Free Tire Size Comparison Calculator

Compare two tire sizes side by side. Get overall diameter, sidewall height, speedometer error at 60 mph, and revs per mile so you can plus-size or downsize safely.

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Enter your details on the left, then press Calculate.

What is this calculator for?

You're shopping for tires and your stock 235/45R18 set is $1,160 for four. The shop says 245/40R18 is "essentially the same size" and runs $720 — significant savings. Or you're considering bigger tires for a rugged look on your truck. The tire size calculator tells you whether the alternate size actually fits, what impact it has on your speedometer accuracy, and the practical differences in ride and performance.

Tire size code: e.g., 235/45R18. First number (235) = section width in millimeters. Second number (45) = aspect ratio (sidewall height as percentage of width — so 45% × 235mm = 105.75mm sidewall). R = radial construction (universal for modern tires). Last number (18) = wheel diameter in inches. Overall tire diameter: (2 × 105.75mm) + (18 × 25.4mm) = 668.7mm, or 26.3 inches.

Two tires are "equivalent" if their overall diameters match within ~3% — beyond that and you affect speedometer accuracy, gear ratios, ground clearance, and fitment in the wheel well. This calculator converts tire codes to actual inches, compares two tire sizes, and tells you whether they're functionally interchangeable.

How to use this calculator

Enter your current tire size in the standard format: 235/45R18. The calculator parses width, aspect ratio, and wheel diameter, computing overall tire diameter, sidewall height, and circumference.

Enter an alternate tire size for comparison. The calculator computes the difference in overall diameter and outputs whether the alternate is "compatible" (within 3%), "marginal" (3-5%), or "not recommended" (over 5%).

For vehicle modification scenarios (lift kits, plus-sizing wheels): the calculator can output the diameter and circumference difference, helping you decide if your speedometer will read accurately, whether the new tires will rub against fenders, and how ground clearance changes.

For tire replacement in stock size: the calculator confirms you're searching for the correct size. Many vehicles have multiple OEM tire sizes for different trim levels — your sticker (driver door jamb) shows your exact size.

Understanding your results

The calculator returns overall tire diameter in inches and mm, sidewall height, circumference (relevant for speedometer impact), and for comparisons, the percentage difference between two sizes.

Reading compatibility. 235/45R18 vs 245/40R18: diameters 26.32" vs 25.72" — 2.3% difference. Compatible. Same outside diameter means same revolutions per mile, same speedometer accuracy, same fender clearance. The 245/40 has slightly more width (245 vs 235 mm) and lower sidewall — slightly stiffer ride, slightly better cornering grip, less sidewall give for absorbing potholes.

The speedometer impact. Speedometer reads from wheel rotation — if your tires are larger than stock, you're actually going faster than the speedometer shows. 5% larger tires: actually going 73 MPH when speedometer shows 70. 10% larger: going 77. This matters for: speeding tickets (radar measures actual speed), accurate trip distance recording, fuel economy calculations (odometer underreports mileage if tires are bigger), gear ratio effects on acceleration. Most cars can absorb 2-3% tire size variance without noticeable impact; 5%+ requires speedometer recalibration or living with the inaccuracy.

The aspect-ratio trade-offs. Lower aspect ratio (40 vs 45 in the example): shorter sidewall = stiffer ride, better cornering response, more visible wheel for aesthetics, BUT higher risk of pothole/curb damage to wheel, harsher ride, less protection against road imperfections. Higher aspect ratio: more sidewall give, smoother ride, better off-road capability, BUT less precise cornering. Sports cars typically use 30-40 series; sedans 45-55; SUVs 55-65; trucks 65-80.

The width trade-offs. Wider tire (245 vs 235): more grip in dry conditions, more rolling resistance (slight MPG hit), more aquaplaning risk in heavy rain, requires wider wheel to fit properly, slightly more weight. Narrower tire: better fuel economy, better in snow (cuts through to pavement), slightly less dry grip. For most everyday driving, 10-20mm width variance is unnoticeable. The OEM tire size was chosen as a compromise for your specific vehicle.

A worked example

Marcus drives a 2019 Honda Accord with stock 235/45R18 tires. They're worn after 50K miles and need replacement. He's comparing three options at Tire Rack:

Option A: Stock-spec 235/45R18 Continental DWS06 Plus = $1,128 for set of 4. Same size as OEM.

Option B: 245/40R18 Continental DWS06 Plus = $1,082. Width up 10mm, aspect ratio down 5. Overall diameter: 25.72" vs stock 26.32" — 2.3% smaller. Compatible but actually-going-faster-than-speedometer-shows by 2.3%.

Option C: 235/50R18 Michelin Defender 2 = $872. Aspect ratio up 5 — taller sidewall. Overall diameter: 27.25" vs stock 26.32" — 3.5% larger. Marginally compatible; speedometer would slightly under-report speed.

Decision factors: Option A preserves OEM specs exactly. Option B saves $46 but introduces small speedometer error. Option C saves $256 but introduces 3.5% diameter difference (marginal) and changes the ride character slightly (taller sidewall, softer ride, slightly less precise cornering).

Marcus picks Option A despite higher price — preserving exact OEM specs is worth the $46-256 premium for his use case (mostly daily commute, occasional highway trips, no autocross or off-road). For someone driving a beat-up daily commuter where exact specs matter less, Option C's savings would make more sense.

Variation: Daniel has a 2017 Toyota 4Runner he wants to "plus-size" for an off-road appearance. Stock 265/65R17 (30.6" diameter). He's considering 265/75R17 (32.7" diameter — 7% larger) or 275/70R17 (32.2" — 5% larger). Both are too large for stock specs — speedometer would under-report by 5-7%. To accommodate larger tires, his options: (1) live with speedometer inaccuracy, (2) get speedometer recalibrated ($150-300), (3) install lift kit to handle wheel-well fitment, (4) install gear-ratio change to compensate for larger tire's effect on final drive ratio. Plus-sizing trucks is a $500-2,500 project beyond just the tires — many enthusiasts under-budget this.

Related resources

For car cost-of-ownership context, see Auto Loan Calculator. For fuel economy effects of tire changes, the Fuel Cost Calculator and Gas Mileage Calculator. Tire Rack publishes fitment guides for nearly every US vehicle along with consumer-reviewed tire performance data; Consumer Reports tire ratings covers independent testing.

Related calculators

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Frequently asked questions

Can I use a different tire size on my car?

Yes — within limits. The general rule is to keep overall diameter within ±3% of the OEM size. Going wider improves grip but can rub fenders or suspension at full lock; going narrower improves fuel economy slightly. Plus-sizing (larger rim, lower-profile tire) preserves diameter but stiffens ride. Always confirm load index and speed rating meet or exceed the original.

What does a tire size like 225/45R17 mean?

Three numbers, in order: 225 = section width in millimeters (sidewall to sidewall). 45 = aspect ratio — sidewall height as a percentage of width (45% of 225 mm = 101 mm sidewall). R = radial construction (the standard for passenger cars). 17 = wheel diameter in inches. The trailing letters and numbers (e.g., 94H) encode load index and speed rating.

Will bigger tires affect gas mileage?

Yes, modestly. Larger and wider tires weigh more (rotating mass), have more rolling resistance, and create more aerodynamic drag. A typical plus-2 upgrade (e.g., 16 → 18) can drop fuel economy by 1–3 mpg. Going to a much taller tire on a truck (e.g., for off-road) can drop economy by 10% or more.

What is plus-sizing?

Plus-sizing means increasing rim diameter while decreasing tire aspect ratio to keep overall diameter the same. Plus-one = 1 inch larger rim (e.g., 17 → 18); plus-two = 2 inches larger. The benefit is sharper steering response and a more aggressive look; the trade-off is harsher ride and higher rim damage risk on potholes.

How do I read a tire sidewall?

Beyond the size, the sidewall shows: load index (a number like 94 = 1,477 lb max load per tire), speed rating (a letter like H = 130 mph max), DOT code (manufacturing date — last 4 digits are week/year, e.g., 2823 = week 28 of 2023), and treadwear/traction/temperature ratings. UTQG numbers like 500AA mean treadwear grade 500, A-traction, A-temp.

What does the tire size code mean?

Format: 235/45R18. 235 = tire section width in millimeters. 45 = aspect ratio (sidewall height as % of width); 45 means sidewall is 45% of the 235mm width = 105.75mm sidewall. R = radial construction (modern standard). 18 = wheel diameter in inches. Optional letter after R indicates speed rating (H=130mph, V=149, W=168, Y=186, Z=149+). Sometimes a load index number appears too (e.g., 92T) indicating maximum load capacity. The full code on a tire sidewall might read 235/45R18 94H — 94 is load index (about 1,479 lbs), H is 130 MPH speed rating.

Can I put bigger tires on my car?

Within limits, yes. Most cars tolerate 2-3% tire diameter increase without modifications. Beyond that: speedometer becomes inaccurate, fender wells may need clearance (rubbing on full lock or over bumps), gear ratios are effectively changed (slower acceleration, slightly higher top speed), suspension geometry can be affected. For trucks and SUVs designed for plus-sizing: 5-10% increases are common, often paired with lift kits. For sedans and sports cars: stick close to OEM unless you're modifying for specific purpose (autocross, drag racing, off-road). Always verify the alternate size fits before buying — Tire Rack and similar sites have fitment guides; tire shops can advise on physical clearance for your specific vehicle.

How long do tires last?

Depends heavily on tire type and driving. All-season tires: 40,000-70,000 miles typical for the lifetime warranty mileage, often 50,000-60,000 in practice. Performance tires: 25,000-45,000 miles (softer rubber for grip). All-terrain tires: 40,000-60,000 miles. Run-flat tires: 25,000-35,000 (shorter life due to stiffer construction). Tire age matters too — even unused tires degrade due to rubber oxidation; tires older than 6-10 years should be replaced regardless of tread depth. Check tread depth with a penny: insert with Lincoln's head down; if you can see all of his head, tread is below 2/32 inch — legally bald, replace immediately. 4/32 inch is wet-weather warning level; many drivers replace at this point.

Do I need to replace all four tires at once?

Depends on vehicle type. All-wheel drive (AWD) and full-time 4WD: yes, replace all four together — mismatched tire diameters can damage the AWD system over time. Front-wheel drive and rear-wheel drive cars: can replace two at a time, but always put the newer tires on the rear (most cars; counterintuitive — the front normally wears faster but new tires belong in back for stability in emergency maneuvers). For typical scheduled replacement: do all four every 4-7 years. For unexpected damage (one tire ruined by a curb hit or nail): match-pair on that axle; rotate older same-pair to opposite axle if treadwear is similar.

What's the difference between all-season and winter tires?

Rubber compound and tread pattern. All-season tires: hard compound that lasts longer (40-70K miles), tread pattern for wet/dry/light snow. Winter tires: soft compound that stays flexible below 40°F (provides grip), aggressive tread for snow and ice, deeper sipes (small slits in tread). Winter tires dramatically outperform all-seasons below 40°F — stopping distances on cold pavement reduce 30-40%. But winter tires wear quickly in warm weather (the soft rubber overheats), so they're seasonal-only. In areas with significant winter (Northeast, Midwest, Mountain West, Plains, parts of South for occasional ice), dedicated winter tires on a second set of wheels is significantly safer than relying on all-seasons. In areas with minimal winter (FL, GA, southern CA, TX, AZ): all-seasons are fine year-round.

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