"Proper" scaling of big-rig truck beds?

[Winchester]

I'm trying to work out the "proper" length scale for my various 6-wide trucks and trailers to make sure they look realistically proportioned, and after talking to my dad (who is a retired long-haul trucker) and looking at the actual regs for truck myself, it turns out that European trucks are actually built from modules to make it easier to satisfy all the rules - and these rules are built around 80 x 120 cm cargo pallets, that will fit either two or three in width inside the cargo box.

My immediate thought was that if you then go by exterior scale only, and assume the real trucks have negligible thickness of the walls, you can basically use 2 x 3 tiles to represent the cargo pallets, and determine the length in studs fairly simply by counting the number of pallets that fits. 

In Europe, the standard modules are 7.2 meters (for 18 pallets), which translates to 6 x 18 studs; and 13.6 meters (for 33 pallets), which translates to 33 studs. The entire vehicle including all trailers is normally limited to 25.25 meters, which is 63.125 studs. Basically, it has to fit on two 32 x 32 baseplates, and it's road legal in the EU.

In Canada, they allow trailers up to 16.2 meters, which is 40.5 studs - I'd call it 40, easier to work with as that's two 6 x 16 and one 6 x 8 plate, and a 23 meter total vehicle length restriction (57.5 studs).

The US regs are confusing, because all the ones I can find are for *minimum* lengths. A single trailer has a federal minimum length of 48 feet, which translates to 36 studs; some states have old trailers that conform to longer form factors (up to 59 feet, which would be 40 studs!) They also have a minimum length of 28 feet (21 studs) per trailer in a multi-trailer rig. 

And of course there are the exceptions, for when your cargo doesn't fit in neat packages. 

All of this assumes that you're comfortable with minifig-scale fudging, by the way - once you put siding on your trailer there's no way to actually stick that many 2 x 3 tiles in there...

(Now, to rebuild my truck MOCs to fit the correct lengths...)