@final gulch
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60 gallon tank with a full tank of fuel will add an extra 350-450lbs to the car and fitting a tank that big in the back of the car is gonna be rough. you're gonna lose all your trunk space and possible even needing to remove the rear seats. highly unrealistic. if you want a car with better fuel economy, i recommend something else.
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yes twin charging it will cause the gas mileage to decrease drastically. it can make the car any more faster but its not practical whatsoever.
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after you add that auxiliary tank and twin charge it, you'd probably get 5-8mpg max on a good day.
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you realistically could put the awd system from police interceptors but its gonna require a lot of modification to the car and you'd have to use a different transmission which may not handle the power of your "twin charged hellcat"
overall, you'd spend 60-70k on the car and an additional 50-60k (generously) to do everything properly including labor
