135/65R15 vs 125/65R15
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 | 135/65R15 | 125/65R15 | Change |
|---|---|---|---|
| Overall height | 21.9" | 21.4" | -2.3 % |
| Sidewall | 3.45" | 3.20" | -7.4 % |
| Section width | 135 mm | 125 mm | -7.4 % |
| Circumference | 68.8" | 67.2" | -2.3 % |
| Revs per mile | 921 | 943 | +2.3 % |
| Axle height | 10.95" | 10.70" | -6.5 mm |
| Approved rim width | 3.5–5" | 3.5–4.5" | overlap 3.5–4.5" |
Both are approved on a rim 3.5" to 4.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 135/65R15. Fit 125/65R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,766 on the road, so it over-counts by 234. At a true 70 mph the first turns 1074 times a minute, the second 1100.