215/75R18 vs 195/80R18
Going from one to the other changes the overall height by -1.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 215/75R18 | 195/80R18 | Change |
|---|---|---|---|
| Overall height | 30.7" | 30.3" | -1.3 % |
| Sidewall | 6.35" | 6.14" | -3.3 % |
| Section width | 215 mm | 195 mm | -9.3 % |
| Circumference | 96.4" | 95.1" | -1.3 % |
| Revs per mile | 657 | 666 | +1.3 % |
| Axle height | 15.35" | 15.14" | -5.3 mm |
| Approved rim width | 6–8" | 5.5–7" | overlap 6–7" |
Both are approved on a rim 6" to 7" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 215/75R18. Fit 195/80R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.1 mph | -0.9 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,865 on the road, so it over-counts by 135. At a true 70 mph the first turns 767 times a minute, the second 777.