I can attest to the 30mpg rating on the Marquis's. I regularly pull high 20's on highway trips in my '93 with 146K, I average 16-18mpg running around town all week.
I know I have 3.08 gears, and run steady....aka Cruise control assisted....speeds on the highway. Other than that, I don't really push too hard in normal driving.....easy starts and stops, that sort of thing. Oh yeah, and regular maintance.
The one quirk I have noticed, and I may be crazy here, but if I run the same brand of gas everytime my MPG will level off then drop slightly. If I vary fuels....a different brand each time....then the mileage stays up. I would say it's all in my head, but I am picky about my gas mileage and I reset my 2nd trip at every fill up, so I really notice a variance.
1993 Grand Marquis w/170,000+ miles and counting. Now it's time to make her FAST!!!
12" conical K&N filter, Custom side exit exhaust with X-pipe and Dynomax Super Turbo mufflers, replaced stock fan and clutch with PI electric fan and PSM fan controller, Aluminum PI driveshaft.
Body mods include a '95 Cobra rear spoiler, tinted windows, 17" Mustang Split 5-spoke rims w/ P255/50-17 BFG g-Froce TA radials, and a 14,000lb Auxillary Trans Cooler.
Interior: Custom A-pillar guage pod w/ Digital oil pressure and Trans. Temp. guages.