Tire size comparison
275/55R20 vs 305/50R20
305/50R20 is +0.3% taller than 275/55R20: 32.0 in vs 31.9 in (+0.10 in), and 1.2 in wider. With 305/50R20 on a vehicle set up for 275/55R20, the speedometer shows 60 mph at an actual 60.2 mph. That is within the ±3% difference many shops use as a rule of thumb (not a manufacturer or legal limit).
Set up for: checking a replacement size (305/50R20 instead of 275/55R20) · Different situation?
Your current size
e.g. 265/70R17, 2657017 or 33x12.50R20
Where do I find my size?
It's on the tire sidewall, e.g. P265/70R17 115T, and on the tire placard sticker on the driver's door jamb (also in the owner's manual). The placard shows the size the vehicle came with; the sidewall shows what is fitted now. If they differ, use the placard as your starting point. Digits are enough here: 2657017 or 33125020.
Size you're considering
275/55R20 vs 305/50R20: full comparison
| 275/55R20 | 305/50R20 | Difference | |
|---|---|---|---|
| Overall diameter | 31.91 in | 32.01 in | +0.10 in (+0.3%) |
| Width | 10.83 in | 12.01 in | +1.18 in (+10.9%) |
| Sidewall height | 5.95 in | 6.00 in | +0.05 in |
| Circumference | 100.2 in | 100.6 in | +0.3 in |
| Revs per mile (nominal) | 632 | 630 | −2 |
| Wheel diameter | 20 in | 20 in | ±0 in |
| Diameter in mm | 810 | 813 | +2 mm |
Speedometer difference
Your speedometer is calibrated for 275/55R20. Fitted with 305/50R20, the actual speed is the reading × 32.01 ÷ 31.91.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 20.1 mph | +0.1 mph |
| 30 mph | 30.1 mph | +0.1 mph |
| 45 mph | 45.1 mph | +0.1 mph |
| 55 mph | 55.2 mph | +0.2 mph |
| 60 mph | 60.2 mph | +0.2 mph |
| 70 mph | 70.2 mph | +0.2 mph |
| 80 mph | 80.2 mph | +0.2 mph |
Real tires: published diameters
275/55R20: 31.8–32.13 in published by 7 makers; 305/50R20: 32–32.05 in published by 4 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 275/55R20 · 305/50R20
Related comparisons
FAQ
Which is bigger, 275/55R20 or 305/50R20?
305/50R20 is +0.3% taller than 275/55R20: 32.0 in vs 31.9 in (+0.10 in).
How much will my speedometer be off with 305/50R20 instead of 275/55R20?
The speedometer is calibrated for 275/55R20. With 305/50R20 it shows 60 mph when you are doing 60.2 mph, and 70 mph at 70.2 mph (+0.3%). Your speedometer also has its own factory tolerance.
Can I replace 275/55R20 with 305/50R20?
The diameter difference is +0.3%. Whether it fits depends on things this calculation can't see: wheel width and offset, clearance at full lock and over bumps, load index and speed rating, and your vehicle maker's approval. Check your door-jamb placard and owner's manual and ask a tire professional.