turbo 400 tranny

If you are willing to spend money and want to increase your mileage, install fast burn heads. Increase the compression to about 10:1. The optimum most efficient point to push on the piston is 14 degrees after TDC. Any push before or after that point, is not as effective in pushing the piston down. This being understood, if the burn is faster, more energy can be focused to occur at that optimum point in the engine rotation. This significantly increases the efficiency of the engine. If you look at engines running fast burn heads, you will notice they require a lot less timing advance - like 10 degrees less. This is because the flame gets going that much faster. Not only is it more efficient, but it also means you can run higher compression, as there is less time for random uneven burn - detonation. So the higher compression also helps efficiency. That is one of the big reasons why the new modern engines get so much better fuel mileage.