245/50R21 vs 235/50R21
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 | 245/50R21 | 235/50R21 | Change |
|---|---|---|---|
| Overall height | 30.6" | 30.3" | -1.3 % |
| Sidewall | 4.82" | 4.63" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 96.3" | 95.0" | -1.3 % |
| Revs per mile | 658 | 667 | +1.3 % |
| Axle height | 15.32" | 15.13" | -5.0 mm |
| Approved rim width | 7–9" | 6.5–8.5" | 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 245/50R21. Fit 235/50R21 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 9,872 on the road, so it over-counts by 128. At a true 70 mph the first turns 768 times a minute, the second 778.