I've got one of those MPG calculators in my car.  It monitors fuel consumption second by second and can tell me what my MPG is at the moment or over time.

I also have a system that alerts me when my tires get too low.

Recently, the tire pressure alert went off, so I stopped and aired up the tires.  They were all only 3 or 4 pound low, but after I did it, my MPG went up half a percent.  I was getting about 44 mpg, and it went up to 46 MPG.  Not much on the surface, but if everybody kept their tires at the proper pressure, just imagine how much fuel we could save overall.

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