You may be able to save how much gas you use, but for every amount of gasoline you save you will use twice that amount meth. As mentioned, tuning will be a issue as will the logistics of doing this. To pull off your theory you would need at min a 10gal meth tank to use for every tank of gas.

 

This.

And lets not forget the premium you will pay for meth over gasoline. It would be a very costly endeavor to increase miles driven on the gasoline pumped. For every gallon of gas you save, you would use 2 gallons of meth at nearly twice the cost. That gallon of gas you save would literally cost you nearly 4x to replace with meth.