245/40R15 vs 185/55R15
185/55R15 is a safe swap for 245/40R15 by the usual ±3% rule: the diameter differs by +1.30%.
Speedometer and odometer
With 185/55R15 on a car calibrated for 245/40R15, the speedometer reads 59.2 mph when you're really doing 60, and the odometer under-counts by 1.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.4 mph |
| 45 mph | 45.6 mph |
| 60 mph | 60.8 mph |
| 70 mph | 70.9 mph |
| 80 mph | 81.0 mph |
Other sizes close to 245/40R15
Questions
Can I replace 245/40R15 with 185/55R15?
185/55R15 is a safe swap for 245/40R15 by the usual ±3% rule: the diameter differs by +1.30%.
What's the difference between 245/40R15 and 185/55R15?
185/55R15 is 0.30" taller and 60 mm narrower; the sidewall is +0.15". At an indicated 60 mph you'd actually be doing 60.8 mph.
Will 185/55R15 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.