"at a speed of XXX mph" and "at XXX mph"

Ume

Banned
Japanese
speed
(at) a speed of 60 mph/80 kmph etc
- The truck was travelling at a speed of 50 mph.

I've seen a lot of "at XXX mph" [without a speed of]. I find the phrase understandable, but do you regard it as grammatically correct??
 
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