for efficiency you want to get the most power out of your engine without adding any more fuel than you are already using. The cheap and easy stuff first: set the timing up one degree at a time until it pings then back it off 3 degrees. make sure you are using free flowing air filters and exhaust. gap spark plugs at .028 to .030 (as small a gap as you can and still idle smoothly) Clean all the electrical connections that are exposed to the elements; Battery terminals, ground wires, alternator battery and ground connections. Make sure you have a ground connection to the frame and to the engine block run as hot a thermostat as you can without overheating, 185 to 195 should be about right - a cylinder head temp of 180 degrees is best for power and 185 is best for economy. Make sure your air cleaner snorkel is hooked up to the manifold preheater - the incoming air chage is best when it is 150 degrees but you want to make sure the engine doesn't start to surge at cruising speeds of 50 to 70 mph. If it does then restrict the air valve in the snorkel so that it doesn't close as much - just a little at a time until the surge goes away. Set the idle as low as it will run well and as lean as it will stay smooth. Run 10-30 oil or lighter so long as your temperatures (outside air temp) are within the tollerance range for the oil and change it at 5000 miles. Don't spend the money you save on fuel for oil changes at 3000 miles. Use 8mm wires even if your engine doesn't call for it. You can do this by using the rotor and cap (and the adapter) from a later model engine. Replace the power valve in your Holley or Autolite carb with one that will open at 3.5 inches of mercury (Holley's are stamped "3.5") this will delay the the opening of the power vavle and reduce moderate throttle fuel consumption. Keep your tires at 30 to 32 psi (if you are running stock tires) if you are running fat tires you are losing mpg because of drag. you have to run lower pressures with fatter tires to prevent wearing out the middle of the tire. At lower pressures the tires roll harder (they are stickier), taking more horse-power to just roll down the road. If you have the know how and the desire for better milage than this will give you then pull the heads. Mill the heads .040 and reinstall them with the thinest shim type head gaskets you can find. Raising the compression is the fastest way to gain HP without using any more fuel. then start over with the other mods mentioned above. You will probably have to change everything that you have already done. Finally the way to save the most fuel for the least amount of money: drive with your brain and not the accellerator. Push it down slow and take your time getting up to speed (install a vacuum guage and see how high you can keep the manifold vacuum) Take your foot off the gas pedal sooner when you need to slow down - use your brake less. Each time you use your brakes you have to use your gas pedal to get back the speed you took away with the brakes. If you have an automatic slip it into neutral when going down hills, if you have a manual then press the clutch in and let it coast down those hills - use your brakes to keep it at the speed limit. Keep the car at 5mph under the limit or right at the limit instead of 5 over - at 60mph you will lose 2-3 mpg if you go 65. The way you drive can affect your milage by as much as 50%. No other change will affect your milage as much as changing your habits. To get the best milage you want your engine to produce the most horse-power that it can without increasing the amount of fuel you burn to do it. The way to get the best milage is not to use the extra horse-power that you have made.
