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"Is it normal in a 93 240 for the guage to fluctuate about 1/8 tank up and down as you drive..."
It depends on how you characterize "fluctuate." The temperature and fuel gauges are thermal gauges. In other words, they deflect the needle from the thermally-induced warpage in a tiny bimettalic strip. The gauges are not the normal type of electrical meter (d'Arsonval movement). The advantages are: cheap, rugged, and a long time constant, which helps damp out "fluctuations."
The time constant of these meters is several seconds -- notice how long it takes them to drop when you turn off the key.
This dampens out sender variations due to sloshing around, but if your car tilts sideways or up 'n down hill, you might see a variation. An apparent 1/8 tank variation wouldn't surprise me.
"...is it 'normal' for the needle to drop rather quickly, compared to the first half consumption rate, after passing the half-full mark..."
Every 240 I've owned, and some 140s before that, have all done this. The sender appears to be non-linear, although it may be related to the shape of the tank. I generally drive off my odometer -- around town, I think about filling at 275-300 miles. On the highway, I get itchy at 300-325 miles.
"...what's normal mpg, if in good shape mechanically..."
Much depends on the configuration of your car and the configuration of your (lead) foot. My '86 245, A/T, combination town 'n highway, gets about 22-25 mpg.
My old '82 POS rustbucket beater bomber 245 4spd-OD (with the MPG engine) could get upwards of 33-34 mpg on the highway. Partially, that was because so much rust and parts had fallen off, it lost 1000 lbs. And partially it's because I always chose the route that allowed me to go downhill both out and back. And partially it's because I ran about 250 psi in the tires. And partially it's because the motor was sooo loose it had no friction.
OTOH, my '82 245 turbo 4spd-OD gets about 17-20 fun mpg combo town 'n highway. I wonder why?
--
Don Foster (near Cape Cod, MA)
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