Crossword Genius

Traveller's instrument on journey shows highest possible mph (5,2,5)

Ross

I believe the answer is:

speed of light

I'm a little stuck... Click here to teach me more about this clue!

'highest possible mph' is the definition.
The definition and answer can be both related to time as well as being singular nouns.
Maybe you can see a link between them that I don't see?

'traveller's instrument on journey' is the wordplay.
'traveller's instrument' becomes 'speedo' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another.
'journey' becomes 'flight' (flight is a kind of journey).
'speedo'+'flight'='SPEED OF LIGHT'

'shows' is the link.

Can you help me to learn more?

(Other definitions for speed of light that I've seen before include "The c in Einstein's equation" , "299,792,458 meters per second" , "The c in E=mc 2" , "299,792,458 metres per second" .)

I've seen this clue in The Times.
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