Tire size comparison
315/70R17 vs 37x12.50R17
37x12.50R17 is +7.7% taller than 315/70R17: 37.0 in vs 34.4 in (+2.64 in), and 0.1 in wider. With 37x12.50R17 on a vehicle set up for 315/70R17, the speedometer shows 60 mph at an actual 64.6 mph. That is more than the ±3% difference many shops use as a rule of thumb, so check carefully before switching.
Set up for: going bigger (37x12.50R17 instead of 315/70R17) · 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
315/70R17 vs 37x12.50R17: full comparison
| 315/70R17 | 37x12.50R17 | Difference | |
|---|---|---|---|
| Overall diameter | 34.36 in | 37.00 in | +2.64 in (+7.7%) |
| Width | 12.40 in | 12.50 in | +0.10 in (+0.8%) |
| Sidewall height | 8.68 in | 10.00 in | +1.32 in |
| Circumference | 108.0 in | 116.2 in | +8.3 in |
| Revs per mile (nominal) | 587 | 545 | −42 |
| Wheel diameter | 17 in | 17 in | ±0 in |
| Diameter in mm | 873 | 940 | +67 mm |
Speedometer difference
Your speedometer is calibrated for 315/70R17. Fitted with 37x12.50R17, the actual speed is the reading × 37.00 ÷ 34.36.
| Speedometer shows | Actual speed | Difference |
|---|---|---|
| 20 mph | 21.5 mph | +1.5 mph |
| 30 mph | 32.3 mph | +2.3 mph |
| 45 mph | 48.5 mph | +3.5 mph |
| 55 mph | 59.2 mph | +4.2 mph |
| 60 mph | 64.6 mph | +4.6 mph |
| 70 mph | 75.4 mph | +5.4 mph |
| 80 mph | 86.1 mph | +6.1 mph |
Real tires: published diameters
315/70R17: 34.4–34.7 in published by 5 makers; 37x12.50R17: 36.54–37 in published by 5 makers. Calculated figures assume a nominal tire; the exact difference between two specific tires depends on their models.
More about each size: 315/70R17 · 37x12.50R17
Related comparisons
FAQ
Which is bigger, 315/70R17 or 37x12.50R17?
37x12.50R17 is +7.7% taller than 315/70R17: 37.0 in vs 34.4 in (+2.64 in).
How much will my speedometer be off with 37x12.50R17 instead of 315/70R17?
The speedometer is calibrated for 315/70R17. With 37x12.50R17 it shows 60 mph when you are doing 64.6 mph, and 70 mph at 75.4 mph (+7.7%). Your speedometer also has its own factory tolerance.
Can I replace 315/70R17 with 37x12.50R17?
The diameter difference is +7.7%. 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.