Tire Size Calculator
Compare two tire sizes using the ETRTO format (e.g. 225/45R17). Shows sidewall height, overall diameter, circumference, revolutions per mile, and speedometer error.
Tire Size Calculator
🔵 Tire 1 (Original / OEM)
🔴 Tire 2 (Replacement / New)
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How to use the Tire Size Calculator
Read your current tire size from the sidewall
Look at the sidewall of any current tire — you'll see a code like 225/45R17, 205/55R16, or 245/40R18. The first number is tire width in millimeters; the second is aspect ratio (sidewall height as % of width); the R means radial (essentially all modern tires); the last number is rim diameter in inches. Type these three numbers into the "Tire 1" inputs. This is your OEM / current size — the baseline.
Enter the replacement tire size
Type the new tire size you're considering into "Tire 2". Common scenarios: Plus 1 (bigger rim, lower-profile tire, same overall diameter) — e.g. 225/45R17 → 225/40R18. Plus 2 (even bigger rim) — 225/45R17 → 225/35R19. Winter sizing (narrower for snow traction) — 225/45R17 → 205/55R16. Stretch fit (narrower tire on existing rim) — same width as OEM but pushed slightly off the rim for show purposes.
Check the overall diameter difference
The single most important comparison is OVERALL DIAMETER. Industry tolerance: ±1.5% is fine (interchangeable for speedometer + ABS purposes); 1.5-3% requires caution (noticeable speedometer error); 3-5% is significant; 5%+ generally not recommended without engineering review. Plus-sizing aims to keep diameter within 1.5% of OEM. If your replacement has 3%+ diameter difference, the speedometer will display wrong, ABS modules may be confused, and the wheel-arch clearance may not be sufficient.
Read the speedometer-error output
If the new tire is larger, it travels further per rotation — so the speedometer (calibrated for the old size) will UNDER-READ. Show 50 km/h, actual 53 km/h. If new is smaller, speedo OVER-READS. The math: actual_speed = displayed_speed × (new_circ / old_circ). This affects: speeding tickets (cops use radar, not your speedo, so you're at risk if speedo shows under), cruise control accuracy, lap-time data, odometer mileage tracking. Some jurisdictions (UK, EU) regulate maximum speedometer error to ±10%; many ASEAN markets have stricter requirements for vehicle inspections.
Tire sizing — why "Plus sizing" is the only safe way to upsize wheels
The European Tire and Rim Technical Organisation (ETRTO) size system that every tire uses today — 225/45R17 — encodes three numbers that fully describe a tire's dimensions: section width in mm, aspect ratio as a percentage, and rim diameter in inches. From these you can calculate overall diameter, circumference, and (using vehicle gearing) speedometer accuracy. The math is rigid: change any of the three numbers and you change overall diameter. The art of tire selection is to change those numbers in COMPENSATING ways — bigger rim with lower aspect = same overall diameter. This is called "Plus sizing", a term coined by Goodyear in the 1980s, and it's the standard methodology for upsizing wheels without breaking the rest of the vehicle.
The 1.5% tolerance rule
Industry consensus across automotive engineering organisations (SAE, ETRTO, JATMA, TRA) is that tires within ±1.5% overall diameter of OEM are functionally interchangeable. Within this tolerance: speedometer error is under 1% (legally compliant everywhere); ABS / traction control / stability control modules read correct wheel speed; transmission shift points don't drift; cruise control speed-holding is accurate; tire-pressure-monitoring systems still calibrate. Outside ±1.5%: things start to break in subtle ways. At 3%, ABS modules are noticeably off at limit conditions (emergency braking on ice). At 5%, some manufacturers explicitly void warranty. At 10%+, you're in "track-only" territory unless professionally re-tuned. Plus sizing's purpose is to fit larger / smaller wheels while staying within 1.5%.
Plus sizing exists because changing wheel size while keeping overall diameter constant is the ONLY way to upsize without breaking speedometers, ABS, traction control, and warranty terms. Stay within 1.5% diameter or accept the consequences.
Why tire width affects more than just looks
A wider tire (e.g. 245 vs 225) means more contact patch on dry roads — better grip, faster lap times, more positive steering feel. But it also means: more aerodynamic drag (1-2% fuel economy penalty per 20mm wider), more weight (rotational mass affects acceleration AND braking), more aquaplaning risk in heavy rain (wider patch displaces more water but with less pressure per unit area), and potentially more road noise (wider patch = more rubber contacting road imperfections). Narrower tires (e.g. 205 vs 225) reverse all of these — less grip but better aquaplaning, slightly better fuel economy, lighter and more nimble. For winter / monsoon driving, narrower is often actually safer; for summer / dry sport driving, wider gives better grip. Plus sizing typically keeps width the same and only changes aspect / rim.
The ASEAN tire-market angle
Tire selection across ASEAN involves some region-specific considerations. Singapore: strict LTA vehicle inspection requires tires within OEM specifications — significant upsizing typically requires LTA approval. Speedometer error beyond ±5% can fail inspection. Premium European tires (Continental, Michelin, Pirelli, Bridgestone Potenza) dominate the market; budget Chinese tires (Sailun, Linglong) increasingly competitive. Malaysia / Indonesia / Philippines / Vietnam / Thailand: more permissive regulatory environment — aftermarket "Plus 2" and "Plus 3" sizing is common in car culture. Japanese brands (Bridgestone, Yokohama, Dunlop, Toyo) dominate alongside European premium. Tropical rainfall considerations: heavy monsoon rains across ASEAN make tread depth + tread pattern matter enormously. Worn tires (under 3mm tread) hydroplane easily in heavy rain. Tire labeling: EU tire labels (rolling resistance grade A-G, wet braking grade A-G, noise dB) are increasingly required across ASEAN exports — look for high A/B grades on wet braking for monsoon-zone driving safety. Stretched tires (narrower-than-rim mounting for visual effect) are popular in Malaysia / Indonesia / Thailand JDM-influenced car culture but ILLEGAL in Singapore and increasingly inspected elsewhere — the safety risk of bead separation is real.
10 Things to Know About Tire Sizing
The ETRTO size code 225/45R17 means: 225mm wide, 45% aspect ratio (sidewall height = 45% of width), R = radial, 17-inch rim.
Industry tolerance for tire size matching: ±1.5% overall diameter = interchangeable. Beyond this, speedometer + ABS + TC are affected.
"Plus 1" sizing means bigger rim + lower aspect ratio, same overall diameter. E.g. 225/45R17 → 225/40R18 keeps diameter within 1%.
A larger tire makes your speedometer UNDER-READ (shows lower than actual speed). Smaller tire = speedometer OVER-READS.
The "R" in tire codes stands for radial construction. Bias-ply ("B") tires existed until 1980s; today, virtually all street tires are radial.
Wider tires generally have better dry grip but worse aquaplaning resistance. Narrower tires displace water better in heavy rain.
Tire tread depth below 3mm dramatically increases hydroplaning risk. Most jurisdictions require minimum 1.6mm to pass inspection.
The load index on a tire (e.g. "91" = 615 kg/tire) and speed rating (V = 240 km/h max) are separate codes after the size — important for replacement matching.
EU tire labels grade tires A through G on rolling resistance + wet braking. An "A" wet-braking tire stops 18m shorter than a "G" at 80 km/h.
Most tires expire after 6-10 years regardless of tread depth. Rubber compounds degrade chemically — check the DOT date code on the sidewall.
Frequently Asked Questions
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It's the standard ETRTO tire size code. 225 = section width in millimeters (the tire is 225mm wide at its widest point). 45 = aspect ratio as a percentage — the sidewall is 45% of the width, so 225 × 0.45 = 101.25mm tall. R = radial construction (the standard for modern street tires). 17 = rim diameter in inches that this tire fits. Overall diameter is computed as: (17 × 25.4) + 2 × 101.25 = 633.3mm or about 24.9 inches.
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The standard methodology for upsizing wheels without breaking speedometer, ABS, or warranty. The rule: larger rim diameter requires lower aspect ratio to keep overall tire diameter the same. Plus 1: +1 inch rim, -5 to -10% aspect — e.g. 225/45R17 → 225/40R18 (diameter change: <1%). Plus 2: +2 inch rim, -10 to -15% aspect — e.g. 225/45R17 → 225/35R19. Plus 3: +3 inch rim — getting aggressive, harder to find correct aspect ratios that stay within tolerance. Coined by Goodyear in the 1980s; now the universal upsizing standard.
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Yes. Speedometers measure wheel rotation, not actual ground speed. A larger tire travels further per rotation, so the speedometer (calibrated for the original size) UNDER-READS — shows lower speed than actual. A smaller tire OVER-READS. The error is directly proportional to circumference difference. For a 3% larger tire: speedo shows 50 km/h, you're actually doing 51.5 km/h. For a 10% larger tire: speedo 50, actual 55. This affects: speeding tickets (police use radar, your speedo is irrelevant for the fine), cruise control accuracy, odometer mileage tracking, fuel economy calculations from trip computers.
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Depends on jurisdiction. Singapore: LTA inspection requires tires within OEM tolerance; significant upsizing needs approval. EU: speedometer must read within ±10% of actual (ECE Regulation 39); tire size changes within Plus 1/2 typically pass. US: state-by-state, generally permissive but Mod-friendly states allow Plus 3+. Malaysia / Indonesia / Thailand / Philippines: generally permissive, but vehicle inspections increasingly check for OEM-compliant fitments. UK: speedometer must NOT read slower than actual (legal regulation). Always check local rules before changing. The calculator's speedometer-error output tells you what you'd see on a stock dashboard with the new tire.
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Wider tires improve dry grip but worsen aquaplaning resistance, fuel economy, and noise. In tropical climates with frequent heavy rain (ASEAN, Florida, Caribbean), narrower tires can actually be SAFER because they displace water better — the contact pressure per unit area is higher, so water gets pushed out from under the tire instead of forming a lubricating film. Winter tires are also typically narrower than summer tires for the same reason (snow + slush displacement). The OEM tire size for your car was chosen by engineers who balanced these tradeoffs — going significantly wider (more than +20mm) requires accepting tradeoffs they avoided.
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Two separate codes after the size. Load index: a number (e.g. 91) corresponding to max load per tire from a standardised table — 91 = 615 kg/tire, 95 = 690 kg, 100 = 800 kg. Total vehicle weight must be supported by the four tires combined. Speed rating: a letter for the maximum sustained speed — H = 210 km/h, V = 240 km/h, W = 270 km/h, Y = 300 km/h. Always replace with EQUAL or HIGHER load + speed ratings than OEM. Going lower can void insurance and is illegal in many jurisdictions. Performance tires (Y, W, ZR) are not always needed but are common on sports cars.
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The calculator gives overall diameter change. Add half of that to the OEM tire's clearance margin to know what physical space the new tire needs. But: tire WIDTH and offset also affect clearance at the inner fender (steering lock) and outer fender (suspension travel). For width changes >20mm or offset changes, do a physical fit check by mounting one wheel, then turning steering to full lock both directions, and compressing suspension to bump-stop while you observe clearance. Many cars need fender rolling or trimming for aggressive fitments. Forums for your specific car model are the best source of "what fits" data.
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"Stretched" or "stretch fitment" mounts a narrower tire than the rim is designed for — popular in JDM and stance car culture for visual effect. Risks: bead can separate from rim under hard cornering or pothole impact (immediate loss of air pressure at speed), sidewall flex characteristics change, and load capacity is reduced. The aesthetic appeal is real; the safety compromise is real. Singapore explicitly bans stretched fitments in LTA inspection. Most ASEAN markets are increasingly inspecting for this. If you must run stretched, keep it mild (1-2 size narrower at most), only on high-quality forged wheels with strong beads, and accept the increased risk.
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No. All calculations run entirely in your browser via JavaScript. There's no server roundtrip — open DevTools → Network and confirm zero outbound requests. Your tire specs stay on your device. Safe for fleet upgrade planning, customisation comparisons, or any vehicle data that shouldn't leave your machine.
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Two criteria: tread depth and age. Tread depth: minimum legal is 1.6mm in most jurisdictions, but stopping distance increases dramatically below 3mm in wet conditions (40%+ longer than new). Replace when wear bars (raised rubber within tread grooves) become flush with the surface. Age: 6-10 years from manufacture date regardless of tread depth — rubber compounds harden chemically and lose grip even when stored unused. Check the 4-digit DOT date code on the sidewall (e.g. "2421" = week 24 of 2021). Tropical climates (ASEAN, Florida) age tires faster due to UV + heat; check the date code annually. Track use, frequent emergency stops, or aggressive driving shortens tire life significantly.
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