285/70R21 vs 305/65R21
Going from one to the other changes the overall height by -0.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 285/70R21 | 305/65R21 | Change |
|---|---|---|---|
| Overall height | 36.7" | 36.6" | -0.3 % |
| Sidewall | 7.85" | 7.81" | -0.6 % |
| Section width | 285 mm | 305 mm | +7.0 % |
| Circumference | 115.3" | 115.0" | -0.3 % |
| Revs per mile | 549 | 551 | +0.3 % |
| Axle height | 18.35" | 18.31" | -1.3 mm |
| Approved rim width | 8–10.5" | 8.5–11" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/70R21. Fit 305/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.9 mph | -0.1 |
| 50 mph | 49.9 mph | -0.1 |
| 70 mph | 69.8 mph | -0.2 |
| 90 mph | 89.8 mph | -0.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,973 on the road, so it over-counts by 27. At a true 70 mph the first turns 641 times a minute, the second 643.