325/40R21 vs 345/35R21
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 | 325/40R21 | 345/35R21 | Change |
|---|---|---|---|
| Overall height | 31.2" | 30.5" | -2.3 % |
| Sidewall | 5.12" | 4.75" | -7.1 % |
| Section width | 325 mm | 345 mm | +6.2 % |
| Circumference | 98.1" | 95.8" | -2.3 % |
| Revs per mile | 646 | 661 | +2.3 % |
| Axle height | 15.62" | 15.25" | -9.3 mm |
| Approved rim width | 9–12" | 9.5–12.5" | overlap 9.5–12" |
Both are approved on a rim 9.5" to 12" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/40R21. Fit 345/35R21 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 29.3 mph | -0.7 |
| 50 mph | 48.8 mph | -1.2 |
| 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 9,767 on the road, so it over-counts by 233. At a true 70 mph the first turns 753 times a minute, the second 771.