225/75R20 vs 215/80R20
Going from one to the other changes the overall height by +0.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/75R20 | 215/80R20 | Change |
|---|---|---|---|
| Overall height | 33.3" | 33.5" | +0.8 % |
| Sidewall | 6.64" | 6.77" | +1.9 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 104.6" | 105.4" | +0.8 % |
| Revs per mile | 606 | 601 | -0.8 % |
| Axle height | 16.64" | 16.77" | +3.3 mm |
| Approved rim width | 6–8.5" | 6–8" | overlap 6–8" |
Both are approved on a rim 6" to 8" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 225/75R20. Fit 215/80R20 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.4 mph | +0.4 |
| 70 mph | 70.5 mph | +0.5 |
| 90 mph | 90.7 mph | +0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,077 on the road, so it under-counts by 77. At a true 70 mph the first turns 707 times a minute, the second 701.