295/40R20 vs 305/40R20
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 | 295/40R20 | 305/40R20 | Change |
|---|---|---|---|
| Overall height | 29.3" | 29.6" | +1.1 % |
| Sidewall | 4.65" | 4.80" | +3.4 % |
| Section width | 295 mm | 305 mm | +3.4 % |
| Circumference | 92.0" | 93.0" | +1.1 % |
| Revs per mile | 689 | 681 | -1.1 % |
| Axle height | 14.65" | 14.80" | +4.0 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 295/40R20. Fit 305/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 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 10,108 on the road, so it under-counts by 108. At a true 70 mph the first turns 803 times a minute, the second 795.