05 Dakota, 4.7 V-8,57,000 mi.Was getting 18-20 mpg highway mileage. Mileage has dropped to around 13 mpg, no matter highway or city driving.
Have tried "Lucas" etc. No help. Any suggestions would be appreciated..

Post some more information are you getting any codes does the vehicle run rough.
A defective thermostat will cause the computer to enrich the fuel mixture because the engine is not operating at the proper temperature for the computer to run the system.

Had the same problem with my 98 Jimmy. I believe (opinion only) that the problem goes back to the problem at the filling stations. As gas companies put more ethanol in the fuel and dilute it down, it then requires more fuel to make the engine perform (ie.. less mileage). Newer vehicles should be affected as badly but it really seems to make a difference on the older engines.

o2 sensor bad?