POOR Gas Mileage - Ideas????

[3 DOGS]

My gas mileage has dropped - BAD!!!
When I bought the truck (used 39,000 in June) I was getting 18/20.
I then took it in for an oil change and check up (at the dealer) and my mileage went down a little to 16/19-20 - nothing too bad but a definate differance that I noticed.
Then I got my new tires - 265/75/16 - up from 265/70/16 mileage dropped to 16/18 - expected I guess.
I traveled out to Colorado and ONLY got 15-17 on the interstate across Nebraska. I figured it was just the strong cross wind. Still very unacceptable though.
In Colorado I got 19/21+ !! I was amazed. This included traveling from Colorado Springs to the White River National Forrest.
Driving back to Nebraska I dropped to 16/19 again. And now I am getting 15/18
I have tried fuel injector cleaner, right foot control and everything else but my mileage keeps dropping.
I knew with the tires it'd go down but the rest of the info I've given doesn't add up to the mileage dropping so badly.
I use 87 octane gas. NO ETHANOL!
Any ideas why?? Any ways to help correct the problem? I don't have nor do I want a K&N system. I am trying to keep from going overboard with "bolt on" this and "modified that".
I can live with 16/18 or better - this 15/17 and less is almost as bad as the Chevy I traded in for the Taco.
HELP!!!!

[Ursidae69]

With each new mod, mine drops more. I average 16mpg. I wouldn't think that your tire change would have that much of an effect.

[flyingwil]

I have noticed that mine changes too, I fluff it off to summer and winter gases. Also with the larger tires, as small as a 2 psi change I notice a change. Try this... try 35 psi for one tank (I did this to prevent the cupping), then go down to 32 psi for the next tank... you will see a change in MPG. I also notice that windows up/down makes a difference too.

Start mechanically, air filter clean, MAF cleaned, and work your way on down, fuel filter, ect.. That might have something to do with your performance.

[bigreen505]

... then move down to the O2 sensor.

[Ric]

I use http://www.seafoamsales.com/ works great, really cleans the engine out.

[erin]

I dropped about 1 1/2 mpg with the new tires, but after regear, got it back. But when I took off the air dam and added a winch bumper, I have now lost about 2 mpg. Like Chuck said, the more you add, the more you lose. Your air pressure will definetely help though.

[Mlachica]

In addition to what's been said...

Has your driving style changed? cruising speed?

have you kept up with your maintenance? Such as your air filter, fuel filter, spark plugs?

[Nullifier]

Wieght has a huge efect. Were you loaded with a bunch of stuff? even 250lbs can really make a difference in the mountains.

[3 DOGS]

I plan on the fuel filter next and I guess the O2 sensor.
As far as weight I had more gear/weight in Colorado and got 21+ mpg - just really odd.
As far as the a/c or windows no significant changes when I checked the mileage before.
I just don't know. I am going in for the oil change and tire rotation so I will have them re-balance the tires and try a different air pressure.
Everything else is stock - or the way I bought it - except for the gas mileage going DOWN.
Thanks for the input

[Grim Reaper]

Did you take into account that the larger tires makes your odometer run slow?