155/80R15 vs 165/70R15
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/80R15 | 165/70R15 | Change |
|---|---|---|---|
| Overall height | 24.8" | 24.1" | -2.7 % |
| Sidewall | 4.88" | 4.55" | -6.9 % |
| Section width | 155 mm | 165 mm | +6.5 % |
| Circumference | 77.8" | 75.7" | -2.7 % |
| Revs per mile | 814 | 837 | +2.7 % |
| Axle height | 12.38" | 12.05" | -8.5 mm |
| Approved rim width | 4–5.5" | 4.5–6" | overlap 4.5–5.5" |
Both are approved on a rim 4.5" to 5.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 155/80R15. Fit 165/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.6 mph | -2.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,730 on the road, so it over-counts by 270. At a true 70 mph the first turns 950 times a minute, the second 977.