305/35R23 vs 285/40R23
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 | 305/35R23 | 285/40R23 | Change |
|---|---|---|---|
| Overall height | 31.4" | 32.0" | +1.8 % |
| Sidewall | 4.20" | 4.49" | +6.8 % |
| Section width | 305 mm | 285 mm | -6.6 % |
| Circumference | 98.7" | 100.5" | +1.8 % |
| Revs per mile | 642 | 631 | -1.8 % |
| Axle height | 15.70" | 15.99" | +7.3 mm |
| Approved rim width | 8.5–11" | 8–10.5" | overlap 8.5–10.5" |
Both are approved on a rim 8.5" to 10.5" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 305/35R23. Fit 285/40R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.9 mph | +0.9 |
| 70 mph | 71.3 mph | +1.3 |
| 90 mph | 91.6 mph | +1.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,182 on the road, so it under-counts by 182. At a true 70 mph the first turns 749 times a minute, the second 736.
What drivers reported
Nobody has filed a complaint naming 305/35R23. 285/40R23 carries 1.