Your Mileage May Differ
On another matter entirely, you'd think you should take those golf clubs out of the trunk unless your trip is truly links-related. Actually, though, I learned that weight has a surprisingly minor effect. Add 100 lb. to a 3000-lb. package, and this 3-percent increase in load invokes at most a 1.5-percent penalty at the fuel pump. Of course, it's more pronounced in accelerating and braking in city traffic, least so in highway cruise.
By contrast, especially at road speeds, the effects of vehicle aerodynamics can be profound. Ski racks are particularly bad in that they're double hits: They screw up a car's CD and increase its frontal area. (Total drag, you'll recall, is CD x A.) Original-equipment luggage racks are somewhat sleeker, until, of course, you use them to carry something.
Aero vs. A/C
The importance of aero has given rise to a good folk legend: You're better off, so goes the myth, running with your car's air conditioning on and the windows up, rather than invoking the added drag of what we used to call "440 a/c" (that is, 4 windows down, 40 mph).
The tradeoff is car- and velocity-sensitive, but my sources tell me that it's generally not beneficial at anything resembling legal speeds. Air conditioning can be as much as a 20-percent hit in fuel consumption. Unless heat and humidity call for it, you're better leaving the a/c off and the windows rolled down. (Purely as an aside, do young people wonder why we "roll down" a window?)
There's a corollary to all this concerning a climate control's Defrost setting. This mode typically activates the a/c as a dehumidifier and is certainly essential in clearing the windshield and side windows. Once they're clear, though, you're better off switching the system from Defrost to Heat; otherwise you may be running the a/c all winter long.
Revs Aren't Free - and, Alas, Neither Is Speed
It's only in racing and with exceptions even there that "revs are free." In fact, entertaining though they may be and sonorous though they may sound revs pay a penalty in fuel consumption and in wear.
Also, among the things under your control, one with a most profound effect on fuel consumption is your car's speed. Power requirements grow with the square of vehicle velocity (e.g., double the speed, quadruple the required power). And, obviously, power demanded is directly related to fuel consumed.
Whatever is an enthusiast to do?
When I'm canyon running, corner strafing or, just for the fun of it, beating that fellow next to me across the intersection, I think of my earlier comment about "liquid entertainment." And certainly we have the least expensive gasoline anywhere I'd want to live.
On the other hand, a lot of my motoring is rather more mundane. And there's genuine satisfaction in performing it efficiently.
Remember Coach Grimbly's dictum about "driving with an egg under your foot"? Forget it. The most efficient way to reach cruising speed is wide-open-throttle (WOT) short-shifting. That is, not only do revs cost money, but so does prolonged motoring in lower gears, when throttling and pumping losses are their greatest.