I have been misusing my batteries in my 2003 Ram. I have fixed this issue but am wondering if driving the truck while the batteries are low would cause the truck mileage to suffer? I have noticed the mileage decrease from reliable 20 mpg to closer to 18 and have kept the oil, filters changed and nothing has changed regarding driving conditions. I suppose I am wondering if the alternator has had to work hard to charge the drained batteries thereby causing poorer mpg?
Thanks
Howard

 

If your batteries wont charge your alternater will be working over time, same as running lights on air conditioning etc. so yes it will drop your mileage some, also winter fuel should be coming in pretty soon, that will drop it also,