295/45R21 vs 305/45R21
Going from one to the other changes the overall height by +1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 295/45R21 | 305/45R21 | Change |
|---|---|---|---|
| Overall height | 31.5" | 31.8" | +1.1 % |
| Sidewall | 5.23" | 5.40" | +3.4 % |
| Section width | 295 mm | 305 mm | +3.4 % |
| Circumference | 98.8" | 99.9" | +1.1 % |
| Revs per mile | 641 | 634 | -1.1 % |
| Axle height | 15.73" | 15.90" | +4.5 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 295/45R21. Fit 305/45R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,113 on the road, so it under-counts by 113. At a true 70 mph the first turns 748 times a minute, the second 740.