245/70R18 vs 225/80R18
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 | 245/70R18 | 225/80R18 | Change |
|---|---|---|---|
| Overall height | 31.5" | 32.2" | +2.1 % |
| Sidewall | 6.75" | 7.09" | +5.0 % |
| Section width | 245 mm | 225 mm | -8.2 % |
| Circumference | 99.0" | 101.1" | +2.1 % |
| Revs per mile | 640 | 627 | -2.1 % |
| Axle height | 15.75" | 16.09" | +8.5 mm |
| Approved rim width | 7–9" | 6–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/70R18. Fit 225/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.5 mph | +1.5 |
| 90 mph | 91.9 mph | +1.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,212 on the road, so it under-counts by 212. At a true 70 mph the first turns 747 times a minute, the second 731.