I know we've all heard this a million times, but at $4.00 a gallon for gas, it really hits home.
Check your tire pressure.
I found that I get an extra 1 1/2 to 2 miles to the gallon when I keep my tire pressure up to 32 PSI. When my tires get down around 25 PSI, I start seeing 17 -18 MPG, but when I keep the pressure up to 32 PSI, I enjoy a sweet 19-20 MPG in my 4.3 L 2000 Jimmy. So, I get an extra 30-40 miles per tank and that adds up quick!
Here is a good spot check. Using your dashboard tach, drive at 60 MPH and see what your RPM's are, Mine is usually a click below 2000 RPM. Now drive at 2000 RPM and see what your speed is, Mine is around 65 MPH. Every vehicle will be slightly different, but if you establish a baseline for your truck, You will have a mental red flag to tell you that something needs attention.
Hope this helps.
Rick