Feets,

This is something my sons and I have discussed before, as I like keeping "long legs" in my '71 R/T as well as my other vehicles. Here are some random thoughts, for what its worth:

-Someone brought up the early-mid 1990's Roadmasters. The same could be said for the 9C1 Caprices I drove during that era. Though they were in itself a bit lighter, once all of our equipment was added they weighed more than the Roadmaster. These vehicles got some really good gas mileage, high 20s at a steady highway cruise. They had something like a 3.08 rear and that steep TH700R4 overdrive

-A better example might be a Cadillac Fleetwood Brougham of the same era. These came with around a 3.23 towing package and were very close to the weight of your Imperial. Their mileage was not quite as good as the Roadmasters and our Caprices.

-Something to think about in reference to your Imperial is the corresponding aerodynamics. At speeds over 70 MPH, it has been my experience that the drag coefficient and frontal area makes a real difference in fuel mileage. Then again, you are running a 440 V8 with a lot more torque than an LT1 Chebby. Another observation of mine was that engines with peak torque down low are usually more efficent as far as gas mileage goes, if all other things are equal.

Finally, and not to steal my younger son's thunder, we just finished a run from Ozona to Annapolis in his '71 Charger SE. It is a 383 Magnum with stock innards, 727, and we installed a 2.76:1 Sure Grip unit in the rear end. It has headers and dual exhuast, a Pertronix ignition with our own advance curve, a Quadrajet I set up for him, and a low restrictive air cleaner. Our best for the trip was 18.7 MPG at around 75 MPH, with a couple of 80-85 MPH bursts.

May God bless America,

Paladin