In late May, I added a tachometer to my 2007 Yaris. I figured it'd be a good addition to the vehicle for autocross, and just an overall good thing to have. However, I didn't expect one side effect: Increased mileage.
No, my tachometer isn't giving me the mileage increase per se, but having it has changed the way I've driven, at least over the last three tanks of gas.
Since installing the tach, I've gone from getting about 32-33 mpg to 36-39 mpg.
So what's going on? I'm more aware of my engine speed. No longer am I going by the sound of the engine and using the speedometer to determine when to shift. Now I'm able to see the true engine speed, and I find myself shifting earlier and driving in a higher gear to conserve fuel. Thus far it's worked pretty well. I was able to achieve 345 miles on my last tank—the most in nearly two years of ownership—and I got 39 mpg doing so.
There's a more fun side to the tach, too. I know exactly where my redline is, which is great for passing on two-lane roads. I know how much juice I still have left if I need to "punch it." No bumping into the rev limiter.
Most cars come with tachometers nowadays, the Yaris is an exception (unless you get the sedan or buy the S model). Adding the tach is almost like rediscovering how to drive the car.