125/80R17 vs 145/70R17
Going from one to the other changes the overall height by +0.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 125/80R17 | 145/70R17 | Change |
|---|---|---|---|
| Overall height | 24.9" | 25.0" | +0.5 % |
| Sidewall | 3.94" | 4.00" | +1.5 % |
| Section width | 125 mm | 145 mm | +16.0 % |
| Circumference | 78.1" | 78.5" | +0.5 % |
| Revs per mile | 811 | 807 | -0.5 % |
| Axle height | 12.44" | 12.50" | +1.5 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 125/80R17. Fit 145/70R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.1 mph | +0.1 |
| 50 mph | 50.2 mph | +0.2 |
| 70 mph | 70.3 mph | +0.3 |
| 90 mph | 90.4 mph | +0.4 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,047 on the road, so it under-counts by 47. At a true 70 mph the first turns 946 times a minute, the second 941.