Originally Posted by Philatio
I guess that makes sense - but I've always heard you get your best mileage when it's full. I'm not sure of the reasoning behind it though.
That's probably true on rear wheel drive cars with a longitudinal transmission and driveshaft shaft. For every one degree in universal joint angle, it causes a three hp parasitic drag, and there are two or three univeral joints on those type of vehicles. Most car manufacturers of these type of cars have factored in universal joint angles for a vehicle with a full tank of fuel. A vehicle with lighter fuel load would have a steeper universal joint causing more parasitic drag, and a decrease in fuel mileage on steady, flat surface driving. In constant stop and go driving, the lighter fuel load should negate the decrease in fuel mileage caused by the parasitic drag of the steeper driveshaft and universal joint angle.
Automotive basics 101 is now dismissed.
