My gas mileage isnt very good and my engine is in tip top shape. I was wondering if a gear swap would help.
Here is my axle code: C8.
Would switching to another gear help at all?

 

this post shows you where to look for the gear setup. If you have the 2.73 gears, then you're probably set for mileage. If you have the 3.73 (truck gears) then swapping out with 2.73 should get your mileage up quite a bit. lower the number, the better the mileage, but the worse the performance (won't get off the line as fast). You will have to change the speedometer gear in the transmission to reflect the gear ratio so the speedo will show the correct speed.

by the way, the 98 mgm is only supposed to get around 15 to 22 mpg (as posted by http://www.fueleconomy.gov/). Only the 92-95 cars got 16-24 (EPA but usually 18-26 actual).

 

I cant figure that chart out. My sticker in the door says c8. By the way the car drives i would say its safe to say that i have the 2.73 gear. I feel like Im losing more gas in the start and stop driving bc the car is geared to high. When you first take off, the car doesnt feel like its in a low gear at all. It feels like more like its starting in overdrive. Although I know the transmission is shifting through the gear perfectly. It just seems like it needs a little bit lower gear on the take off.

 

that's the way the restrictive V8s are in the boats. Especially the MAF systems. They lag off the line, but pick up good in the mid-range. The old speed density systems from the box bodies would jump more off the line, but the mid-range was weak. I'm pretty sure the transmission already has the wide range gears in it (since those have been standard since at least 93 I think). The Grand Marquis is just more of a hog than the vic with all the luxury features (vinyl top, power everything, air suspension will add more weight too). If you're getting mileage below 15 mpg in the city, I would suggest a higher flow air filter and cleaning the MAF sensor (contact cleaner or MAF cleaner should be used... and no touchy the wire bits... just hose them off real good). A top end tune up may be in order too (plugs, wires, etc) and dual exhaust doesn't hurt if it's not already equipped (with high flow mufflers of course). You just gotta remember to keep your foot off the floor. I have a hard time with that.

 

Thanks for the replies slymer.
I just did all new pulgs last month when I put the new intake manifold. I do have a check engine p0430 I dont know if thats affecting my gas mileage or not. Car doesnt misbehaive so I havent looked into that code too much.
I will look into the air filter and see if t hat helps any.
Mine doesnt have any luxury options except leather and power windows, locks and seats. No big deal I guess. I have been wanting to learn to drive speed limit anyway. Since owning this car I dont think I have broken the limit very often. Now in my 95 buick park ave its a different story. Shes quick off the line and quick all the rest of the way and barely uses any gas.

 

p0430 means the catalytic converter (or the post cat oxygen sensor aka catalytic sensor) is bad. If you're in an emission controlled area, you will have to get that fixed before it will pass inspection.