285/80R19.5 vs 285/75R19.5
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 | 285/80R19.5 | 285/75R19.5 | Change |
|---|---|---|---|
| Overall height | 37.5" | 36.3" | -3.0 % |
| Sidewall | 8.98" | 8.42" | -6.3 % |
| Section width | 285 mm | 285 mm | 0.0 % |
| Circumference | 117.7" | 114.1" | -3.0 % |
| Revs per mile | 538 | 555 | +3.0 % |
| Axle height | 18.73" | 18.17" | -14.3 mm |
| Approved rim width | 8–10.5" | 8–10.5" | overlap 8–10.5" |
Both are approved on a rim 8" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 285/80R19.5. Fit 285/75R19.5 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 9,700 on the road, so it over-counts by 300. At a true 70 mph the first turns 628 times a minute, the second 648.