315/35R24 vs 305/40R24
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 | 315/35R24 | 305/40R24 | Change |
|---|---|---|---|
| Overall height | 32.7" | 33.6" | +2.8 % |
| Sidewall | 4.34" | 4.80" | +10.7 % |
| Section width | 315 mm | 305 mm | -3.2 % |
| Circumference | 102.7" | 105.6" | +2.8 % |
| Revs per mile | 617 | 600 | -2.8 % |
| Axle height | 16.34" | 16.80" | +11.8 mm |
| Approved rim width | 8.5–11.5" | 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 315/35R24. Fit 305/40R24 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 10,283 on the road, so it under-counts by 283. At a true 70 mph the first turns 720 times a minute, the second 700.