IMHO, the idea that drives come in even, 1G steps, from 16 to 6G, is a holdover from Traveller's heritage as a board game played on a hex grid.  :)

At the player level, it's a game mechanism.  It is  a casual approximation, because most players aren't interested in doing fractional-G calculations every time the mass of their ship changes.  If drive ratings were really calculated based on mass and thrust, they'd be continually varying, and potentially, something that everyone would have to account for.  All sorts of minor variability, that doesn't really make the game more enjoyable. 

At the character level, it's also a casual approximation.   The characters all understand it to be a casual approximation, calculated under certain circumstances, and will vary depending on conditions.  (Much like, say, fuel economy on cars....)

Gearheads and engineers would spec G ratings out to several decimal places, compete for sprint g's, and do all sorts of other entertaining things.  

People buying ships might read the papers closely to see how many actual gs the ship pulls, loaded vs unloaded gs, etc.

Real ships would have all sorts of fractional G ratings.  A free trader would have a g rating for both loaded mass and unloaded mass, and possibly a maximum rated payload mass, etc etc etc.  A ship would theoretically pull more gs with empty holds and nearly empty tanks, than it would fully loaded and fueled.  Ships arriving from jump with empty tanks would be faster than outbound ships with full tanks. 

But for most *players*, most of the time, nobody cares.  Couple dice rolls to see what's available for hauling, couple dice rolls to see what's available to buy on speculation, couple dice rolls to see if any of the crew went missing, take off and head for the next planet, couple dice rolls for encounters, roll for jump, etc.

Nobody* wants to calculate mass of the cargo, calculate the g's the ship can pull with that extra mass, then recalc time to jump spot based on the new G rating, etc etc. 

So we just call it 1g, 2g, whatever, it's close enough, and get about the game.

Dan

*I'm sure *someone* wants to do those calculations.  Maybe even me, every so often.  

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