195/80R18 vs 195/85R18
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/80R18 | 195/85R18 | Change |
|---|---|---|---|
| Overall height | 30.3" | 31.1" | +2.5 % |
| Sidewall | 6.14" | 6.53" | +6.3 % |
| Section width | 195 mm | 195 mm | 0.0 % |
| Circumference | 95.1" | 97.6" | +2.5 % |
| Revs per mile | 666 | 650 | -2.5 % |
| Axle height | 15.14" | 15.53" | +9.8 mm |
| Approved rim width | 5.5–7" | 5.5–7" | 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 195/85R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,254 on the road, so it under-counts by 254. At a true 70 mph the first turns 777 times a minute, the second 758.