For the OP,
Forget all the temperature experts and statisticians etc etc etc.
and especially all the know-nothings and know-it-all’s above
Fridge transport is very critical, to keep fresh - fresh, frozen - frozen, and even ripen fruit in transit.
If you are loading somewhere, lets say dairy produce, and they want it to be at +2, you’ve been sitting in a lay-by having your break,
turn on the fridge at +2, as long as it’s not next door, by the time you get there, it will be near or as ■■■■ it +2
You have to turn up at the supplier with the fridge at the required temperature,
When you turn up at a supplier, they will check the temperature of the inside of the trailer, if it isn’t low enough, they will not (in theory) load you,
This was your comment:
The scenario is: you load up chilled products, its +25c outside, so no matter what the temperature was in the trailer, by the time its loaded it is +16c inside. Your running temp is supposed to be +4c
Forget the outside temperature, it’s what is inside the reefer that is important
it’s up to you to get to the supplier with a suitable internal temperature,
you can’t just turn up without the fridge running, then think OK i’ll just shove it on -25
so if you’re loading fresh at +2 to +6 turn up with an internal temp within that range
if it’s frozen at -25, turn up with your internal temp at at least -18
edited to add
if it’s frozen at -25, turn up with your internal temp at at least -18
if you want to be loaded straight away, otherwise be prepared for a wait till it is low enough