Vac Hose size #'s in FA???

[Ameer Antar]

Anyone know what those numbers for vac hose size in the FA mean? I usually see something like 3.5x3 or 4x3, but I'm not sure if the first would be ID or OD... I'm guessing the second number is wall thickness??? Anyone know what it's s'posed to be? 

Also I'd love to hear of any good sources of metric vacuum hoses for our cars. I'm going to try the dealer, but I have  bad feeling about it...I'm hoping to find some other place for that. I've tried standard US hoses, but they seem to work better on the larger hoses, the vac hoses are harder to get a good fit. TIA.

-Ameer