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.
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.
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.
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.
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.
Calculate horsepower from torque and RPM using HP = Torque x RPM / 5252.
Calculate your miles per gallon from fill-up data and see annual fuel costs.
Calculate your annual tax savings from the new EV loan interest deduction that replaces the $7,500 credit.
Compare the total cost of leasing versus buying a car.
Compare fuel costs between vehicles by MPG, gas price, and annual miles driven.
See how extra payments help you pay off your car loan faster.
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
| 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 |