High Idle and Horrible mpg plus bonus gas tank question

I've got a 90 st185 w/ ~160k miles on the engine. Just had lots of major maintenance items taken care of but there's still something bugging me with the car...

When I start it up it idles around 1k, a little bit over actually. Once it's running and I'm driving around town at stoplights and such it idles around 800rpm. Does this seem absurdly high?

I also get horrible mpg. Even when I drove an entire tank calmly I only ended up with a sub 20 mpg, like around 17 if I remember correctly. Typically I get only I know this car isn't great on milage but this seems a bit excessive.

Also for my bonus question I'd like to know what the capacity of the gas tank should be. According to my owners manual it should hold ~18 gallons but it seems as if it only holds 16? If I refill it when the gas light comes on it only takes about 14 gallons and when I've run it to what seemed like fairly close to empty it still only took 15.something gallons?

your idle is fine, your mpg sounds about right for city driving. As far as what the tank will take, your light doesnt come on when the tank is bone dry and that would be why it doesnt take as much as it should. I wouldnt suggest running your tank dry either to find out what the "true" capacity is, unless you want to replace your fuel filter because it sucked up all the deposits in the bottom of your tank.

check your tire pressure.

I am running Falkens on stock rims and run 42psi front and 34 psi back. It helps offset the poor weight distribution some as well as saves gas.

I ran 32psi all the way around and did 21mpg on the highway and had abnormal tire-wear concurrent with low-pressure in the front.

I ran the above psi and got almost 28 on the highway under the same driving conditions.

Drop 15-20% for city driving.

If I am often in heavy boost, my city mpg drop to 14-18mpg depending on the outside temp.

Oh, the temperature probably has alot to do with it, I live in the desert known as Tucson AZ. I will check my tire pressures too, I don't think they're underinflated but it could be.

Heh, and I'll definitely avoid running it bone dry. It just seems like it's only a 16 gallon tank because it lets out a really loud hissing noise (like depressurizing) when I open the gas cap yet it only takes 15 gallons?