OP- So the PO "says" it got 15-18 or you have observed 15-18 since you've owned it? Big difference there.
So here are some more questions to answer:
1) If indeed it got 15-18, was the PO driving like grandma and now you drive like grandpa after he's had a couple shots?
2) Did the PO drive mostly highway and now you are driving mostly city?
3) Have you owned a diesel before? I ask because our truck are notorious for not being able to get the last few gallons of fuel in the tank easily. Consistent filling of the fuel tank is important to accurate fuel economy readings.
4) Have you checked odometer accuracy with the 33-inch tires? Run a couple of measure miles in a straight line (mile markers on straight sections of the highway). If you read near 1 mile, you can eliminate that as a possible source of inaccuracy. If you cover only 8/10s of a mile, or some number less than 1, then your odometer is reading less miles than the actual miles traveled and that will make the mpg calculation read less than actual (the opposite is true if it reads more). If that is the case (forgive me if you already know this), and let's say it's 8/10, then all you do is multiply your miles per tank times .8... 100 miles X.8= 80 miles actually traveled and then you will get your real MPG.
Beyond the above, who knows. The E4OD trucks are usually the most economical of the ones in our era due to the OD trans. Usually if the truck runs "good" (by an objective standard) it doesn't have any serious fuel system problems. You could toss a lot of money at it trying to figure it out and that why it's best to start with the free and easy, basic stuff first. Like checking the odo and making sure you are getting a full tank (an extra five minutes squirting the fuel in a quart at a time until the tank is visibly full). If you put the 33s on and the PO was using stock tires, that is a strong indication of odo inaccuracy being part of the problem. Not intending to insult anyone but I wouldn't necessarily take the PO's word 100% on the mpg either. Unless you've observed the same using accurate methods of checking and then suddenly it changed....? When I was a dealer tech, we used to get complaints of poor mpg. 75 percent of the time it was either inaccurate mpg checking or the driving situation and the owner "expecting" more (like a guy who cruised at 100 mph and still expected to get 16 mpgon the highway. Of the remaining 25 percent there was a mix of legit vehicle problems from EFI issues, dragging brakes, low air pressure, fuel leaks, etc.
