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?
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.
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.