295/65R20 vs 305/60R20
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/65R20 | 305/60R20 | Change |
|---|---|---|---|
| Overall height | 35.1" | 34.4" | -2.0 % |
| Sidewall | 7.55" | 7.20" | -4.6 % |
| Section width | 295 mm | 305 mm | +3.4 % |
| Circumference | 110.3" | 108.1" | -2.0 % |
| Revs per mile | 575 | 586 | +2.0 % |
| Axle height | 17.55" | 17.20" | -8.8 mm |
| Approved rim width | 8.5–11" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 295/65R20. Fit 305/60R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,804 on the road, so it over-counts by 196. At a true 70 mph the first turns 670 times a minute, the second 684.