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).
Set up for: checking a replacement size (245/50R20 instead of 235/55R20) · 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
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.