Slant Six mpg's?

I calculated my fuel mileage farely accurately. I got up on the interstate to see if my odometer followed the mile markers. It was consistant the whole 20 miles I was on the interestate...so it must be pretty close. I built a seperate tank with a clear level on the side and measure the amount of fuel that was in it. I drove 20 miles and could clearly see how much fuel I burned. My '66 Charger with the '76 360 (rebuilt, nice and fresh), grounded head (resurfaced to add a little bit of compression) 2.73 rear gear (I believe..haven't checked) at 55-60 mph, and coasting down the hills I got 22.5 mpgs! I bet, a fresh slant 6, 170, or 225 could manage ALMOST 30 with an OD transmission, and skinny tires. Weight does not matter on the highway so much. Think about it..down the hills...weight would help when coasting. It would only really hurt you in the city having to get up to speed all the time.