Tire Size Difference
Find the exact diameter difference between two tire sizes — and whether that difference is safe for your vehicle
REAL-WORLD EXAMPLE
265 vs 285 is the most-searched tire size difference question. Going from 265/70R17 to 285/70R17 adds just +1.10 inches (+3.5%) of diameter — right at the marginal threshold. But stepping up to 285/75R17 adds +2.22 inches (+7.0%) — crossing clearly into "re-gear advised" territory, with your speedometer reading 59.4 mph when you're actually doing 63.7 mph. The calculator below shows you the exact difference and safety verdict for any two sizes, including AWD tolerance checks.
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
These thresholds apply to overall diameter change. Use the calculator above to find your exact percentage.
| Original | New Size | Dia. Diff | % Diff | Speedo Err | Verdict |
|---|---|---|---|---|---|
| 265/70R17 | 275/70R17 | +0.55" | +1.7% | +1.7% | Acceptable |
| 265/70R17 | 285/70R17 | +1.10" | +3.5% | +3.4% | Marginal |
| 265/70R17 | 285/75R17 | +2.22" | +7.0% | +6.6% | Re-gear advised |
| 275/70R17 | 285/70R17 | +0.55" | +1.7% | +1.7% | Acceptable |
| 245/65R17 | 265/65R17 | +1.02" | +3.5% | +3.3% | Marginal |
| 235/55R18 | 245/55R18 | +0.43" | +1.5% | +1.5% | Acceptable |
| 225/65R17 | 235/65R17 | +0.39" | +1.4% | +1.4% | Acceptable |
| 275/65R18 | 295/65R18 | +0.79" | +2.4% | +2.4% | Marginal |
| 245/75R16 | 265/75R16 | +0.79" | +2.6% | +2.5% | Marginal |
| 31x10.5R15 | 33x12.5R15 | +2.00" | +6.5% | +6.1% | Re-gear advised |
- Enter your current tire size in the Original Tire panel. Use P-metric (e.g. 265/70R17) or Imperial/flotation format (e.g. 31x10.5R15).
- Enter the size you are considering in the Compare Tire panel.
- Click 'Calculate & Compare' to get the full breakdown.
- Check the safety badge at the top of results — it shows whether the difference is Excellent, Acceptable, Marginal, or Not Recommended.
- Review the Stats tab for exact diameter difference in inches and percentage.
- Check the Speedometer tab to see how far off your speedo will read at highway speeds.
- Use the gear ratio section in Stats to assess whether a re-gear is needed for trucks and 4x4s.
Changing tire size is one of the most common vehicle modifications — and one of the most misunderstood. The physical size difference between two tires affects ground clearance, wheel-well fitment, speedometer accuracy, odometer readings, fuel economy, and on trucks and 4x4s, whether a re-gear is necessary.
The 3% Rule
A widely used guideline: stay within ±3% overall diameter difference to avoid meaningful speedometer error and drivetrain stress. For passenger cars, ±2% is a safer target. For dedicated off-road trucks, some builders push to 8–10%, but always paired with a matching axle re-gear to maintain drivability.
Speedometer & Odometer Impact
A 7% larger tire means your speedometer reads 60 mph when you're actually doing 64.2 mph — that's 4.2 mph fast at every highway speed. The odometer reads 7% fewer miles than you actually drive, which accelerates all mileage-based service intervals if you're tracking the wrong number. Recalibration (via a tuner on modern vehicles) corrects both.
AWD and 4WD Tolerance by Brand
On full-time AWD systems, mismatched tire diameters between axles creates a continuous speed difference that the center differential must constantly compensate for — generating heat, wear, and potentially binding. Published tolerances by system:
- Subaru Symmetrical AWD — max 1/4" tread depth difference between any two tires; effectively limits diameter variance to ~0.5%. Replace all four or measure remaining tread carefully.
- BMW xDrive — tolerates very small differences; BMW recommends replacing all four tires simultaneously. Temporary spare use is restricted to low speeds.
- Audi quattro (Torsen) — similarly strict; Audi specifies matching tires on all four corners. Even 3/32" tread difference can cause wear.
- Mercedes 4MATIC — recommends all four tires identical; mismatched sizes can trigger 4WD error codes.
- Ford F-150 4x4 / part-time systems — generally more tolerant (2–3% diameter difference acceptable) when 4WD is disengaged during normal driving.