235/40R15 vs 225/45R15
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 | 235/40R15 | 225/45R15 | Change |
|---|---|---|---|
| Overall height | 22.4" | 23.0" | +2.5 % |
| Sidewall | 3.70" | 3.99" | +7.7 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 70.4" | 72.2" | +2.5 % |
| Revs per mile | 900 | 878 | -2.5 % |
| Axle height | 11.20" | 11.49" | +7.3 mm |
| Approved rim width | 6.5–8.5" | 6–8.5" | 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 235/40R15. Fit 225/45R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,255 on the road, so it under-counts by 255. At a true 70 mph the first turns 1050 times a minute, the second 1024.
What drivers reported
Nobody has filed a complaint naming 235/40R15. 225/45R15 carries 2.