205/75R17.5 vs 205/70R17.5
Going from one to the other changes the overall height by -2.7 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 205/75R17.5 | 205/70R17.5 | Change |
|---|---|---|---|
| Overall height | 29.6" | 28.8" | -2.7 % |
| Sidewall | 6.05" | 5.65" | -6.7 % |
| Section width | 205 mm | 205 mm | 0.0 % |
| Circumference | 93.0" | 90.5" | -2.7 % |
| Revs per mile | 681 | 700 | +2.7 % |
| Axle height | 14.80" | 14.40" | -10.3 mm |
| Approved rim width | 5.5–7.5" | 5.5–7.5" | overlap 5.5–7.5" |
Both are approved on a rim 5.5" to 7.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 205/75R17.5. Fit 205/70R17.5 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.2 mph | -0.8 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.1 mph | -1.9 |
| 90 mph | 87.5 mph | -2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,727 on the road, so it over-counts by 273. At a true 70 mph the first turns 795 times a minute, the second 817.