235/75R20 vs 215/80R20
Going from one to the other changes the overall height by -1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/75R20 | 215/80R20 | Change |
|---|---|---|---|
| Overall height | 33.9" | 33.5" | -1.0 % |
| Sidewall | 6.94" | 6.77" | -2.4 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 106.4" | 105.4" | -1.0 % |
| Revs per mile | 595 | 601 | +1.0 % |
| Axle height | 16.94" | 16.77" | -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 235/75R20. Fit 215/80R20 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 | 89.1 mph | -0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,901 on the road, so it over-counts by 99. At a true 70 mph the first turns 695 times a minute, the second 701.