I do mainly highway driving and average 19 or so. My ST is a completely stock 4x4. I have seen as high as 22; typically in the summertime.
To improve mileage?
Increase tire pressure from factory 34 to 38 or 40lbs, helps a bit.
Remove the factory air silencer on the intake.........may help...a little bit.......
Wanna spend money?
The exhaust on the ST is somewhat restrictive. Replacing the factory exhaust with on of the many available aftermarket units typically yields a MPG or 2.
Xcal2 is a computer device that can be programmed to add a MPG or 2.
You can also replace the belt driven fan with a flexalite electric 16" unit. This way the fan only turns when it needs to, thus removing a bit of drag from the motor. I put an electric fan on another car and it only ran in traffic. On the highway, it didn't even come on....
There are many other things you can do to enhance the efficiency/performance of the ST, but I think these would yield the most in MPG....
The reason you will see lower mileage on the ST vs. the car with a larger engine is due to wind resistance. The ST has got a fairly large wind profile, and this will keep it's mileage more truck like and less car like.
cheers,
3.73 gears with 32" tires too, I had a 4.0 SOHC Ranger with 4.10's before and it was screaming on the highway so I wanted something to slow the motor down a little on this one.
