I drive an 03 Corolla, and I track my gas mileage per tank, just to know what I'm getting and to be able to think back on that tank's history and see what may have caused it to go up or down. It is usually around 32 mpg. This even helped me diagnose a problem once when it dropped to around 24 mpg for a couple weeks with no evident change in performance. Turns out a $20 O2 sensor had died. I swapped it out and got back up to my usual 32 mpg.
Recently we switched over to Progressive Insurance because they had the best rate at my 6 month price check. As part of that, they sent me a "Snapshot" device as seen in the "Don't mess with my discount" commercials. It beeps at me every time I have a "hard stop," which I quickly realized was a very broad definition by the device's standards. I was getting 2-3 beeps every day, even while trying my hardest to keep my braking to a gentle slowdown!
2 weeks later, I have learned the art of the slow brake (Three days beep free as of this morning), with several added benefits. First of all, my wife keeps complimenting me on how smooth my driving is. Secondly, my car is getting about 5 more mpg, bringing it up from 32 to ~37 mpg! I have to keep this snapshot device plugged in for 6 months, so I would imagine my driving habits will be fairly firmly changed for the better by the end of it.
Thanks Progressive, for helping my wallet, my safety, and my marriage.