/45R18 is a classic "plus one." It sharpens steering response at the cost of ride comfort. Use the comparison fields here to confirm your plus-size lands within the 3% band before buying.
Does a wider tire always mean a bigger diameter?
No. Width and diameter are independent — because aspect ratio is a percentage of width, a wider tire with a lower aspect ratio can have the same or a smaller overall diameter. That's exactly why comparing the computed diameters matters more than eyeballing the size codes.
What if I need plain unit conversion instead?
This tool always reports lengths in both millimetres and inches, so you shouldn't need to convert anything by hand. If you're working with a measurement it doesn't cover, the Length Converter handles mm/cm/inch/foot conversions directly, and the Speed Converter will turn the mph figures here into km/h.
Privacy
This calculator runs entirely in your browser. Your tire sizes and speeds are never uploaded, logged, or stored — the math happens locally on your device.
Use it from code
From 3 credits per callREST API
curl -X POST https://api.iotools.cloud/v1/tool/tire-size-calculator \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"widthMm": "225",
"aspectRatio": "50",
"rimInches": "17",
"speedMph": "60"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `tire-size-calculator` tool (Tire Size Calculator) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.