...if the AC is on or off? Will it compute better mileage (as it should) with the AC off?

 

I do not think this holds true...in the past the AC systmes were engine driven using up more fuel however in the newer cars they are battery driven, using the power from the electrical system and not fuel...i would say the fuel consumption is about the same in not exactly the same...

 

I think you need to understand how the AC system works. The compressor is still driven by a belt off of the crankshaft pulley. So the AC will put a load on the engine when it is on which will require more torque . This will burn a little more fuel but the newer AC compressor are more efficent . The mileage CPU does not care if the AC is on or off and will give you a reading based on the the amount of fuel being used. So you will get less MPG with the AC on unless you drive with the windows open. This will cause more drag and use more fuel. So because I don't care about how much fuel I burn I drive with the AC on auto and set at 72*.

 

Huh? That's news to me...

AFAIK, there's still an A/C compressor -- which puts a drag on the engine when engaged.

Regardless, the fuel consumption computer doesn't need to "know" about it anyway. The calculation is determined based on consumption and the travel distance of the vehicle -- which is impacted by a number of factors that the computer doesn't need to "know" about. If you're towing something, that calculation should reflect the consumption -- without any input by you. Same thing is you have a trunk full of cinder blocks, etc...


<edit> jagtoes -- you beat me to the same point! So much for posting at the same time