185/70R18 vs 185/75R18
Going from one to the other changes the overall height by +2.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 185/70R18 | 185/75R18 | Change |
|---|---|---|---|
| Overall height | 28.2" | 28.9" | +2.6 % |
| Sidewall | 5.10" | 5.46" | +7.1 % |
| Section width | 185 mm | 185 mm | 0.0 % |
| Circumference | 88.6" | 90.9" | +2.6 % |
| Revs per mile | 715 | 697 | -2.6 % |
| Axle height | 14.10" | 14.46" | +9.3 mm |
| Approved rim width | 5–6.5" | 5–6.5" | overlap 5–6.5" |
Both are approved on a rim 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 185/70R18. Fit 185/75R18 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 10,258 on the road, so it under-counts by 258. At a true 70 mph the first turns 834 times a minute, the second 813.