265/35R18 vs 245/40R18
Going from one to the other changes the overall height by +1.6 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/35R18 | 245/40R18 | Change |
|---|---|---|---|
| Overall height | 25.3" | 25.7" | +1.6 % |
| Sidewall | 3.65" | 3.86" | +5.7 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 79.5" | 80.8" | +1.6 % |
| Revs per mile | 797 | 784 | -1.6 % |
| Axle height | 12.65" | 12.86" | +5.3 mm |
| Approved rim width | 7.5–9.5" | 7–9" | overlap 7.5–9" |
Both are approved on a rim 7.5" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 265/35R18. Fit 245/40R18 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.5 mph | +0.5 |
| 50 mph | 50.8 mph | +0.8 |
| 70 mph | 71.1 mph | +1.1 |
| 90 mph | 91.5 mph | +1.5 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,163 on the road, so it under-counts by 163. At a true 70 mph the first turns 930 times a minute, the second 915.