325/60R20 vs 37x12.50R20
37x12.50R20 is outside the ±3% rule as a replacement for 325/60R20: the diameter differs by +4.65%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Side by side
| 325/60R20 | 37x12.50R20 | Difference | |
|---|---|---|---|
| Overall diameter | 35.35" | 37.00" | +1.65" (+4.65%) |
| Section width | 325 mm | 318 mm | -7 mm |
| Sidewall | 7.68" | 8.50" | +0.82" |
| Rim | 20" | 20" | same |
| Circumference | 111.1" | 116.2" | +5.2" |
| Revs per mile | 570 | 545 | -25 |
| Ride height | — | — | +0.82" |
| Speedometer reads 60 mph | 60.0 | 62.8 | +2.8 mph |
Speedometer and odometer
With 37x12.50R20 on a car calibrated for 325/60R20, the speedometer reads 57.3 mph when you're really doing 60, and the odometer under-counts by 4.7%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.1 mph |
| 60 mph | 62.8 mph |
| 70 mph | 73.3 mph |
| 80 mph | 83.7 mph |
Other sizes close to 325/60R20
Questions
Can I replace 325/60R20 with 37x12.50R20?
37x12.50R20 is outside the ±3% rule as a replacement for 325/60R20: the diameter differs by +4.65%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 325/60R20 and 37x12.50R20?
37x12.50R20 is 1.65" taller and 7 mm narrower; the sidewall is +0.82". At an indicated 60 mph you'd actually be doing 62.8 mph.
Will 37x12.50R20 fit on 325/60R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 318 mm tire and the fender clearance.