255/35R23 vs 275/30R23
Going from one to the other changes the overall height by -1.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/35R23 | 275/30R23 | Change |
|---|---|---|---|
| Overall height | 30.0" | 29.5" | -1.8 % |
| Sidewall | 3.51" | 3.25" | -7.6 % |
| Section width | 255 mm | 275 mm | +7.8 % |
| Circumference | 94.3" | 92.7" | -1.8 % |
| Revs per mile | 672 | 684 | +1.8 % |
| Axle height | 15.01" | 14.75" | -6.8 mm |
| Approved rim width | 7–9.5" | 7.5–10" | overlap 7.5–9.5" |
Both are approved on a rim 7.5" to 9.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 255/35R23. Fit 275/30R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.5 mph | -0.5 |
| 50 mph | 49.1 mph | -0.9 |
| 70 mph | 68.8 mph | -1.2 |
| 90 mph | 88.4 mph | -1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,823 on the road, so it over-counts by 177. At a true 70 mph the first turns 784 times a minute, the second 798.