245/75R21 vs 235/75R21
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/75R21 | 235/75R21 | Change |
|---|---|---|---|
| Overall height | 35.5" | 34.9" | -1.7 % |
| Sidewall | 7.23" | 6.94" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 111.4" | 109.6" | -1.7 % |
| Revs per mile | 569 | 578 | +1.7 % |
| Axle height | 17.73" | 17.44" | -7.5 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 245/75R21. Fit 235/75R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.2 mph | -0.8 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.5 mph | -1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,833 on the road, so it over-counts by 167. At a true 70 mph the first turns 663 times a minute, the second 675.