305/40R20 vs 295/40R20
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 | 305/40R20 | 295/40R20 | Change |
|---|---|---|---|
| Overall height | 29.6" | 29.3" | -1.1 % |
| Sidewall | 4.80" | 4.65" | -3.3 % |
| Section width | 305 mm | 295 mm | -3.3 % |
| Circumference | 93.0" | 92.0" | -1.1 % |
| Revs per mile | 681 | 689 | +1.1 % |
| Axle height | 14.80" | 14.65" | -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 305/40R20. Fit 295/40R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,894 on the road, so it over-counts by 106. At a true 70 mph the first turns 795 times a minute, the second 803.