Tire Size Comparison
Compare two tire sizes side by side to see the differences in overall diameter, width, sidewall height, circumference, and speedometer error. Enter tire sizes in standard format (width/aspect ratio + rim diameter) to compare.
Tire size markings encode three critical dimensions: width (in millimeters), aspect ratio (sidewall height as a percentage of width), and wheel/rim diameter (in inches). A tire labeled "225/55R17" means 225mm tread width, 55% aspect ratio (sidewall height = 225 × 0.55 = 124mm), fitting a 17-inch rim. The "R" indicates radial construction (the standard since the 1980s; older "bias-ply" tires used "B" or no letter). Other letters appear in specialty sizes (D = diagonal, ZR = high speed-rating).
Tire size matters for more than fitment. Overall diameter affects: speedometer/odometer accuracy (larger tires cover more distance per revolution, so actual speed is higher than displayed), gear ratios (effectively change with diameter), ground clearance, and aesthetic stance. Width affects: contact patch area (more grip, more rolling resistance), aquaplaning behavior (wider = more prone to hydroplaning), and ride feel. Aspect ratio affects: sidewall flex (lower = stiffer, more responsive but harsher ride), curb damage risk (lower = more wheel vulnerability), and steering response.
This calculator compares two tire sizes across all key metrics: overall diameter, width, sidewall height, circumference, and resulting speedometer error if you swap from one to the other. Use it for: plus-sizing (going to larger rim with lower-profile tire while maintaining overall diameter), evaluating winter tire swaps, planning truck/SUV "leveled" looks, and quantifying speedometer impact of any tire change. The 3% rule: most automotive manufacturers recommend staying within ±3% of original tire diameter to avoid speedometer error, ABS/traction control miscalibration, and clearance issues. Larger deviations require professional recalibration and clearance verification.
Inputs
Results
Tire 1 Diameter
26.74"
225/55R17
Tire 2 Diameter
26.68"
245/45R18
Diameter Difference
-0.06" (-0.2%)
Speedometer Error
-0.2%
At indicated 60 mph
Side-by-Side Comparison
| Metric | Tire 1 (225/55R17) | Tire 2 (245/45R18) | Difference |
|---|---|---|---|
| Width (in) | 8.86 | 9.65 | 0.79 |
| Sidewall (in) | 4.87 | 4.34 | -0.53 |
| Diameter (in) | 26.74 | 26.68 | -0.06 |
| Circumference (in) | 84.02 | 83.82 | -0.20 |
| Revs/Mile | 754 | 756 | 2 |
Formula
How to use this calculator
- Enter Tire 1 specifications (your current or reference tire).
- Enter Tire 2 specifications (the alternative or upgrade you're comparing).
- Tire format: width(mm)/aspectratio + rim(inches). Example: 225/55R17 means width 225mm, aspect 55%, rim 17 inches.
- Review the comparison: overall diameter difference, sidewall difference, width difference, speedometer error %.
- For plus-sizing decisions: aim for diameter difference within ±3% to avoid recalibration.
- For truck/SUV upgrades: 5-10% diameter changes require recalibration and clearance check.
- For off-road tires: factor in lift kit requirements if going significantly larger.
- For winter tire swaps: many drivers go to narrower, taller-profile winter tires for snow handling. Calculator verifies diameter match to avoid speedometer issues.
- For replacement: when current tires are unavailable, find alternative size with similar diameter to maintain calibration.
Worked examples
Classic plus-one upgrade
Stock: 225/55R17 (mid-size sedan typical). Upgrade: 245/45R18 (sport package wheels). Stock diameter: 26.74 in Upgrade diameter: 26.68 in Difference: −0.2% (essentially identical) Result: speedometer/odometer remain accurate. Wider contact patch improves grip. Lower profile sharpens steering response but stiffens ride. Larger rim costs more (replacement tires $50-$150 more per tire). Curb damage risk increases with lower sidewall. Classic plus-sizing approach — improve aesthetics and handling without recalibration. Used widely in factory sport packages.
Truck off-road tire upgrade
Stock: LT265/70R17 (typical pickup, 31.6 in diameter). Upgrade: LT285/75R17 (slight off-road bump, 33.8 in diameter). Difference: +2.2 in / +7% larger. Result: Speedometer reads 60 mph when actually going 64.2 mph (7% under) Odometer accumulates 7% fewer miles than actual Effective gear ratio increases (highway RPM drops ~7%, low-end pulling power reduced) Ground clearance increases ~1.1 in Slightly worse fuel economy due to higher rolling resistance Should recalibrate speedometer with programmer or specialty tuner For larger jumps (LT35"+ tires on lifted trucks), gear ratio change typically also needed to maintain drivability. Many off-road builders re-gear from 3.55 to 4.10 or 4.56 to compensate for taller tires.
Winter tire size choice
Summer setup: 245/40R19 (low-profile performance). Winter setup: 225/45R18 (narrower + smaller wheel for snow performance). Summer diameter: 26.72 in Winter diameter: 25.97 in Difference: −2.8% (within acceptable 3% range) Result: Speedometer reads slightly high (actual 60 mph = displayed 58.3 mph) Modest odometer error (~2.8%) Significantly better winter handling — narrower tire cuts through snow rather than floating on top Smaller wheel allows taller sidewall to absorb impacts (curbs, frozen potholes) Often $400-$1,000 cheaper to source winter tires in this smaller size Winter setup decisions: narrower is better for snow traction; smaller-rim/taller-profile provides better curb damage resistance and cost savings. Stay within 3% diameter to avoid speedometer recalibration.
When to use this calculator
Use this calculator when planning a wheel/tire upgrade, comparing winter vs. summer tire options, evaluating off-road tire sizing for trucks/SUVs, replacing tires when exact original size isn't available, or verifying speedometer impact of a tire change.
Pair with horsepower (engine performance) and mpg-calculator (fuel economy modeling).
Important tire size considerations:
1. **Stay within 3% diameter rule.** Most automotive manufacturers recommend ±3% to avoid: speedometer error, ABS/traction control miscalibration, transmission shift point inaccuracies, and warranty concerns. Beyond 3%, recalibration is recommended.
2. **Plus-sizing maintains diameter while changing wheel size.** "Plus-one" = +1" rim with corresponding −2 aspect ratio points keeps overall diameter same. Plus-two, plus-three follow same pattern. Improves aesthetics and handling without speedometer issues.
3. **Wider isn't always better.** Wider tires increase grip but also: rolling resistance (worse fuel economy), road noise, hydroplaning risk, and steering effort. Manufacturer-spec widths balance these tradeoffs.
4. **Lower aspect = better handling, worse ride.** Lower-profile tires (30, 35, 40 series) provide sharper steering response but harsher ride, more wheel damage risk, and worse vibration absorption. High-profile (60, 65, 70+) are comfort-oriented.
5. **Off-road tires demand attention.** Larger tires on trucks/SUVs require: speedometer recalibration (>3% diameter change), wheel well clearance check, potential gear ratio change for drivability, and verification of suspension/wheel arch interference at full turn and full compression.
6. **Winter tires often run narrower.** Narrower tires (lower width) cut through snow more effectively than floating on top. Many drivers go down 1-2 widths for winter setup (e.g., 245 summer → 215-225 winter).
7. **Load and speed ratings matter.** Beyond size, tires have load index (weight capacity) and speed rating (max safe speed). Replacement tires must meet OE specifications. Lower ratings reduce safety margin.
8. **Tire diameter affects gearing.** Larger tires effectively raise gear ratios (lower RPM at given speed, less pulling power). 10%+ tire diameter increase often warrants gear ratio change to restore drivability.
9. **All four tires should match.** Tires with different sizes or significant tread differences can cause drivetrain stress (especially AWD/4WD), ABS confusion, and uneven handling. Replace in sets or pairs as recommended.
10. **OEM vs. plus-sized maintenance differs.** Plus-sized tires (lower profile, larger rim) are typically more expensive to replace, more vulnerable to road hazard damage, and may require professional installation due to specialized equipment requirements.
11. **Speedometer error is consistent if no recalibration.** A 3% larger tire produces 3% under-reading at ALL speeds — not just at certain speeds. Smartphone GPS apps can verify actual speed.
12. **Some vehicles allow programmer recalibration.** Many trucks/SUVs accept programmer-based speedometer recalibration after tire size changes. Cars often require dealer reprogramming, sometimes not officially supported.
Common mistakes to avoid
- Going more than 3% larger in diameter without recalibrating. Speedometer, odometer, and ABS systems all assume original tire size.
- Mixing tire sizes on AWD/4WD vehicles. Different sizes can damage drivetrain — replace all four to same size.
- Choosing tire size based only on appearance. Wider/lower-profile looks good but affects ride, fuel economy, and curb damage risk.
- Ignoring load and speed ratings. Replacement must meet OEM ratings minimums for safety.
- Plus-sizing without considering replacement cost. Lower-profile tires for larger rims often cost 50-100% more than OEM equivalents.
- Forgetting winter tire benefits. Narrower winter tires often outperform wider summer tires in snow despite "smaller looking."
Frequently Asked Questions
Sources & further reading
- Tire Safety and Performance — U.S. National Highway Traffic Safety Administration
- Tire Information and Specifications — Tire Rack
- Tire Safety — Consumer Guide — Consumer Reports