215/65R18 vs 205/70R18
Going from one to the other changes the overall height by +1.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/65R18 | 205/70R18 | Change |
|---|---|---|---|
| Overall height | 29.0" | 29.3" | +1.0 % |
| Sidewall | 5.50" | 5.65" | +2.7 % |
| Section width | 215 mm | 205 mm | -4.7 % |
| Circumference | 91.1" | 92.0" | +1.0 % |
| Revs per mile | 695 | 688 | -1.0 % |
| Axle height | 14.50" | 14.65" | +3.8 mm |
| Approved rim width | 6–8" | 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 215/65R18. Fit 205/70R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.5 mph | +0.5 |
| 70 mph | 70.7 mph | +0.7 |
| 90 mph | 90.9 mph | +0.9 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,102 on the road, so it under-counts by 102. At a true 70 mph the first turns 811 times a minute, the second 803.