325/50R24 vs 305/55R24
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 | 325/50R24 | 305/55R24 | Change |
|---|---|---|---|
| Overall height | 36.8" | 37.2" | +1.1 % |
| Sidewall | 6.40" | 6.60" | +3.2 % |
| Section width | 325 mm | 305 mm | -6.2 % |
| Circumference | 115.6" | 116.9" | +1.1 % |
| Revs per mile | 548 | 542 | -1.1 % |
| Axle height | 18.40" | 18.60" | +5.3 mm |
| Approved rim width | 9–12" | 8.5–11" | overlap 9–11" |
Both are approved on a rim 9" to 11" wide. If yours is in that band, this is a tire change and nothing else.
What the speedometer will say
Calibrated for 325/50R24. Fit 305/55R24 and the needle stops telling the truth.
| Needle says | You are doing | Out by |
|---|---|---|
| 30 mph | 30.3 mph | +0.3 |
| 50 mph | 50.6 mph | +0.6 |
| 70 mph | 70.8 mph | +0.8 |
| 90 mph | 91.0 mph | +1.0 |
The odometer drifts with it. Ten thousand miles on the dial are really 10,112 on the road, so it under-counts by 112. At a true 70 mph the first turns 639 times a minute, the second 632.