Tire size comparison
235/55R20 vs 245/50R20
245/50R20 is −1.8% shorter than 235/55R20: 29.6 in vs 30.2 in (−0.53 in), and 0.4 in wider. With 245/50R20 on a vehicle set up for 235/55R20, the speedometer shows 60 mph at an actual 58.9 mph. That is within the ±3% difference many shops use as a rule of thumb (not a manufacturer or legal limit).
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). If your current tires aren't the original size, the placard shows what the vehicle came with. Digits are enough here: 2657017 or 33125020.
Size you're considering
235/55R20 vs 245/50R20: full comparison
| 235/55R20 | 245/50R20 | Difference | |
|---|---|---|---|
| Overall diameter | 30.18 in | 29.65 in | −0.53 in (−1.8%) |
| Width | 9.25 in | 9.65 in | +0.39 in (+4.3%) |
| Sidewall height | 5.09 in | 4.82 in | −0.27 in |
| Circumference | 94.8 in | 93.1 in | −1.7 in |
| Revs per mile (nominal) | 668 | 680 | +12 |
| Wheel diameter | 20 in | 20 in | ±0 in |
| Diameter in mm | 766 | 753 | −14 mm |
Speedometer difference
Your speedometer is calibrated for 235/55R20. Fitted with 245/50R20, the actual speed is the reading × 29.65 ÷ 30.18.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 19.6 mph | −0.4 mph |
| 30 mph | 29.5 mph | −0.5 mph |
| 45 mph | 44.2 mph | −0.8 mph |
| 55 mph | 54.0 mph | −1.0 mph |
| 60 mph | 58.9 mph | −1.1 mph |
| 70 mph | 68.8 mph | −1.2 mph |
| 80 mph | 78.6 mph | −1.4 mph |
Real tires: published diameters
235/55R20: 29.9–30.2 in published by 7 makers; 245/50R20: 29.4–29.8 in published by 7 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 235/55R20 · 245/50R20
FAQ
Which is bigger, 235/55R20 or 245/50R20?
245/50R20 is −1.8% shorter than 235/55R20: 29.6 in vs 30.2 in (−0.53 in).
How much will my speedometer be off with 245/50R20 instead of 235/55R20?
The speedometer is calibrated for 235/55R20. With 245/50R20 it shows 60 mph when you are doing 58.9 mph, and 70 mph at 68.8 mph (−1.8%). Your speedometer also has its own factory tolerance.
Can I replace 235/55R20 with 245/50R20?
The diameter difference is −1.8%. 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.