175/75R15 vs 185/70R15
Going from one to the other changes the overall height by -0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/75R15 | 185/70R15 | Change |
|---|---|---|---|
| Overall height | 25.3" | 25.2" | -0.5 % |
| Sidewall | 5.17" | 5.10" | -1.3 % |
| Section width | 175 mm | 185 mm | +5.7 % |
| Circumference | 79.6" | 79.2" | -0.5 % |
| Revs per mile | 796 | 800 | +0.5 % |
| Axle height | 12.67" | 12.60" | -1.8 mm |
| Approved rim width | 5–6" | 5–6.5" | overlap 5–6" |
Both are approved on a rim 5" to 6" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 175/75R15. Fit 185/70R15 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 9,946 on the road, so it over-counts by 54. At a true 70 mph the first turns 929 times a minute, the second 934.