225/65R18 vs 205/70R18
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 | 225/65R18 | 205/70R18 | Change |
|---|---|---|---|
| Overall height | 29.5" | 29.3" | -0.7 % |
| Sidewall | 5.76" | 5.65" | -1.9 % |
| Section width | 225 mm | 205 mm | -8.9 % |
| Circumference | 92.7" | 92.0" | -0.7 % |
| Revs per mile | 683 | 688 | +0.7 % |
| Axle height | 14.76" | 14.65" | -2.8 mm |
| Approved rim width | 6–8.5" | 5.5–7.5" | overlap 6–7.5" |
Both are approved on a rim 6" to 7.5" 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 205/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.8 mph | -0.2 |
| 50 mph | 49.6 mph | -0.4 |
| 70 mph | 69.5 mph | -0.5 |
| 90 mph | 89.3 mph | -0.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,927 on the road, so it over-counts by 73. At a true 70 mph the first turns 797 times a minute, the second 803.