265/70R17.5 vs 245/80R17.5
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/70R17.5 | 245/80R17.5 | Change |
|---|---|---|---|
| Overall height | 32.1" | 32.9" | +2.6 % |
| Sidewall | 7.30" | 7.72" | +5.7 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 100.9" | 103.5" | +2.6 % |
| Revs per mile | 628 | 612 | -2.6 % |
| Axle height | 16.05" | 16.47" | +10.5 mm |
| Approved rim width | 7.5–9.5" | 7–9" | overlap 7.5–9" |
Both are approved on a rim 7.5" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/70R17.5. Fit 245/80R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,258 on the road, so it under-counts by 258. At a true 70 mph the first turns 733 times a minute, the second 714.