125/70R18 vs 125/75R18
Going from one to the other changes the overall height by +2.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/70R18 | 125/75R18 | Change |
|---|---|---|---|
| Overall height | 24.9" | 25.4" | +2.0 % |
| Sidewall | 3.44" | 3.69" | +7.1 % |
| Section width | 125 mm | 125 mm | 0.0 % |
| Circumference | 78.2" | 79.7" | +2.0 % |
| Revs per mile | 810 | 795 | -2.0 % |
| Axle height | 12.44" | 12.69" | +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 125/70R18. Fit 125/75R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.6 mph | +0.6 |
| 50 mph | 51.0 mph | +1.0 |
| 70 mph | 71.4 mph | +1.4 |
| 90 mph | 91.8 mph | +1.8 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,198 on the road, so it under-counts by 198. At a true 70 mph the first turns 945 times a minute, the second 927.
What drivers reported
125/70R18 carries 12 complaints. Nobody has filed one naming 125/75R18.