questions to the experts.

First is is it normal for our cars to dip a bit in rpm and vibrate more when the heater is on or the car has the lights and heater on ? Also I read some on mpg on a few threads here but I seam to get around 200 miles on a full tank of gas I drive 16 miles total to and from work and that 90 percent freeway is that normal ? I know a lot goes into the mpg but I can never seam to crack over 200 also I fill up when the fuel light comes on I end up putting 13.5 gallons of fuel into this beast I figure I am getting around 14 to 15 mpg.