FedEx uses both weight as well as dimensions.
They will charge the higher of 2 scenarios
The example in a previous post suggested a 12 lbs. box with 12x12x18 dimensions.
barring any special negotiated rates:
FedEx will use (12x12x18/166)=15.61 (round up to 16 lbs.) for air shipments.
FedEx will use (12x12x18/194)=13.36 (round up to 14 lbs.) for ground shipments.
I can all but guarantee that Amazon has a seriously more favorable factor (in other words much better than /166 or /184.
The trouble with FedEx module on Magento is, that it will be super accurate through 5 or 6 cartons in the shopping cart, but if there are more (lets say 20 boxes) shipping ground.. that is where it all falls apart...
When you have that many boxes, your account will likely calculate freight using FedEx's multiweight (it is a big discount). Magento on the other hand, would take every individual box, calculate the dimensional weight and just keep adding them all up (ignoring your multiweight discount)...
The result is one furious online customer who 100% will abandon the shopping cart.
We had FedEx IT support tell us they could see the multiweight results being calculated on the Magento backend, while displaying a very high dollar amount on the front end (shopping cart)
If anyone has a solution for that... please DO share...