235/45R21 vs 245/40R21
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/45R21 | 245/40R21 | Change |
|---|---|---|---|
| Overall height | 29.3" | 28.7" | -2.1 % |
| Sidewall | 4.16" | 3.86" | -7.3 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 92.1" | 90.2" | -2.1 % |
| Revs per mile | 688 | 702 | +2.1 % |
| Axle height | 14.66" | 14.36" | -7.8 mm |
| Approved rim width | 6.5–8.5" | 7–9" | overlap 7–8.5" |
Both are approved on a rim 7" 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 235/45R21. Fit 245/40R21 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 | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,792 on the road, so it over-counts by 208. At a true 70 mph the first turns 802 times a minute, the second 819.