Maybe I have my terminology confused ...the sprinter would go into overdrive on most hills if I left cruise on... I could make it without cruise and it wouldn't go into o/d... especially Jellico, lookout Mt or Eagle Mt
You do have your terminology confused
Overdrive is any gear ratio numerically lower than 1 to 1 (direct drive) such as .74 to 1...usually just expressed as .74.
On your magical Sprinter the only overdrive gear was 5th (.83) so on those mountain climbs with the cruise control on you van was actually going OUT of overdrive.
On MANY of the newer vans with smaller engines and 6 or more speed transmissions, there can be multiple overdrive gears. These vans will also downshift at the slightest load...and are designed to...and it's normal. While these engines are beneficial in many ways, they do not possess much meaningful torque or horsepower at lower RPM's...like under 2500 etc. Under low load on flat land they will happily purr along at low RPM because your van may only need as little as 30 or 40 hp to maintain 65 mph. As soon as some load is applied..a headwind, a hill, passing or even maybe just a jump up to 75mph..the required horsepower will exceed what the smaller engine can provide at the lower RPM. The ECM will initiate a downshift to get the engine RPM up to a point where it will produce the required hp or torque.
The days of old with 4 speed transmissions and 5 liter plus engines where the transmission got into top gear around 40mph...and it stayed there unless you needed rocket mode to pass someone...are gone.
Moot's engine for reference...doesn't produce appreciable torque until over 3500 RPM and the horsepower is even higher than that. Small engines need to rev to make horsepower relative to a larger one. Turbo's can help alter this...a bit.
Your other question...warmer temps don't only mean different fuel blends...warm air produces less aero drag than cold air due to it's lower density. Dry roads also use less fuel (lower rolling resistance).
