245/35R21 vs 255/30R21
Going from one to the other changes the overall height by -2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/35R21 | 255/30R21 | Change |
|---|---|---|---|
| Overall height | 27.8" | 27.0" | -2.6 % |
| Sidewall | 3.38" | 3.01" | -10.8 % |
| Section width | 245 mm | 255 mm | +4.1 % |
| Circumference | 87.2" | 84.9" | -2.6 % |
| Revs per mile | 727 | 746 | +2.6 % |
| Axle height | 13.88" | 13.51" | -9.3 mm |
| Approved rim width | 7–9" | 7–9.5" | overlap 7–9" |
Both are approved on a rim 7" 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 255/30R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,738 on the road, so it over-counts by 262. At a true 70 mph the first turns 848 times a minute, the second 871.