305/35R23 vs 295/40R23
Going from one to the other changes the overall height by +2.8 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 305/35R23 | 295/40R23 | Change |
|---|---|---|---|
| Overall height | 31.4" | 32.3" | +2.8 % |
| Sidewall | 4.20" | 4.65" | +10.5 % |
| Section width | 305 mm | 295 mm | -3.3 % |
| Circumference | 98.7" | 101.4" | +2.8 % |
| Revs per mile | 642 | 625 | -2.8 % |
| Axle height | 15.70" | 16.15" | +11.3 mm |
| Approved rim width | 8.5–11" | 8.5–11" | overlap 8.5–11" |
Both are approved on a rim 8.5" to 11" 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 295/40R23 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.8 mph | +0.8 |
| 50 mph | 51.4 mph | +1.4 |
| 70 mph | 72.0 mph | +2.0 |
| 90 mph | 92.5 mph | +2.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,282 on the road, so it under-counts by 282. At a true 70 mph the first turns 749 times a minute, the second 729.