215/80R20 vs 235/70R20
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/80R20 | 235/70R20 | Change |
|---|---|---|---|
| Overall height | 33.5" | 33.0" | -1.8 % |
| Sidewall | 6.77" | 6.48" | -4.4 % |
| Section width | 215 mm | 235 mm | +9.3 % |
| Circumference | 105.4" | 103.5" | -1.8 % |
| Revs per mile | 601 | 612 | +1.8 % |
| Axle height | 16.77" | 16.48" | -7.5 mm |
| Approved rim width | 6–8" | 6.5–8.5" | 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 215/80R20. Fit 235/70R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,824 on the road, so it over-counts by 176. At a true 70 mph the first turns 701 times a minute, the second 714.