245/55R21 vs 235/55R21
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/55R21 | 235/55R21 | Change |
|---|---|---|---|
| Overall height | 31.6" | 31.2" | -1.4 % |
| Sidewall | 5.31" | 5.09" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 99.3" | 97.9" | -1.4 % |
| Revs per mile | 638 | 647 | +1.4 % |
| Axle height | 15.81" | 15.59" | -5.5 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 245/55R21. Fit 235/55R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,863 on the road, so it over-counts by 137. At a true 70 mph the first turns 744 times a minute, the second 755.