255/35R21 vs 245/40R21
Going from one to the other changes the overall height by +2.5 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 255/35R21 | 245/40R21 | Change |
|---|---|---|---|
| Overall height | 28.0" | 28.7" | +2.5 % |
| Sidewall | 3.51" | 3.86" | +9.8 % |
| Section width | 255 mm | 245 mm | -3.9 % |
| Circumference | 88.1" | 90.2" | +2.5 % |
| Revs per mile | 720 | 702 | -2.5 % |
| Axle height | 14.01" | 14.36" | +8.8 mm |
| Approved rim width | 7–9.5" | 7–9" | overlap 7–9" |
Both are approved on a rim 7" to 9" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 255/35R21. Fit 245/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.7 mph | +0.7 |
| 50 mph | 51.2 mph | +1.2 |
| 70 mph | 71.7 mph | +1.7 |
| 90 mph | 92.2 mph | +2.2 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,246 on the road, so it under-counts by 246. At a true 70 mph the first turns 840 times a minute, the second 819.