255/75R18 vs 245/80R18
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 | 255/75R18 | 245/80R18 | Change |
|---|---|---|---|
| Overall height | 33.1" | 33.4" | +1.1 % |
| Sidewall | 7.53" | 7.72" | +2.5 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 103.9" | 105.0" | +1.1 % |
| Revs per mile | 610 | 603 | -1.1 % |
| Axle height | 16.53" | 16.72" | +4.8 mm |
| Approved rim width | 7–9.5" | 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 255/75R18. Fit 245/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,113 on the road, so it under-counts by 113. At a true 70 mph the first turns 712 times a minute, the second 704.