225/45R15 vs 235/40R15
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/45R15 | 235/40R15 | Change |
|---|---|---|---|
| Overall height | 23.0" | 22.4" | -2.5 % |
| Sidewall | 3.99" | 3.70" | -7.2 % |
| Section width | 225 mm | 235 mm | +4.4 % |
| Circumference | 72.2" | 70.4" | -2.5 % |
| Revs per mile | 878 | 900 | +2.5 % |
| Axle height | 11.49" | 11.20" | -7.3 mm |
| Approved rim width | 6–8.5" | 6.5–8.5" | overlap 6.5–8.5" |
Both are approved on a rim 6.5" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/45R15. Fit 235/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.3 mph | -1.7 |
| 90 mph | 87.8 mph | -2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,751 on the road, so it over-counts by 249. At a true 70 mph the first turns 1024 times a minute, the second 1050.
What drivers reported
225/45R15 carries 2 complaints. Nobody has filed one naming 235/40R15.