125/70R15 vs 125/75R15
Going from one to the other changes the overall height by +2.2 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/70R15 | 125/75R15 | Change |
|---|---|---|---|
| Overall height | 21.9" | 22.4" | +2.2 % |
| Sidewall | 3.44" | 3.69" | +7.1 % |
| Section width | 125 mm | 125 mm | 0.0 % |
| Circumference | 68.8" | 70.3" | +2.2 % |
| Revs per mile | 921 | 901 | -2.2 % |
| Axle height | 10.94" | 11.19" | +6.3 mm |
| Approved rim width | 3.5–4.5" | 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 125/70R15. Fit 125/75R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.1 mph | +1.1 |
| 70 mph | 71.6 mph | +1.6 |
| 90 mph | 92.0 mph | +2.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,225 on the road, so it under-counts by 225. At a true 70 mph the first turns 1075 times a minute, the second 1051.