245/35R15 vs 235/40R15
Going from one to the other changes the overall height by +3.0 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/35R15 | 235/40R15 | Change |
|---|---|---|---|
| Overall height | 21.8" | 22.4" | +3.0 % |
| Sidewall | 3.38" | 3.70" | +9.6 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 68.3" | 70.4" | +3.0 % |
| Revs per mile | 927 | 900 | -3.0 % |
| Axle height | 10.88" | 11.20" | +8.3 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 245/35R15. Fit 235/40R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.9 mph | +0.9 |
| 50 mph | 51.5 mph | +1.5 |
| 70 mph | 72.1 mph | +2.1 |
| 90 mph | 92.7 mph | +2.7 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,299 on the road, so it under-counts by 299. At a true 70 mph the first turns 1082 times a minute, the second 1050.