235/45R17 vs 235/40R18
235/40R18 is a safe swap for 235/45R17 by the usual ±3% rule: the diameter differs by +0.30%.
Speedometer and odometer
With 235/40R18 on a car calibrated for 235/45R17, the speedometer reads 59.8 mph when you're really doing 60, and the odometer under-counts by 0.3%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.1 mph |
| 45 mph | 45.1 mph |
| 60 mph | 60.2 mph |
| 70 mph | 70.2 mph |
| 80 mph | 80.2 mph |
Other sizes close to 235/45R17
- 235/45R17 vs 225/45R17-1.40%
- 235/45R17 vs 225/40R18-0.95%
- 235/45R17 vs 225/50R17+2.10%
- 235/45R17 vs 215/45R17-2.80%
- 235/45R17 vs 205/50R17-1.01%
- 235/45R17 vs 215/55R17+3.89%
- 235/45R17 vs 215/50R17+0.54%
- 235/45R17 vs 245/45R17+1.40%
- 235/45R17 vs 215/55R16-0.06%
- 235/45R17 vs 245/40R17-2.41%
- 235/45R17 vs 205/45R17-4.20%
- 235/45R17 vs 275/35R18+0.99%
Questions
Can I replace 235/45R17 with 235/40R18?
235/40R18 is a safe swap for 235/45R17 by the usual ±3% rule: the diameter differs by +0.30%.
What's the difference between 235/45R17 and 235/40R18?
235/40R18 is 0.07" taller and 0 mm wider; the sidewall is -0.46". At an indicated 60 mph you'd actually be doing 60.2 mph.
Will 235/40R18 fit on 235/45R17 wheels?
No — 235/45R17 needs a 17" wheel and 235/40R18 a 18" wheel. This is a plus/minus-size swap that changes wheels too.