245/35R21 vs 265/30R21
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/35R21 | 265/30R21 | Change |
|---|---|---|---|
| Overall height | 27.8" | 27.3" | -1.8 % |
| Sidewall | 3.38" | 3.13" | -7.3 % |
| Section width | 245 mm | 265 mm | +8.2 % |
| Circumference | 87.2" | 85.6" | -1.8 % |
| Revs per mile | 727 | 740 | +1.8 % |
| Axle height | 13.88" | 13.63" | -6.3 mm |
| Approved rim width | 7–9" | 7.5–9.5" | overlap 7.5–9" |
Both are approved on a rim 7.5" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/35R21. Fit 265/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,823 on the road, so it over-counts by 177. At a true 70 mph the first turns 848 times a minute, the second 863.