165/65R15 vs 145/75R15
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 | 165/65R15 | 145/75R15 | Change |
|---|---|---|---|
| Overall height | 23.4" | 23.6" | +0.5 % |
| Sidewall | 4.22" | 4.28" | +1.4 % |
| Section width | 165 mm | 145 mm | -12.1 % |
| Circumference | 73.7" | 74.0" | +0.5 % |
| Revs per mile | 860 | 856 | -0.5 % |
| Axle height | 11.72" | 11.78" | +1.5 mm |
| Approved rim width | 4.5–6" | 4–5.5" | 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 165/65R15. Fit 145/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.2 mph | +0.2 |
| 50 mph | 50.3 mph | +0.3 |
| 70 mph | 70.4 mph | +0.4 |
| 90 mph | 90.5 mph | +0.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,050 on the road, so it under-counts by 50. At a true 70 mph the first turns 1004 times a minute, the second 999.