Those numbers aren’t really goofy: that’s actually what you’re getting at that very moment.
Stick your hand out the window and try to push it forward at those speeds. Of course, if you’ve got a strong tailwind, the negative impact of speeding will be mitigated to some extent.
Not only does the extra weight hurt your fuel economy, it upsets the balance of the car.
Numbers can be manipulated any which way you want, but the simple truth is that MPG doesn’t really tell you how much money you’re spending on fuel. You’ve heard this one before about a million times, and you know what? Every time I see some moron in a Prius out accelerate me off a light I wish I had the legal right to pull him or her over, pry their eyes open Clockwork Orange-style and force them to watch Jeremy Clarkson getting better fuel economy in a BMW M3 than the Stig in a Prius. Obviously you'll have to stop at some point, but if you know you’re turning soon or there’s a sharp bend in the road, coast for a bit. 1) all your brakes do is convert your forward momentum into heat energy, and even the best energy recovery systems don’t capture 90% of that energy.
Even the worst fuel-sipping cars made will become gas guzzlers when you floor it. 2) if you don’t slow down for a turn you have to take it faster, which is not only fun, it means you don’t have to accelerate as much.
No matter what kind of car you drive, you can be getting better fuel economy than what you're getting now.