Decode 225/45R17 into diameter, sidewall, circumference & revs/km — then compare two tires for speedometer error and clearance change. Browser-only.
- Runs locally
- Category Calculator
- Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.
Compare two tires
| Indicated | Actual |
|---|---|
| 40 km/h | 40.2 km/h |
| 60 km/h | 60.2 km/h |
| 80 km/h | 80.3 km/h |
| 100 km/h | 100.4 km/h |
| 120 km/h | 120.5 km/h |
Most installers treat ±3% diameter change as the safe limit before the speedometer, ABS, and gearing drift too far.
What this tool does
Type a tire size like 225/45R17 (metric) or 33x12.5R15 (inch / flotation) and read back what the sidewall hides: section width, aspect ratio, sidewall height (width × aspect), overall diameter in mm and inches, rolling circumference, and revolutions per kilometre and per mile. The second half answers the pre-swap question: "how wrong does my speedometer get?" Enter the original (speedometer reference) tire and the new one, and you get the diameter difference as a percent, a table of true road speed for each indicated reading, the ground-clearance change, and the circumference change. A ±3% banner flags swaps that push the speedometer, ABS, and gearing past the rule of thumb. Switch km/h ⇄ mph, copy a summary, or share via URL. All computed in your browser — no database, no upload.
Tool details
- Input
- Text + Numbers
- The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
- Output
- Live result + Copy
- The result area focuses on usable output, with copy, download, or preview actions when supported.
- Privacy
- Browser-side processing
- The main tool logic does not call an external API, so inputs normally stay in the current tab.
- Save / share
- Shareable URL state
- Key settings are encoded in the URL so another person can reopen the same setup.
- Performance budget
- Initial JS <= 11 KB
- No WASM budget is declared, keeping the tool quick to open on mobile.
- Best fit
- Calculator · Operations
- Category and role tags drive related tools, internal links, and quick fit checks.
How to use
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1. Input
Paste or drop your content into the tool panel.
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2. Process
Click the button. All processing is local in your browser.
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3. Copy / Download
Copy the result or download to disk in one click.
How Tire Size Calculator fits into your work
Use it for fast estimates, comparisons, and planning numbers before you make the final call.
Calculation jobs
- Getting a realistic range before a purchase, plan, workout, or schedule decision.
- Comparing scenarios by changing one input at a time.
- Turning rough assumptions into a number you can discuss.
Calculation checks
- Double-check units, dates, rates, and rounding assumptions.
- Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
- Save the inputs that produced an important result so you can reproduce it later.
Good next steps
These links move the current task into a more complete workflow.
- 1 Unit Converter Convert between length, weight, temperature, area, volume, speed, time — instant, browser-only Open
- 2 Aspect Ratio Calculator Calculate aspect ratio — fix any 1 of [width, height, ratio], get the other two. Open
- 3 Ratio Calculator Solve a:b = c:x, simplify ratios, split a total by a ratio, scale a recipe up or down — with full steps, browser-only Open
Real-world use cases
Decode a tire before buying online
You found a deal on 245/40R18 but the listing only shows the string. Paste it here and you instantly know the tire is 614.6 mm tall with a 98 mm sidewall — enough to check it physically fits your wheel well before you commit to a set of four.
Plan a plus-size wheel upgrade
Going from 17-inch to 18-inch wheels means dropping the aspect ratio to keep the overall diameter constant. Type your stock size as the original, try 245/40R18 as the new tire, and read the diameter difference. Aim for a number near 0% so the speedometer and gearing stay honest while the rim gets bigger.
Recalibrate after a winter-tire swap
Winter sets are often a size narrower on a smaller rim. Enter both sizes and the speedometer table tells you whether your indicated highway speed is now reading 2-3 km/h off, so you can mentally correct or adjust a GPS-based reference until spring.
Size up off-road tires within a safe lift
Bigger meats for the trail also raise the body and skew the speedometer. Compare 265/70R17 against 285/70R17 and watch the ground-clearance change and the ±3% banner so you upgrade for clearance without blowing past what the ABS and fenders tolerate.
Settle a 'these aren't the same size' dispute
A shop fitted 225/50R17 when your car spec'd 225/45R17. Put both in the comparison and you have the exact diameter gap (about +3.5%) and the resulting speedometer drift in one screenshot — objective evidence instead of an argument.
Common pitfalls
Reading the aspect ratio as millimetres. The 45 in 225/45R17 is a percentage of the width, not 45 mm. The actual sidewall is 225 × 0.45 = 101.25 mm. Confusing the two makes every downstream number wrong.
Assuming a wider tire is automatically taller. Width and diameter are independent. 225/45R17 and 245/40R17 have nearly the same diameter despite the 20 mm width gap — because the wider tire uses a lower aspect ratio. Always compare overall diameter, not section width, when you care about the speedometer.
Mixing rim diameter into the diameter math. The 17 in 225/45R17 is the wheel, not the tire. The overall tire diameter adds two sidewalls on top of the rim. People who eyeball '17 inches' as the tire size are off by more than 8 inches.
Privacy
Every figure — the size parser, the diameter and circumference math, the speedometer-error table, and the clearance delta — runs as plain JavaScript in your browser tab. No tire database is queried, no size string is uploaded, and nothing about your vehicle is logged. The one caveat: the shareable URL encodes both size strings in the query string (e.g. ?o=205/55R16&n=225/45R17), so a "share link" pasted into chat lands in the recipient's server access log. Tire sizes are not sensitive, but if you would rather not leave any trail, use the copy-summary button instead of the share URL.
FAQ
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