99' pont. gtp sc 3.8l miserable mpg???

so, my wife owns a 99 gtp sc 3.8l sII. we have owned this car for over 3 yrs now. when we start it the crank time is pretty enlongated before it starts. keep in mind it does have a k&n air filter and aftermarket k&n breather tube. she does need a lower intake manifold gasket (i saw oil in her radiator filler neck, and visible external leak on LIM). her current mpg is 15.6 according to her avg. econ. its usually between 15-16 mpg. this is horrible compared to the apx. 25 mpg that she should have. alot will ask does she get into her boost often? she never gets into her boost and drives easy and slow on her take off. we can put 20 in gas and within an hour her low fuel light is on. any ideas why her mpg is so low??? the car runs fine other than a slight chug, and long crank, on cold starts. no misfires, she runs 91 + octane, and never gets into the boost.

is the sc model really that much worse on gas than the normal sII 3.8l?
ANY ideas where i should check that would cause to lose so much fuel???

also there are no fuel leaks, nor do i smell fuel or exhaust. the cat flows freely as well. i wondered if she has an injector stuck open but the car runs fine. its got new plugs and wires as well as coils and ignition module.

does anyone have the specs. for the injectors on this model handy???
do you think that the lower intake leak would cause this kind of fuel loss???
or would a faulty fuel press. regulator cause this fuel loss?

btw... the car does have 175k mi.

ANY and ALL knowledgeable answers are greatly appreciated!!! :runaround:
I WILL try to add pics. to thread when i do the lower manifold gasket on the r&r procedure for the SC model.

16 city/25 highway/19 combined........only about 1 gal difference in a naturally aspirated engine....

Have seen them get higher mileage on the highway, on long trips(closer to 30)........

K&N filters, which are oiled, can have an affect on the MAF sensor.....

New ignition parts....... ever replace the fuel filter? Check vac line to f/p reg for fuel? Sometimes O2 sensors become "lazy" and affect mileage....

These are heavy vehicles......city, stop and go, and hilly terrain driving will lower mileage.....

Our old '92 Bonneville would get about 29-30 mph on trips to the Cape.......driving around town, it was 22-25, depending on which lead foot was driving....