235/75R17.5 vs 215/80R17.5
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 | 235/75R17.5 | 215/80R17.5 | Change |
|---|---|---|---|
| Overall height | 31.4" | 31.0" | -1.1 % |
| Sidewall | 6.94" | 6.77" | -2.4 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 98.6" | 97.5" | -1.1 % |
| Revs per mile | 643 | 650 | +1.1 % |
| Axle height | 15.69" | 15.52" | -4.3 mm |
| Approved rim width | 6.5–8.5" | 6–8" | overlap 6.5–8" |
Both are approved on a rim 6.5" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 235/75R17.5. Fit 215/80R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.3 mph | -0.7 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,893 on the road, so it over-counts by 107. At a true 70 mph the first turns 750 times a minute, the second 758.