the alternator generates electrcity while the engine runs. doesn't it use the same amount of fuel whether u use the stored up electricity or not?

 

[QUOTE=dyhppy,Dec 3 2007, 12:59 PM] the alternator generates electrcity while the engine runs.

 

right, except there is typically more juice being generated by the alternator than the system is using, so the gas is a sunk cost. Using the heater or other electronic accessories won't change your mileage.

One thing to note here - if you use your heat but have it in defrost mode, it DOES use the AC unit to dry out the air (in most new cars) - so you will see slightly worse mileage.

 

I'd say that running the AC full bore with top down will have a negative effect on MPG. Seems to for me.

 

Seems to be solid logic but it is not. Like traveller said (you probably missed his post), the more electrical load, the harder the alternator is to spin. The lighter the electrical load, the easier.

 

I don't know what "cumbustion" is, but the heater is powered by the battery plus alternator The engine has a passive influence by way of its cooling system. The spirit of the OP's question was if the heater affected the engine the same way A/C does when it's on. It does not because it doesn't require an extra piece in the puzzle in order for it to function.

 

The real questions needs to be - how do the fan and temperature settings effect the durations of the AC compressor on/off cycle?