145/75R15 vs 135/75R15
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 | 145/75R15 | 135/75R15 | Change |
|---|---|---|---|
| Overall height | 23.6" | 23.0" | -2.5 % |
| Sidewall | 4.28" | 3.99" | -6.9 % |
| Section width | 145 mm | 135 mm | -6.9 % |
| Circumference | 74.0" | 72.2" | -2.5 % |
| Revs per mile | 856 | 878 | +2.5 % |
| Axle height | 11.78" | 11.49" | -7.5 mm |
| Approved rim width | 4–5.5" | 3.5–5" | overlap 4–5" |
Both are approved on a rim 4" to 5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 145/75R15. Fit 135/75R15 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 9,749 on the road, so it over-counts by 251. At a true 70 mph the first turns 999 times a minute, the second 1024.