225/65R18 vs 215/65R18
Going from one to the other changes the overall height by -1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 225/65R18 | 215/65R18 | Change |
|---|---|---|---|
| Overall height | 29.5" | 29.0" | -1.7 % |
| Sidewall | 5.76" | 5.50" | -4.4 % |
| Section width | 225 mm | 215 mm | -4.4 % |
| Circumference | 92.7" | 91.1" | -1.7 % |
| Revs per mile | 683 | 695 | +1.7 % |
| Axle height | 14.76" | 14.50" | -6.5 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 225/65R18. Fit 215/65R18 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 9,827 on the road, so it over-counts by 173. At a true 70 mph the first turns 797 times a minute, the second 811.