235/40R17 vs 225/40R17
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/40R17 | 225/40R17 | Change |
|---|---|---|---|
| Overall height | 24.4" | 24.1" | -1.3 % |
| Sidewall | 3.70" | 3.54" | -4.3 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 76.7" | 75.7" | -1.3 % |
| Revs per mile | 827 | 837 | +1.3 % |
| Axle height | 12.20" | 12.04" | -4.0 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 235/40R17. Fit 225/40R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.4 mph | -0.6 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,871 on the road, so it over-counts by 129. At a true 70 mph the first turns 964 times a minute, the second 977.