235/40R21 vs 245/35R21
Going from one to the other changes the overall height by -2.3 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 235/40R21 | 245/35R21 | Change |
|---|---|---|---|
| Overall height | 28.4" | 27.8" | -2.3 % |
| Sidewall | 3.70" | 3.38" | -8.8 % |
| Section width | 235 mm | 245 mm | +4.3 % |
| Circumference | 89.2" | 87.2" | -2.3 % |
| Revs per mile | 710 | 727 | +2.3 % |
| Axle height | 14.20" | 13.88" | -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 235/40R21. Fit 245/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.9 mph | -1.1 |
| 70 mph | 68.4 mph | -1.6 |
| 90 mph | 87.9 mph | -2.1 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,771 on the road, so it over-counts by 229. At a true 70 mph the first turns 828 times a minute, the second 848.