[spacemanspif]

I'm just now seeing this thread and commend you for doing work to eco-mod that beast. I have a big American car too but I use it for performance and don't really worry about MPG with it. That being said, I also have pretty good experience with older V8 cars of the era and have some insights for you.

Please clarify when you drive the car. If only in the warm months, there is no need for engine/trans heaters, your money will be better invested elsewhere.

Advice:

1. Don't use Lucas oil stabilizers or other products. They are very thick and will turn your 5-20oil into 20-50 oil. Better to run 10-40 on that car if it is only run in the warm weather.

2. Depending on what emissions equipment you have, there is a small gain in MPG to be had. Please take a few pics of the engine compartment for us to see what you are working with...bonus points if you remove the air cleaner to take the pics so we can see carb stuff.

3. A cheap set of headers will last a long time if you don't drive in the salt and will allow the engine to breathe easier making a little more power AND getting better MPG.

4. Look for marks on the carb to identify it. Then go to the library or internet and get a book on carb tuning. Vacuum gauges are cheap and help, but once past the basic tune, wideband O2 sensors will help you with tuning but can be expensive.

5. Buy a cheap tachometer at your local parts store so you at lease have info of what RPM you are cruising at and if a gear change would be beneficial.

My Monte has 3.73 rear in it and for a short time I ran a 2.29 out of a Malibu. 65mph was around 1600rpm and got terrible mileage. Brought the engine (and car) speed up to 2000rpm (~80mph) and was getting 25mpg. RPM "sweet spot" is important and can be overshot. You having an OD trans means you are better off on the high side of the 2.xx gears. So if you do actually have a 2.73, you should probably leave it alone.