235/35R21 vs 225/35R21
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R21 | 225/35R21 | Change |
|---|---|---|---|
| Overall height | 27.5" | 27.2" | -1.0 % |
| Sidewall | 3.24" | 3.10" | -4.3 % |
| Section width | 235 mm | 225 mm | -4.3 % |
| Circumference | 86.3" | 85.5" | -1.0 % |
| Revs per mile | 734 | 741 | +1.0 % |
| Axle height | 13.74" | 13.60" | -3.5 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 235/35R21. Fit 225/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,900 on the road, so it over-counts by 100. At a true 70 mph the first turns 856 times a minute, the second 865.