245/40R15 vs 205/50R15
205/50R15 is a safe swap for 245/40R15 by the usual ±3% rule: the diameter differs by +1.56%.
Speedometer and odometer
With 205/50R15 on a car calibrated for 245/40R15, the speedometer reads 59.1 mph when you're really doing 60, and the odometer under-counts by 1.6%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.5 mph |
| 45 mph | 45.7 mph |
| 60 mph | 60.9 mph |
| 70 mph | 71.1 mph |
| 80 mph | 81.2 mph |
Other sizes close to 245/40R15
Questions
Can I replace 245/40R15 with 205/50R15?
205/50R15 is a safe swap for 245/40R15 by the usual ±3% rule: the diameter differs by +1.56%.
What's the difference between 245/40R15 and 205/50R15?
205/50R15 is 0.35" taller and 40 mm narrower; the sidewall is +0.18". At an indicated 60 mph you'd actually be doing 60.9 mph.
Will 205/50R15 fit on 245/40R15 wheels?
Both use a 15" rim, so the wheel diameter matches; check the wheel's width range for a 205 mm tire and the fender clearance.