245/80R19.5 vs 245/75R19.5
Going from one to the other changes the overall height by -2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/80R19.5 | 245/75R19.5 | Change |
|---|---|---|---|
| Overall height | 34.9" | 34.0" | -2.8 % |
| Sidewall | 7.72" | 7.23" | -6.3 % |
| Section width | 245 mm | 245 mm | 0.0 % |
| Circumference | 109.7" | 106.7" | -2.8 % |
| Revs per mile | 577 | 594 | +2.8 % |
| Axle height | 17.47" | 16.98" | -12.3 mm |
| Approved rim width | 7–9" | 7–9" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/80R19.5. Fit 245/75R19.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,724 on the road, so it over-counts by 276. At a true 70 mph the first turns 674 times a minute, the second 693.