235/80R19 vs 255/70R19
Going from one to the other changes the overall height by -2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/80R19 | 255/70R19 | Change |
|---|---|---|---|
| Overall height | 33.8" | 33.1" | -2.2 % |
| Sidewall | 7.40" | 7.03" | -5.1 % |
| Section width | 235 mm | 255 mm | +8.5 % |
| Circumference | 106.2" | 103.8" | -2.2 % |
| Revs per mile | 597 | 610 | +2.2 % |
| Axle height | 16.90" | 16.53" | -9.5 mm |
| Approved rim width | 6.5–8.5" | 7–9.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/80R19. Fit 255/70R19 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,779 on the road, so it over-counts by 221. At a true 70 mph the first turns 696 times a minute, the second 712.