I've recently become somewhat obsessed with what is mf scale. I've been buying different themes; adventurer, indiana jones, speed racer, etc where cars are built for mfs but at different scales. My conclusion is that MF scale is roughly 1stud = 1foot (300mm). Take the new mf motorbikes; much as I love the design, they're huge! A motorbike wheel is usually 2ft in dia. and the lego ones are 21mm which gives a scale of 1:28.5 (instead of 1:38 for 1ft=1stud). Then compare a 4stud wide classic town car with a real 6ft wide car. That's a scale of 1:56.
Now let's look at trains. The lego track with 4 studs clear between rails is just about right for a 1ft/stud scale, but would mean the carriages should be 10 studs wide.Mad Physicist
works at 5studs wide for cars & 7 for trucks. But as he acknowledges this is difficult to do. My thinking is lorries in real life are uniformly 8ft wide. Cars are all within a few inches of 6ft. So it's much easier to achieve this ratio with 6 studs for cars & 8 studs for lorries. Lego has been doing this with cars such as 5918 - Scorpion Tracker
& lorries such as 7213 - Off-Road Fire Truck & Fireboat
I mentioned on the modular forum that the 16stud module also neatly matches the historical rod, (pole or perch) building unit of 16.5 feet.
So taking all these things together:
•The gauge (as opposed to the scale) of the trains
•The width of cars using the dual seat base
•The width of new lorries
•The module size of the modular buildings
leads me to believe a scale of 1foot (300mm) per stud (1:38) is just about right for minifig scale.