How does weight not matter in PG? Extra weight could increade the time it takes to pulse back up to speed, and extend the distance covered in glide. I doubt these would balance each other out, so it would matter...
Look at the statistics of F1 race cars. They have extremely high reving huge engines going extreme speeds and get insane mpg. F1 cars are LIGHT, they dont even paint them to avoid the extra weight of a millimeter thick of paint....
I have lightened my car nearly 100 pounds off the factory dry curb weight and have seen an increase in FE and overall power. I can cruise in higher gears than I have before, and that is a direct weight to FE improvement. I can also leave stoplights with less effort on the engine.
There are no flats around here, so I cannot vouche for "flat-driving", but I promise you, weight has a dramatic affect on mpg when you start cruising up a hill.
I think that one of the factors that hasn't been mentioned yet is what I've come to believe as a fundamental "FE driving technique" which is the "when driving up hill, set the power (RPM) and accept whatever MPH you get." For those who mentioned Newton's Second Law of Motion, that's like having a constant Force. So, given a constant Force, if Mass is relatively low, then Acceleration will be relatively high. Similarly, given a constant Force, if Mass is relatively high, then Acceleration will be relatively low. What that means is that for a heavier loaded car, it means it's going to take a longer time to get to say 55 MPH than a lightly loaded car. However, since fuel efficiency is related to the engine RPM reading on the tachometer more than the MPH reading on the speedometer, I think the "set the power (RPM) and accept whatever MPH you get" driving method basically cancels out the weight factor. Now, if you live in a part of the country with minimum speed limits then you wouldn't be able to "accept whatever MPH you get" then that would necessitate a higher throttle setting in order to move the relatively heavier car at a proportionally higher acceleration to achieve and maintain the minimum speed limit. In that case, yes I agree that a heavier car would have a negative effect on your fuel economy.
So to summarize those who posted before me, the answer is "it depends."
Usually it's at 3600 pounds or so (With me in it) and on trips where I'm going a consistent 65 mph, I get an average of 35-45 mpg.
When it was loaded down to nearly 4500 pounds for that vacation, I got that same average of 35-45 mpg at 65 mph.
(And that was over 600 miles of highway cruising, so it was a pretty good way to average. But note, I'm ONLY counting the lengthened interstate trips where I didn't touch the brakes. The total average for the trip was 29.7MPG. Th 100 miles of city driving I did in that one trip lowered my MPG by that much! If I were lighter, they wouldn't have)
Nearly a thousand pounds extra weight, and it didn't change my highway MPG.
Wreaked havoc with my city MPG because I had to use the brakes and I couldn't reclaim that extra fuel used to accelerate in longer coasts.
The physics works. My real world experience works.
On the highway, weight plays little factor in MPG, so long as you don't touch the brakes.
You won't go as fast, but you'll go the same distance on the same set energy. And that is counting friction into the equation. The rolling resistance is higher for heavier cars, yes, but not by enough to really matter. With equal drag coefficients and equal force being applied, the weight won't effect MPG.
If you don't ever touch the brakes.
That's the key sentance. And therefore, since people always will touch the brakes.
Lighter cars will ALWAYS get better real world MPG than heavier cars. Because you will ALWAYS use your brakes.
"Never" is not the upper limit, nor a good choice for a debate. But I use them infrequently enough and as little as possible that I probably have made a few trips around town without using them, sure.
Think about all the variables, and how they might be different from one situation to another (like the distance between hills, how much the wind is gusting, how well trained the driver is, how is the car geared, ad-infinitum),
Once you know everything in the universe then it is safe to use words like always and never