195/40R18 vs 185/45R18
Going from one to the other changes the overall height by +1.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 195/40R18 | 185/45R18 | Change |
|---|---|---|---|
| Overall height | 24.1" | 24.6" | +1.7 % |
| Sidewall | 3.07" | 3.28" | +6.7 % |
| Section width | 195 mm | 185 mm | -5.1 % |
| Circumference | 75.8" | 77.1" | +1.7 % |
| Revs per mile | 835 | 821 | -1.7 % |
| Axle height | 12.07" | 12.28" | +5.3 mm |
| Approved rim width | 5.5–7" | 5–6.5" | overlap 5.5–6.5" |
Both are approved on a rim 5.5" to 6.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 195/40R18. Fit 185/45R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.2 mph | +1.2 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,171 on the road, so it under-counts by 171. At a true 70 mph the first turns 975 times a minute, the second 958.