Sorry for a question that may seem obvious to some... but what does this refer to? first or fourth?

The highest gear.  4th gear in a 4 speed & 5th in a 5 speed.
 
Matt
 
 

Thanks (as always) matt

I would assume it would refer 4th or 5th gear (top gear)depending on your box but I am only guessing as I haven't come across the term  "tallest gear" I will be interested in seeing what others have to say. 

This could really refer to any gear under consideration or when comparing different gearboxs.
For instance first gear is usualy around 3.6 to one, but a close ratio box can have a first gear that is much taller say 2.6 to one. So in this example the close ratio box has the tallest gear.

So applying that to a standard four speed gearbox, a fourth gear is always "taller" than a first gear...?