[elhigh]

I think it's great you want to make a car do better than its original manufacturer specified.

That said, I want to caution you against a few things.

I noticed back near the beginning of the thread "wickydude" posted a link to someone claiming 400+mpg. That sticks in my head, and let me explain why:

I don't believe it is mathematically possible.

I don't have the numbers right here in front of me, and frankly these kind of discussions don't ever encourage me to go find them because it always devolves into a pie fight. But I will lay these facts out:

The maximum heat efficiency of a good automotive engine is about 37%. Diesels are a bit better. If you put 100 BTU worth of heat energy into an engine in the form of fuel, the very best you can get out of it will be 37 BTU worth of work. That's it. No exceptions.

Even if you approach the theoretical maximum for a heat engine, about 58%, (still trying to remember where I saw that for reference) it's still not going to get you close to 400mpg, and I don't think 400mpg is even possible. That would require an energy consumption of less than 90 watt-hours per mile, less energy than is actually consumed in moving the air out of the way of the car.

I have seen only a couple of vehicles even approach that at anything like road-legal speeds, and they were ultra-tiny, ultra-streamlined electric vehicles which enjoyed ridiculously good motive efficiencies compared to a gas engined car. 90 watt-hours per mile, but only in a vehicle that would make a coffin feel roomy and luxurious. Those things could manage the feat because they weren't moving much air aside and weren't moving it very far. The displaced masses and the magnitude of displacement were minute compared to even a compact car.

Do you see my point? It is a violation of physics. You are asking it to put out more work than it has energy to perform.