Tire Size Comparison Calculator
Compare any two tire sizes side-by-side — diameter, width, speedometer error, and a visual chart
REAL-WORLD EXAMPLE
A Toyota Tacoma owner upgrading from the stock 265/70R17 (31.61" diameter) to popular aftermarket 285/75R17 (33.83" diameter) tires gains 2.22 inches of overall diameter (+7.02%). This shifts revolutions per mile from 634.0 to 592.3, causing the speedometer to read 59.4 mph when actually traveling 63.7 mph at highway speeds. Odometer error reaches 6.5%, meaning every 100 miles logged is actually 106.5 miles driven. A re-gear from the stock 3.73 to 4.10 axle ratio is commonly recommended to restore factory-like performance feel.
① Original Tire
→ 265/70R17
② Compare Tire
→ 285/70R17
Enter tire specs above and click Calculate to compare
Supports P-metric (225/65R17) and Imperial/flotation (31x10.5R15) formats
| Original | Compare | Dia. Diff | % Diff | Speedo Error | Verdict |
|---|---|---|---|---|---|
| 225/65R17 | 235/65R17 | +0.76" | +2.4% | +2.35% | Safe |
| 265/70R17 | 285/70R17 | +0.79" | +2.5% | +2.41% | Acceptable |
| 265/70R17 | 285/75R17 | +2.22" | +7.0% | +6.55% | Re-gear advised |
| 275/65R18 | 295/65R18 | +0.79" | +2.4% | +2.36% | Safe |
| 245/75R16 | 265/75R16 | +0.79" | +2.6% | +2.53% | Safe |
| 255/70R18 | 275/65R18 | -0.54" | -1.7% | -1.67% | Safe |
| 31x10.5R15 | 33x12.5R15 | +2.00" | +6.5% | +6.10% | Re-gear advised |
| 285/75R16 | 315/75R16 | +1.22" | +3.7% | +3.55% | Marginal |
| 225/45R18 | 235/45R18 | +0.39" | +1.5% | +1.53% | Excellent |
| 245/65R17 | 265/70R17 | +1.26" | +4.2% | +4.01% | Marginal |
Speedometer error is positive when the new tire is larger (speedo reads high — you travel faster than indicated). Always verify fitment and clearance for your specific vehicle.
- Enter your current (Original) tire size — choose P-metric format (e.g. 265/70R17) or Imperial/flotation format (e.g. 31x10.5R15).
- Enter the Compare tire size you are considering in the second panel.
- Click 'Calculate & Compare' to see all results instantly.
- Check the Visual tab for a to-scale side-by-side tire diagram showing height and width differences.
- Review the Stats tab for exact diameter, sidewall height, circumference, revs/mile, and rim clearance change.
- Check the Speedometer tab to see your corrected actual speed at every common highway speed — critical for safety and legality.
- Expand 'Effective Gear Ratio Impact' in the Stats tab to assess whether a re-gear is recommended for your axle ratio.
Changing tire sizes is one of the most impactful modifications you can make to a vehicle — it affects ground clearance, fuel economy, speedometer accuracy, handling dynamics, and even powertrain stress on AWD systems. This calculator handles the full comparison so you don't have to do the math by hand.
P-Metric vs. Imperial/Flotation Sizes
Most passenger cars, SUVs, and crossovers use P-metric sizing (e.g. 225/65R17). Light trucks, Jeeps, and off-road builds often use Imperial flotation sizing (e.g. 33x12.5R15). Our calculator supports both formats independently for each tire, so you can cross-compare directly.
Why Speedometer Accuracy Matters
Your factory speedometer is calibrated to your stock tire circumference. Larger tires have a bigger circumference and cover more ground per revolution, so the wheel spins fewer times per mile. The speedometer doesn't know this and will undercount — showing a lower speed than you're actually travelling. This creates both a safety concern and potential issues with speed cameras and toll transponders. The Speedometer table in this tool shows every common highway speed corrected for your new tire size.
Gear Ratio Considerations
Bigger tires effectively "taller" your gearing — the engine turns fewer RPM per unit of road speed. For stock trucks this often means sluggish acceleration from stops and the engine lugging under load. A common solution is an axle re-gear (e.g. from 3.73 to 4.10 for a 33" tire upgrade on a Tacoma). The advanced stats panel in this calculator shows effective gear ratio impact for three common axle ratios.
Visual Comparison
The Visual tab renders a to-scale side-by-side SVG diagram of both tires sitting on the ground. The diagram accounts for both tire height (overall diameter) and width (section width) so you get an immediate intuitive feel for the size difference before committing to a purchase.