215/85R18 vs 225/80R18
Going from one to the other changes the overall height by -0.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/85R18 | 225/80R18 | Change |
|---|---|---|---|
| Overall height | 32.4" | 32.2" | -0.7 % |
| Sidewall | 7.19" | 7.09" | -1.5 % |
| Section width | 215 mm | 225 mm | +4.7 % |
| Circumference | 101.8" | 101.1" | -0.7 % |
| Revs per mile | 623 | 627 | +0.7 % |
| Axle height | 16.19" | 16.09" | -2.8 mm |
| Approved rim width | 6–8" | 6–8.5" | 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 215/85R18. Fit 225/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.7 mph | -0.3 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.4 mph | -0.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,933 on the road, so it over-counts by 67. At a true 70 mph the first turns 726 times a minute, the second 731.