245/40R15 vs 185/60R15
185/60R15 is outside the ±3% rule as a replacement for 245/40R15: the diameter differs by +4.51%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 185/60R15 on a car calibrated for 245/40R15, the speedometer reads 57.4 mph when you're really doing 60, and the odometer under-counts by 4.5%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 31.4 mph |
| 45 mph | 47.0 mph |
| 60 mph | 62.7 mph |
| 70 mph | 73.2 mph |
| 80 mph | 83.6 mph |
Other sizes close to 245/40R15
Questions
Can I replace 245/40R15 with 185/60R15?
185/60R15 is outside the ±3% rule as a replacement for 245/40R15: the diameter differs by +4.51%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 245/40R15 and 185/60R15?
185/60R15 is 1.02" taller and 60 mm narrower; the sidewall is +0.51". At an indicated 60 mph you'd actually be doing 62.7 mph.
Will 185/60R15 fit on 245/40R15 wheels?
Both use a 15" rim, so the wheel diameter matches; check the wheel's width range for a 185 mm tire and the fender clearance.