My car already has an instant mpg readout that is similar to the scangauge, and it is the single most effective tool I use. It tells me in real time how my driving habits are affecting the car, and of course changing my driving habits has gained me more mpg than any other mod.
As far as all the actual mods I have slapped on on Turtle are concerned, I use three of them in conjunction with each other to really boost my mpg: Injector kill switch, clutch switch, and brake regen button. If I had to choose between those three and could only have one, it would be the clutch switch, since as a stand-alone mod it would help the most. But the three together have a big impact.
Pump those tires up, too. Those are the only part of the car in contact with the road!
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