195/80R18 vs 205/75R18
Going from one to the other changes the overall height by -0.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/80R18 | 205/75R18 | Change |
|---|---|---|---|
| Overall height | 30.3" | 30.1" | -0.6 % |
| Sidewall | 6.14" | 6.05" | -1.4 % |
| Section width | 195 mm | 205 mm | +5.1 % |
| Circumference | 95.1" | 94.6" | -0.6 % |
| Revs per mile | 666 | 670 | +0.6 % |
| Axle height | 15.14" | 15.05" | -2.3 mm |
| Approved rim width | 5.5–7" | 5.5–7.5" | overlap 5.5–7" |
Both are approved on a rim 5.5" to 7" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 195/80R18. Fit 205/75R18 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 | 69.6 mph | -0.4 |
| 90 mph | 89.5 mph | -0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,941 on the road, so it over-counts by 59. At a true 70 mph the first turns 777 times a minute, the second 782.