245/40R21 vs 235/40R21
Going from one to the other changes the overall height by -1.1 %. That is inside the 3% band, so it fits and the speedometer stays close.
Side by side
| Measure | 245/40R21 | 235/40R21 | Change |
|---|---|---|---|
| Overall height | 28.7" | 28.4" | -1.1 % |
| Sidewall | 3.86" | 3.70" | -4.1 % |
| Section width | 245 mm | 235 mm | -4.1 % |
| Circumference | 90.2" | 89.2" | -1.1 % |
| Revs per mile | 702 | 710 | +1.1 % |
| Axle height | 14.36" | 14.20" | -4.0 mm |
| Approved rim width | 7–9" | 6.5–8.5" | 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 245/40R21. Fit 235/40R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.7 mph | -0.3 |
| 50 mph | 49.5 mph | -0.5 |
| 70 mph | 69.2 mph | -0.8 |
| 90 mph | 89.0 mph | -1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 9,890 on the road, so it over-counts by 110. At a true 70 mph the first turns 819 times a minute, the second 828.