225/80R18 vs 215/80R18
Going from one to the other changes the overall height by -2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/80R18 | 215/80R18 | Change |
|---|---|---|---|
| Overall height | 32.2" | 31.5" | -2.0 % |
| Sidewall | 7.09" | 6.77" | -4.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 101.1" | 99.1" | -2.0 % |
| Revs per mile | 627 | 639 | +2.0 % |
| Axle height | 16.09" | 15.77" | -8.0 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 225/80R18. Fit 215/80R18 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 | 68.6 mph | -1.4 |
| 90 mph | 88.2 mph | -1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,804 on the road, so it over-counts by 196. At a true 70 mph the first turns 731 times a minute, the second 746.