Is this normal for a heavy car?
 

Righto, just something I noticed when I'm driving on nearly any road (Except perfectly flat)

Even the slightest upward grade can have a massive effect on my FE. Going 65 down the highway, I'll get anywhere from 45 to 25 mpg, depending upon the slope. (And we're not talking about massive changes. I live in Northern Indiana for pete's sake. The slopes are barely there at all...)

Is it due to the fact that I have a heavy (3500lb), but aerodynamic(.35 drag coefficient) car that my biggest effect is caused by slope? (From 55 to 65, I see maybe a 2-3 mpg loss, but on a slight uphill, I can get a 10mpg loss)

I've stripped out all loose items and have essentially an empty car.

I've got my tires at around 40 psi. Would upping them to 45(sidewall max) have much an effect? What would likely show the most dramatic effect on my FE that I could do?

Turbo spooling up maybe making you run rich?

Ya, this makes sense to me. But the turbo seems like a possible culprit as well....

Nah, turbo doesn't spool. My vaccuum level goes more towards 0, but only on a very steep hill do I see boost.

Ah yes, and I shall clear up a little what I meant.

I don't cruise at 45mpg, I cruise at around 35. Just with the cruise control on, slight slopes will make it drop down to 25 (going up) and jump to 45(Going down)

Sometimes it can be as much as drop to 20, raise to 50 (And that's not counting when I put it in neutral and coast. Then it jumps to 100+mpg)

The slope effect also hits lighter cars. Slope has a much bigger effect than speed on my civic. I have to either let my speed drop or accept a drop in mileage on a slope. If I am getting 90mpg at 65mph on the flat, the mileage can easily drop to 40 to 50mpg on a slight upslope and rise to 140 to 160mpg on a slight downslope.

what car is it?

My T-bird. 87 turbocoupe.

ahh the ol thunder chicken!

Biffmeistro, I have the same problem as you, On my sable. Fill your front tires up a little more psi, Possibly consider a air dam or belly pan to improve areo.
This is how I can tell if my gas is good or bad, the amount the mpg drops on slight uphills, Good gas I can keep it up around 28-30 uphill, when I get some bad tanks of gas my mpg drops way down to 22-24 on the same hill. I think due to excessive moisture in the ethanol.
Try to speed up slightly before the hill and try to lighten up on the throttle while climbing the hill even if you drop some speed, try to recoup the speed when going down hill.