215/45R15 vs 235/40R15
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/45R15 | 235/40R15 | Change |
|---|---|---|---|
| Overall height | 22.6" | 22.4" | -1.0 % |
| Sidewall | 3.81" | 3.70" | -2.8 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 71.1" | 70.4" | -1.0 % |
| Revs per mile | 892 | 900 | +1.0 % |
| Axle height | 11.31" | 11.20" | -2.8 mm |
| Approved rim width | 6–8" | 6.5–8.5" | overlap 6.5–8" |
Both are approved on a rim 6.5" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/45R15. Fit 235/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,904 on the road, so it over-counts by 96. At a true 70 mph the first turns 1040 times a minute, the second 1050.