205/45R17 vs 205/40R18
205/40R18 is a safe swap for 205/45R17 by the usual ±3% rule: the diameter differs by +0.80%.
Speedometer and odometer
With 205/40R18 on a car calibrated for 205/45R17, the speedometer reads 59.5 mph when you're really doing 60, and the odometer under-counts by 0.8%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.2 mph |
| 45 mph | 45.4 mph |
| 60 mph | 60.5 mph |
| 70 mph | 70.6 mph |
| 80 mph | 80.6 mph |
Other sizes close to 205/45R17
- 205/45R17 vs 225/45R17+2.92%
- 205/45R17 vs 215/45R17+1.46%
- 205/45R17 vs 205/50R17+3.33%
- 205/45R17 vs 215/50R17+4.95%
- 205/45R17 vs 235/45R17+4.38%
- 205/45R17 vs 245/40R17+1.87%
- 205/45R17 vs 255/40R17+3.16%
- 205/45R17 vs 195/55R16+0.75%
- 205/45R17 vs 205/50R16-0.80%
- 205/45R17 vs 205/40R17-3.33%
- 205/45R17 vs 235/40R17+0.57%
- 205/45R17 vs 215/40R17-2.03%
Questions
Can I replace 205/45R17 with 205/40R18?
205/40R18 is a safe swap for 205/45R17 by the usual ±3% rule: the diameter differs by +0.80%.
What's the difference between 205/45R17 and 205/40R18?
205/40R18 is 0.19" taller and 0 mm wider; the sidewall is -0.40". At an indicated 60 mph you'd actually be doing 60.5 mph.
Will 205/40R18 fit on 205/45R17 wheels?
No — 205/45R17 needs a 17" wheel and 205/40R18 a 18" wheel. This is a plus/minus-size swap that changes wheels too.