305/60R21 vs 295/65R21
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 | 305/60R21 | 295/65R21 | Change |
|---|---|---|---|
| Overall height | 35.4" | 36.1" | +1.9 % |
| Sidewall | 7.20" | 7.55" | +4.8 % |
| Section width | 305 mm | 295 mm | -3.3 % |
| Circumference | 111.2" | 113.4" | +1.9 % |
| Revs per mile | 570 | 559 | -1.9 % |
| Axle height | 17.70" | 18.05" | +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 305/60R21. Fit 295/65R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,195 on the road, so it under-counts by 195. At a true 70 mph the first turns 664 times a minute, the second 652.