for a stock vehicle: the alternator will produce enough power at a certain RPM to provide enough amperage to power every component of the vehicle at the same time, for example radio full blast, headlights on, wipers on, heated seats on, etc., etc. will draw say a total of 80 amps, well your alternator should then be designed to produce 100 amps at idle and 120 amps at say 3500 RPM. now the alternator is supposed to be a fixed output but aftermarket alternators will tell you there is a difference between idle and higher RPM's, but keep in mind you are talking about how much strain you are putting on the engine based on how much pull will fluctuate a magnet as it passes coiled wires! I would say worst case scenario less than a couple of cents a year in additional fuel costs to always have the max current draw, (but then again always having max current draw may wear out your alternator a lot faster than planned).

but just on a side note, I would honestly think that if you have more turns on your daily commute than someone who drives straight ahead every day then your gas would be more because of the strain on the power steering pump! and I would imagine that has more effect on the engine than the alternator!

 

You're driving a billboard, I wouldn't worry about it.

 

Gas mileage? Whats that....

 

^What he said. I didn't buy my Disco for gas mileage either.

 

The alternator is simply replacing the electricity drawn off the battery. Anytime the engine is running the alternator is recharging the battery. Since the alternator does not actually see the load directly it can't effect your fuel mileage. As Rover Chris was saying, your alternator only puts out certain amps at a given RPM. A 100 amp alternator is only going to put out 100 amps to charge the battery regardless whether you're pulling 20 amps or 1000 amps off the battery.

Some vehicles fuel mileage is an issue. If you're driving a Disco and worried about mileage save yourself and get a new vehicle.

 

Yeah, like a Prius...or a SMART.