Because I've used my vehicles for business purposes for decades and I like spreadsheets I have kept close track of my gas purchases for both of my Sport Tracs. With the data printed on the receipt at the pump it is easy to compute exact gas mileage. I have over 250 gas purchases logged for the 2010 ST so far.
I bought the 2002 ST new and sold it in 2013 with 103,000 miles on it and bought the 2010 ST used in 2013. The 2010 has the V-8 engine. Both vehicles have 4x4 drivetrains (my best friend still drives the 2002) and both driven as primary vehicles. The 2010 is still running on the original spark plugs at 97,000 miles.
I live in Colorado with the home altitude of both vehicles of 5,300 feet. Summers are hot and winters can be cold. The owner's manual says to use 87 octane gas. At this altitude regular gas is 85 octane; mid grade is 87 octane. The 2002 ST lived on 85 octane gas. My driving is a mix of city, cross-country road trips, cargo trips over the Rocky Mountain continental divide and exploring back roads.
The 2002 ST got anywhere from 14 to 22 mpg over a tank of gas. Since the aerodynamics of the ST are typical of a truck, my gas mileage goes down as my speed increases over 70 mph. The best mileage I typically got was driving east on I-70 downhill across the plains of Kansas - as long as there was no headwind!
With the 2010 ST having the dashboard average mpg computer, I religiously write that number on my gas receipt and compare it to the computed mileage. The V-8 engine actually gets better gas mileage than the V-6 did. In the winter with bitter cold weather, some warmup time to clear the windshield and short trips to the grocery store over snowy streets my winter average mileage has dipped to 12.5 mpg for a tank of gas. On road trips I have gotten as high as 24 mpg.
Four years ago I started making multiple trips over the continental divide each year with up to 850 pounds of cargo one way, I decided to try 87 octane gas. These trips start at 5,300 feet, reaching 11,158 feet elevation at the Eisenhower Tunnel and after going down a long hill to 9,035 elevation in Silverthorne going back up to 10,666 feet elevation over Vail Pass. There are many steep ups and downs with the transmission dropping back to 4th gear on the way up. I drive within 5 miles of the speed limit, which is 75 for parts of the drive.
By the time I get to Grand Junction, elevation 4,583 I typically have averaged 20.5 mpg. On the way home with no load except me, I get about a bit over 22 mpg. When I was doing this trip with 85 octane the numbers were more like 18.6 and 20.1 mpg.
The dashboard mpg calculator can run up to +/- 1 mpg different than the computed mileage, typically overestimating it. My average overestimate so far this year is 0.5 mpg.
I found that the increase in gas mileage by switching to 87 octane for these trips over the Rocky Mountains is a break even financially when the price difference between regular (85 octane) and mid-grade (87 octane) is around 30 cents per gallon. Lately that differential has jumped up to 40 cents per gallon. But the higher octane gives me better performance, which I see in fewer and shorter drops down to 4th gear on the steep uphill road segments.
Bottom line for all of this is that your dashboard mpg average number will usually give too optimistic a reading but that number is NOT consistent, depending on conditions. Some times it can also read low.
I can't offer technical or troubleshooting advice but did want to post this information as a reference as to what a properly operating ST can do. As noted by others, 6-7 mpg means something is dramatically wrong with your engine and is not typical of these vehicles.
Apparently they are also full of fun stuff like asbestos :/ ....
