I would use the 273 before the slant because parts are way easier to get, only need custom pistons. The slant is pretty much imposible to get good quench and because of that it will not tolerate as much compression. If you really want to go 6cyl get a 3.9 mag V6, you could use the same pistons I did and use the factory heads that are really just like 302s and after 92 they were magnums.

As for what mpg the new mags get it is a joke. They need more quench, KB 167 or 107s depending on 318 or 360 are needed. That will raise comp and quench. A good fuel injector would be bound to help, the factory ones squirt out a stream right on the side of the pushrod bump that washes a nice clean path across the bottom of the port and never gets atomized. A switch to some accels with the cone type spray pattern should help fix that. A more complete burn will make the O2s rear rich so the computer will compensate by leaning it out a little. I bet those changes ther would make a very noticable differance in mpg and probably even help hp and throttle response

Also for the gaskets the mopar gasket will raise the compression on my motor 1/2 point. Not only is it thinner, it has a much smaller bore size. I think if I can get the money and time I will swap them. The reduced bore size should help mpg by eliminateing a big dead area that doesn't really push on the piston.

I would like to get a custom speced cam but that is even more money.

I agree with patrick that the mag head would be better, but only slightly.

Remember it is not just one area like light rotateing assembly that helps the mpg, it is the whole package. Quench, compression, lighter rotateing parts, high gears, tiny duration/overlap cam, very lean, 190 t-stat, electric fan, lowered car, low rolling resistance tires.

I would like to try this combo but about 12.5 compression on e-85. Should make for a real good cost to miles ratio. I bet a high comp 273 on e-85 would make as much power and mpg as my 318

Someone asked about useing a roller cam, I think it would be hard to get custom lobes that small, the 318 2bbl cam from the mid to late 80s would probably be the best readily available roller cam. Just changing that should not hurt much and may even help a tad. I don't think the factory 318 2bbl cam would be a bad choice, I just wanted to be able to play with the lash a little and I already had it sitting here.

I just saw on good morning america some concept cars that got 100 mpg, there is some big 10million prize being given out in 2010 for the best one so I need to get out there and get to work on the cuda