305/75R22.5 vs 305/70R22.5
Going from one to the other changes the overall height by -3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/75R22.5 | 305/70R22.5 | Change |
|---|---|---|---|
| Overall height | 40.5" | 39.3" | -3.0 % |
| Sidewall | 9.01" | 8.41" | -6.7 % |
| Section width | 305 mm | 305 mm | 0.0 % |
| Circumference | 127.3" | 123.5" | -3.0 % |
| Revs per mile | 498 | 513 | +3.0 % |
| Axle height | 20.26" | 19.66" | -15.3 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 305/75R22.5. Fit 305/70R22.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.5 mph | -1.5 |
| 70 mph | 67.9 mph | -2.1 |
| 90 mph | 87.3 mph | -2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,704 on the road, so it over-counts by 296. At a true 70 mph the first turns 581 times a minute, the second 599.