145/75R14 vs 145/70R14
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 | 145/75R14 | 145/70R14 | Change |
|---|---|---|---|
| Overall height | 22.6" | 22.0" | -2.5 % |
| Sidewall | 4.28" | 4.00" | -6.7 % |
| Section width | 145 mm | 145 mm | 0.0 % |
| Circumference | 70.9" | 69.1" | -2.5 % |
| Revs per mile | 894 | 917 | +2.5 % |
| Axle height | 11.28" | 11.00" | -7.3 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 145/75R14. Fit 145/70R14 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 1043 times a minute, the second 1070.