175/70R15 vs 185/70R15
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 175/70R15 | 185/70R15 | Change |
|---|---|---|---|
| Overall height | 24.6" | 25.2" | +2.2 % |
| Sidewall | 4.82" | 5.10" | +5.7 % |
| Section width | 175 mm | 185 mm | +5.7 % |
| Circumference | 77.4" | 79.2" | +2.2 % |
| Revs per mile | 818 | 800 | -2.2 % |
| Axle height | 12.32" | 12.60" | +7.0 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 175/70R15. Fit 185/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,224 on the road, so it under-counts by 224. At a true 70 mph the first turns 955 times a minute, the second 934.
What drivers reported
175/70R15 carries 2 complaints. Nobody has filed one naming 185/70R15.