The HHR was billed as getting 22MPG city / 28MPG highway. I've always tended to get significantly better mileage on my vehicles than the EPA ratings; I keep my vehicles in excellent shape and I understand where the powerband is on the engine.
So I was pretty shocked when, after the first couple of weeks, the HHR was reporting about 17MPG average. Ouch! And wtf?
After that shock, even though I felt like I knew better, I tried driving like most people do when they are trying to save gas - I babied the throttle. Creeping off the line, accelerating very lightly, et cetera. As I've always told people, this isn't the great idea they think it is - if you're accelerating outside the engine's powerband, you're wasting gas. Unsurprisingly, my couple of weeks spent babying the throttle didn't do me any good. I got up to about 18 or 19 MPG, but that was with SERIOUSLY grandmothering the hell out of EVERYTHING - not just accelerating slowly off the line, but also cruising so sedately that people kept diving in ahead of me. Very little improvement in economy, with a SEVERE decrease in the entire driving experience.
So I got thoughtful, and I flipped the HUD on the car to "instant MPG" mode and really paid attention. And I figured out what was biting me in the butt: modern (meaning within the last 2 or 3 years) automatic transmissions are very different from the ones I grew up with.
Traditional automatic transmissions leave the car in gear when you idle. What this means is, when cruising at a relatively stable speed, the best fuel economy you can get is to feather the throttle precisely to what's needed to keep the car moving at that speed - the less surging, the better. Trying to coast by taking your foot off of the gas entirely is generally a mistake, as the drag of "engine braking" with the vehicle still in gear will slow you down rapidly, so accelerating back again is a net waste of fuel.
The automatic in the HHR, however, actually puts the vehicle in neutral when your foot is off the gas. This makes for a very different picture when it comes to maximizing fuel economy: suddenly, coasting and accelerating makes a lot more sense. You have to remember, again, that an engine is most efficient inside its powerband: when it is accelerating pretty strongly. So in the case of a vehicle with a transmission that goes completely neutral when your foot is off the pedal, the most efficient driving profile becomes one in which you accelerate smoothly but fairly strongly off the line to cruising speed, then take your foot off the throttle until you start to slow down, then accelerate again fairly strongly but briefly, then coast... wash, rinse, repeat.
It's an odd way to drive, to my sensibilities. And it requires considerably more skill and attention: the idea, after all, is not to make it obvious to other drivers that you're accelerating in surges; you don't want to vary your speed more than 1mph or so while you're "cruising". But the results of trying it speak for themselves: I went from averaging 17-19 MPG to and from work to averaging 24 MPG on the same route.