235/35R20 vs 215/35R20
Going from one to the other changes the overall height by -2.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/35R20 | 215/35R20 | Change |
|---|---|---|---|
| Overall height | 26.5" | 25.9" | -2.1 % |
| Sidewall | 3.24" | 2.96" | -8.5 % |
| Section width | 235 mm | 215 mm | -8.5 % |
| Circumference | 83.2" | 81.4" | -2.1 % |
| Revs per mile | 762 | 778 | +2.1 % |
| Axle height | 13.24" | 12.96" | -7.0 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 235/35R20. Fit 215/35R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.4 mph | -0.6 |
| 50 mph | 49.0 mph | -1.0 |
| 70 mph | 68.5 mph | -1.5 |
| 90 mph | 88.1 mph | -1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,792 on the road, so it over-counts by 208. At a true 70 mph the first turns 889 times a minute, the second 908.