295/60R21 vs 285/65R21
Going from one to the other changes the overall height by +1.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/60R21 | 285/65R21 | Change |
|---|---|---|---|
| Overall height | 34.9" | 35.6" | +1.9 % |
| Sidewall | 6.97" | 7.29" | +4.7 % |
| Section width | 295 mm | 285 mm | -3.4 % |
| Circumference | 109.8" | 111.8" | +1.9 % |
| Revs per mile | 577 | 567 | -1.9 % |
| Axle height | 17.47" | 17.79" | +8.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | 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 295/60R21. Fit 285/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.7 mph | +1.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,186 on the road, so it under-counts by 186. At a true 70 mph the first turns 673 times a minute, the second 661.