125/75R17 vs 145/70R17
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 | 125/75R17 | 145/70R17 | Change |
|---|---|---|---|
| Overall height | 24.4" | 25.0" | +2.5 % |
| Sidewall | 3.69" | 4.00" | +8.3 % |
| Section width | 125 mm | 145 mm | +16.0 % |
| Circumference | 76.6" | 78.5" | +2.5 % |
| Revs per mile | 827 | 807 | -2.5 % |
| Axle height | 12.19" | 12.50" | +7.8 mm |
| Approved rim width | 3.5–4.5" | 4–5.5" | overlap 4–4.5" |
Both are approved on a rim 4" 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 125/75R17. Fit 145/70R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.3 mph | +1.3 |
| 70 mph | 71.8 mph | +1.8 |
| 90 mph | 92.3 mph | +2.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,250 on the road, so it under-counts by 250. At a true 70 mph the first turns 965 times a minute, the second 941.