how can I improve the gas milage on my mercury grand marquis

tune up, no air conditioning and fuel injector cleaner.

put a 4 cylinder in it

Make sure the car is properly tuned up and the tires are properly inflated.

buy a smaller car..

Get the proper tire pressure in the tires, which should be able to tell you on door jamb sticker, in driver is door area. Also take out unnecessary weight from the trunk, etc.

Acetone In Fuel Said to Increase Mileage (http://pesn.com/2005/03/17/6900069_Acetone/) - Acetone said to improve the fuel is ability to vaporize completely by eliminating the surface tension that causes an increase in particulate vaporization temperature. (PESN; March 18, 2005)
A growing number of people r reporting their results, as tabulated here. Most have noted modest increased mileage (e.g. 2-15%), more power, more stable idle, faster start-up, cleaner emmissions. Part of that improvement is likely to be from the cleaning of the engine that the acetone accomplishes, without impinging lubricity. A few have not seen an increase in mileage at the concentration of acetone they tried. Too much acetone decreases mileage. Some have reported that Alcohol in the fuel tends to reduce the positive effects of acetone. No one has yet reported damage to their engine from acetone being added to the fuel. Several have soaked fuel components in pure acetone for extended periods & have not seen substantial effect other than some minor swelling.

Fill the tires full of air. Slow down & drive the speed limit. Do not do short running around trips. Starting & stopping alot lowers the milage (hense why ''highway milage'' is a selling point). Get a tune up, change the spark plugs & wires, the air filter, & the fuel filter out. Keep the AC off unless absolutely needed. Get any excess weight out (empty the trunk of unneeded things- the heavier the vehical, the more ''drag'' it will have, slowing it down, & using more gas to get u from point 1 to 2).

Make sure you change the air cleaner! change the oil every 3 months or 3,000 miles, ritually! make sure the air pressure in the tires is all equal per MVSS. Throw in a bottle of dry gas, something that takes the water out of the gas, especially following a rainy season, & before the winter.

good question:: first off u can improve ur mileage significantly. now thats the good news. the bad news if that u may have to break some bad driving habits to do it. from the mechanical standpoint u can improve mileage by switching to using synthetic oil, using a high flow air filter & by running ur tire pressure 2 to 3 pounds higher than the recommended pressure, but never exceed the maximum!!! now as for really making a difference well that is up to ur driving habits. here where l drive people regulary exceed the speed limit by 10 to 15 mph all the while bitching about poor mileage???? some ever drive 20 mph over the speed limit & wonder why they only get 10 mpg in that gas hog suv??????????? so my advice is slowdown & drive conservatively. u may get alot of dirty looks from those idiots driving 85 mph in a 55, but u will be smiling all the way to the bank, not them!!!!!!!!!!!!!!!!!!

Buy a free flowing air cleaner like a K&N air filter & a free flowing exhaust. These improvements will increase the engines ability to pass air through & create a better burn for ur fuel & make more horsepower. Next get a throttle body spacer for it. This increases the torque that ur engine produces making it have more low end power so u will not have to press the gas pedal as hard to get up to speed. And always make sure ur tires r full & in alignment. The only problem with doing all of these upgrades is that the vehicle will become more fun to drive & u will probably find yourself hitting the gas a little harder just to feel it accelerate. After the fun wears off though u will notice substancial fuel mileage improvements. Hope l have been helpful.

the least expensive way is to find somebody who does the BG DECARBON SERVICE. It will improve gas mileage and power.

Drive slower, make sure engine is running at operating temperature, and running right.

Change air filter put a K&N one , Spark plugs , Use Syntetic Blend motor oil, put gas treatment and get a better exhaut sistem