The maximum speed a Porter engine can travel while pulling the maximum load is found by dividing the diameter of the driving wheels by a factor determined by the length of the piston stroke. The table for converting piston stroke to the factor is as follows:
| Stroke (in) | 8 | 10 | 12 | 14 | 16 | 18 | 20 | 22 | 24 | 26 | 28 |
| Factor | 5 | 5.2 | 5.4 | 5.6 | 5.8 | 6 | 6.2 | 6.4 | 6.6 | 6.8 | 7 |
Since both engine types have 16" piston strokes and 33" diameter driver wheels, their maximum speed is 33÷5.8 or 5.68 mph
How fast is the engine going on the layout?
Because I use 24 inch tie spacing and no fast clock, an engineer can count the number of ties passed each second for an idea of how fast the engine is going. The maximum speed at maximum draw of 5.68 mph converts to 5.68×88÷60 = 8.33 ft per second (call it 4 ties per second).
How about going faster?
As an engine goes faster, its tractive force decreeses. The following table shows the impact in hauling capacity as the engine goes from 4 ties per second up to 8 ties per second:
| Speed (ties/second) | Four wheeler hauling capacity (tons) | Six wheeler hauling capacity (tons) |
|---|---|---|
| 4 | 68 | 71 |
| 5 | 57 | 60 |
| 6 | 46 | 48 |
| 7 | 39 | 41 |
| 8 | 33 | 35 |