235/40R15 vs 245/35R15
Going from one to the other changes the overall height by -2.9 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/40R15 | 245/35R15 | Change |
|---|---|---|---|
| Overall height | 22.4" | 21.8" | -2.9 % |
| Sidewall | 3.70" | 3.38" | -8.8 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 70.4" | 68.3" | -2.9 % |
| Revs per mile | 900 | 927 | +2.9 % |
| Axle height | 11.20" | 10.88" | -8.3 mm |
| Approved rim width | 6.5–8.5" | 7–9" | 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 235/40R15. Fit 245/35R15 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.1 mph | -0.9 |
| 50 mph | 48.6 mph | -1.4 |
| 70 mph | 68.0 mph | -2.0 |
| 90 mph | 87.4 mph | -2.6 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,710 on the road, so it over-counts by 290. At a true 70 mph the first turns 1050 times a minute, the second 1082.