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I would look into custom engine tuning. To get to 40 mpg, you will want lean burn and optimal efficiency across the board. This one of the ways I am going to use to reach 75 mpg in my car.
Look into one that you can data log yourself and adjust parameters, if one exist. Idk if many suppliers offer much for a 100% focus on mpg. There will be a learning curve, but you have the luxury of doing whatever you want.
How do you feel about emissions? Running lean will increase your NOX output, and idk how you feel about making emissions worse.
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