225/45R21 vs 235/40R21
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/45R21 | 235/40R21 | Change |
|---|---|---|---|
| Overall height | 29.0" | 28.4" | -2.0 % |
| Sidewall | 3.99" | 3.70" | -7.2 % |
| Section width | 225 mm | 235 mm | +4.4 % |
| Circumference | 91.0" | 89.2" | -2.0 % |
| Revs per mile | 696 | 710 | +2.0 % |
| Axle height | 14.49" | 14.20" | -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 225/45R21. Fit 235/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,803 on the road, so it over-counts by 197. At a true 70 mph the first turns 812 times a minute, the second 828.