135/80R15 vs 135/75R15
Going from one to the other changes the overall height by -2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 135/80R15 | 135/75R15 | Change |
|---|---|---|---|
| Overall height | 23.5" | 23.0" | -2.3 % |
| Sidewall | 4.25" | 3.99" | -6.3 % |
| Section width | 135 mm | 135 mm | 0.0 % |
| Circumference | 73.8" | 72.2" | -2.3 % |
| Revs per mile | 858 | 878 | +2.3 % |
| Axle height | 11.75" | 11.49" | -6.8 mm |
| Approved rim width | 3.5–5" | 3.5–5" | overlap 3.5–5" |
Both are approved on a rim 3.5" to 5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 135/80R15. Fit 135/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 88.0 mph | -2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,774 on the road, so it over-counts by 226. At a true 70 mph the first turns 1001 times a minute, the second 1024.