225/35R15 vs 205/40R15
Going from one to the other changes the overall height by +1.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/35R15 | 205/40R15 | Change |
|---|---|---|---|
| Overall height | 21.2" | 21.5" | +1.2 % |
| Sidewall | 3.10" | 3.23" | +4.1 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 66.6" | 67.4" | +1.2 % |
| Revs per mile | 951 | 940 | -1.2 % |
| Axle height | 10.60" | 10.73" | +3.3 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/35R15. Fit 205/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.1 mph | +1.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,121 on the road, so it under-counts by 121. At a true 70 mph the first turns 1110 times a minute, the second 1097.