155/75R15 vs 155/70R15
Going from one to the other changes the overall height by -2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 155/75R15 | 155/70R15 | Change |
|---|---|---|---|
| Overall height | 24.2" | 23.5" | -2.5 % |
| Sidewall | 4.58" | 4.27" | -6.7 % |
| Section width | 155 mm | 155 mm | 0.0 % |
| Circumference | 75.9" | 74.0" | -2.5 % |
| Revs per mile | 835 | 857 | +2.5 % |
| Axle height | 12.08" | 11.77" | -7.8 mm |
| Approved rim width | 4–5.5" | 4–5.5" | overlap 4–5.5" |
Both are approved on a rim 4" 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 155/75R15. Fit 155/70R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.7 mph | -1.3 |
| 70 mph | 68.2 mph | -1.8 |
| 90 mph | 87.7 mph | -2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,747 on the road, so it over-counts by 253. At a true 70 mph the first turns 974 times a minute, the second 999.