Short answer
Trial speed is not service speed. One measures what a ship achieved under test conditions; the other reflects what it could sustain economically and reliably in regular operation.
Common speed figures
Trial speed
A speed achieved during sea trials under controlled conditions, often with a light load and freshly tuned machinery.
Maximum speed
The highest speed the vessel was known or estimated to reach, sometimes briefly.
Service speed
The practical speed maintained in normal scheduled operation, allowing for fuel economy, weather, machinery wear, and timetable margin.
Crossing average
Average speed over a defined voyage or measured record course, which may differ substantially from instantaneous maximum speed.
Why inflated comparisons happen
Publicity often favored the highest impressive number available. Later summaries may place a trial speed beside another ship’s service speed, creating a comparison that looks precise but is methodologically invalid.
Comparison rule: use the same type of speed measurement, over the same kind of interval, before deciding one ship was faster than another.
Speed also changed over a career
Boiler condition, propeller changes, machinery refits, fuel conversion, hull fouling, wartime restrictions, and aging could all affect performance.