145/70R17 vs 145/75R17
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 | 145/70R17 | 145/75R17 | Change |
|---|---|---|---|
| Overall height | 25.0" | 25.6" | +2.3 % |
| Sidewall | 4.00" | 4.28" | +7.1 % |
| Section width | 145 mm | 145 mm | 0.0 % |
| Circumference | 78.5" | 80.3" | +2.3 % |
| Revs per mile | 807 | 789 | -2.3 % |
| Axle height | 12.50" | 12.78" | +7.3 mm |
| Approved rim width | 4–5.5" | 4–5.5" | overlap 4–5.5" |
Both are approved on a rim 4" to 5.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 145/70R17. Fit 145/75R17 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 | 92.1 mph | +2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,228 on the road, so it under-counts by 228. At a true 70 mph the first turns 941 times a minute, the second 920.