Running Coach A/C vs. Cab A/C While Driving
There have been discussions about running the generator and coach A/C versus using the cab A/C. Although the need to cool the coach for passengers is an overriding reason to run the generator and coach A/C, some have mentioned the possibility of doing it to reduce the load on the chassis engine or to save gas.
We have a Sunseeker 3100 on a Ford E450 chassis with a V-10, and I checked the fuel flow with a ScanGauge II to see how much difference the A/C actually makes. Here’s what I found with the transmission in park and an outside temperature of about 90 degrees F:
Idle speed, A/C off: 0.55 gallon per hour
Idle speed, A/C on: 0.67 gallon per hour
2000 rpm, A/C off: 1.85 gallons per hour
2000 rpm, A/C on: 2.02 gallons per hour
2000 rpm was the best speed I could hold by reading the tachometer. It is about the rpm at 50 mph, and I usually drive faster than that, but it gives an idea of the A/C compressor load at normal speeds.
Since the additional gas used at 2000 rpm with the A/C running is only 0.17 gallons per hour compared to about 0.5 gallons per hour for the generator, it is definitely more economical to use the cab air only, if that will give you the cooling you need. As for the horsepower load on the engine, the additional 0.17 gallon per hour equates to only about 2.5 horsepower.
The cab A/C does add to the engine cooling load, but we have towed a 4000 pound car in 95 degrees up a grade more than 7% at 45 mph for more than 5 miles and the engine coolant temperature gauge never moved.