RE: Mileage
This does seem really, really weird. I get around 30 mpg in my 93 doing pure freeway at about 57mph on flat ground and high twenties at seventy. Those freway numbers were before I fixed the AC so they have probably droped 1-2 mpg. In town, with AC, I drive total stop every other block sidestreets and get 15-19 mpg. Overall for me is about 19-20 mpg since I basically spend no time on the freeway in my average week.
My opinion, take it or leave it: Low milage car huh... they are notorious for having nasty carbon build up especially if they have been babied. Be sure not to forget that although streelight racing isn't maybe the best thing for it. Wide open throttle accelerations (with a rolling start rather than from a stop is easier on the car and tires) are absolutely necessary to the car's health long term health. I drive pretty conservatively so I usually punch it to the floor on a relatively empty on-ramp and then I slow down to something like the speed limit when I get to the bottom. Assuming that your car is in good health and doesn't smoke, if you see any smoke at all (that wasn't from the tires) when you punch it to the floor like this, you have carbon/crud build up.
Random thoughts/questions to consider: Have you seafoamed it to remove engine crud and then replaced the normal tune up stuff (plugs, filters, proper oil full synthetic 5w30 or similar (I use Mobil 1 despite not liking the company, it is a great product)? Does it idle smoothly? Is your overdrive lock up working? Have you manually checked your mileage as the mileage computer could be wrong especially if speedo is off. Are you running premium fuel? I don't think many of us (or maybe any of us) use 87 octane gas. Are the wheels stock size? Have you tried shutting off the climate control (or using vent or floor buttons)to see if the AC makes an unusual difference? Any belts squealing or pulley bearings making noise? If you come off the freeway at at over 60 and then stop at the light at the top of the exit and you do it aggressively (IE you barrel up the ramp and then stop somewhat quickly) can you hear or feel the compressor kick in to bring the car back up to normal ride height from the over 55 MPH trim height?
Timothy
PS Hawthorne, my 93, is 100% stock with no mods of any kind (except the heat resistant chrome tape inside the headlight and the aftermarket radio that some Jacka$$ previous owner put in (I am third owner))
PS on a long road trip (without AC) My record was I got 33 for a stretch of I-94. But then again I get the funny feeling that Hawthorne is on the leanest end of the stock spectrum (still goes like a demon when pushed tho so who cares). Also, I was specifically experimenting to get the best mileage possible. I was using the cruise at like 52mph or something absurd like that...
Peak mileage will on any Mark viii will be at whatever is the lowest speed you can slow back down to after getting the car to drop to it's 55 and above height without it raising up again. Usually somewhere around 50 I think. Needless to say after I got the 33MPG I was very happy and immediately went back to driving 75 or 80 since by then I was running like an hour late because of my lollygagging...