225/45R14 vs 235/40R14
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/45R14 | 235/40R14 | Change |
|---|---|---|---|
| Overall height | 22.0" | 21.4" | -2.6 % |
| Sidewall | 3.99" | 3.70" | -7.2 % |
| Section width | 225 mm | 235 mm | +4.4 % |
| Circumference | 69.0" | 67.2" | -2.6 % |
| Revs per mile | 918 | 942 | +2.6 % |
| Axle height | 10.99" | 10.70" | -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 225/45R14. Fit 235/40R14 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,740 on the road, so it over-counts by 260. At a true 70 mph the first turns 1071 times a minute, the second 1099.