in a Ford Econoline, SWFL/AZ .... higher fan flow gives colder temps as the higher flow fills the cockpit ...there's a partition behind seats.

The coldness increases with each of 3 speeds and with Hi speed there is surplus cold flow overcoming heat gain from engine/large windshield...the roof is overlain with a rack floor. Paint is WHITE...there is a 30+ degree difference between WHITE and Safety Blue at the outside surface midmorning here in SWFL

The surplus coldness ...I forget the temp...hot is 150 ..... prob around 42-45..
is too cold unless the outside is too hot.

If your system is constant temp at varying speeds then change the coolant....inspect the system. Design satisfaction would spec excess available heat removal for all speeds. Or, why have variable speeds ?

Ford's test track is down the road below Naples.