Virage - it is confusing - I don't understand it all.
First the decrease in supplies relates to trucks / train carriages in the convoy so 1 in 6 is only 1 in 6 trucks will carry the relevant supplies. What I don't know is :-
1. How many trucks or train carriages are in a convoy / train.??
2. If a convoy is suppling many resources e.g. fuel, ammo and ack then how is the load spilt ??
3. How many trucks of a supply does it take to get the full 30 minute reduction.??
The big question for me is supply drops by C47 LVT or M3 - the help file says :-
The C47, LVT-2 and M3 have the capability to drop supplies on a field to repair the field or shorten the downtime. These object supplies will repair all resupplyable objects in a one mile radius from the point where the supplies land.
This says to me that if you drop supplies on a field then it will repair all the resuppliable objects (ack, ammo, fuel etc) within a 1 mile radius instantly and decrease the downtime of non-suppliable objects by 30 mins. This would explain what people have seen with cities re-building quickly - city buildings are suppliable.
HT - if you're reading this some clarification would be great

Sparks