the real company driver's mpg

Depends on how long the hill is as to whether the fan kicks in,one reason the higher horsepower engines often do better is that they don't have time to get hot so they don't run the cooling fan. 15 horsepower might be an average, but it uses more at high rpm and less at low rpm, the fan is also controlled by a refrigerant pressure switch, if the AC has too much pressure, the fan will be running even though the engine isn't too warm.

If your engine isn't running too slow to run efficiently, slower speeds will help economy, and not take that much more time to run the run.

I drove a setup for a long time that was terrible, 2.64 rears with a straight 10 speed with a 12.7 Detroit. you had to run 64 in high to be turning 1400 rpm, the truck actually did better at pulling a chicken trailer a gear down at 55 mph turning 1500rpm. High gear was lugging it unless you were going at least 68 mph, too fast for those hard pulling chicken trailers. The same tractor pulling a reefer up a long mountain grade would overheat unless it was run near the top of the horsepower curve at about half throttle. A new radiator fixed that

I had to make 2 runs with a Western Star, geared tall with a 13 speed OD. That truck did better in 12th at almost any reasonable speed than it did in 13th at any speed. Half a mpg better fuel economy.

I have about so much time to run out and get loaded, going slow isn't much of an option. the cargo has to be loaded on time and back at the plant almost as soon as possible.

 

The Star has good AC but it also cycles the cooling fan unless you are driving down the Interstate, the Columbia has a bad fan switch but the AC switch works good , it doesn't cycle on and off much unless I'm idling, but I shut that one off at every opportunity. It might be that the switch is off a bit on the pressure side. If the AC works when it is hot that is the main thing right now, not working means you are sitting in a heavyweight sweat lodge. If you get near that Penske shop, you might get them to look into it further.

I drove a 96 FLD small sleeper for a couple of months one winter a few years back, the cooling fan never kicked out and the AC never kicked in, even though I was in Texas on an anti-freeze peddle with it, and the temp reached into the high 70s. That shop wouldn't check it out while I was in it,but they fixed it later, that's when I found out the high limit switch was bad

I miss that darned Columbia. I can back that thing into a shoe box without pulling up