265/35R21 vs 245/40R21
Going from one to the other changes the overall height by +1.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 265/35R21 | 245/40R21 | Change |
|---|---|---|---|
| Overall height | 28.3" | 28.7" | +1.5 % |
| Sidewall | 3.65" | 3.86" | +5.7 % |
| Section width | 265 mm | 245 mm | -7.5 % |
| Circumference | 88.9" | 90.2" | +1.5 % |
| Revs per mile | 713 | 702 | -1.5 % |
| Axle height | 14.15" | 14.36" | +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 265/35R21. Fit 245/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.4 mph | +0.4 |
| 50 mph | 50.7 mph | +0.7 |
| 70 mph | 71.0 mph | +1.0 |
| 90 mph | 91.3 mph | +1.3 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,146 on the road, so it under-counts by 146. At a true 70 mph the first turns 831 times a minute, the second 819.