245/45R17 vs 235/45R17
Going from one to the other changes the overall height by -1.4 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/45R17 | 235/45R17 | Change |
|---|---|---|---|
| Overall height | 25.7" | 25.3" | -1.4 % |
| Sidewall | 4.34" | 4.16" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 80.7" | 79.6" | -1.4 % |
| Revs per mile | 785 | 796 | +1.4 % |
| Axle height | 12.84" | 12.66" | -4.5 mm |
| Approved rim width | 7–9" | 6.5–8.5" | overlap 7–8.5" |
Both are approved on a rim 7" to 8.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 245/45R17. Fit 235/45R17 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.6 mph | -0.4 |
| 50 mph | 49.3 mph | -0.7 |
| 70 mph | 69.0 mph | -1.0 |
| 90 mph | 88.8 mph | -1.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,862 on the road, so it over-counts by 138. At a true 70 mph the first turns 916 times a minute, the second 929.