with that kind of gas mileage, there has to be something wrong...when the last time you did a full tune up? as every one said a custom tune will help but you should be ATLEAST getting 10-12MPG at the worst now...lol Ive heard of people getting better mileage pulling a trailer loaded with 8k on it

 

With that many miles, you need to change the plugs, wires, fuel filter, air filter and add a big bottle of Techron injector cleaner when the tank is pretty low. Seafoaming the top end, cleaning the tb and MAF are probably good ideas as well. There is a good writeup on Seafoaming on LS1Tech, I believe. Then after all of that, get a good tune.

But as was said earlier, 6.4 mpg is way too low, something is not right. I would start with the cheapest fixes first, like plugs, fuel filter, injector cleaner, etc. to see if it starts improving. Best of luck and keep us posted!

 

Seriously though, our 2001 Chevy 2wd at work with a trailer hooked up to it all the time at least pulling 2k-4k in weight gets no worse than 11-12. I would definately say their is something wrong.

 

to answser all my truck has fastory 20's but they did not come on the truck new. The truck has 66000 miles on it and the tires are 275 60 r 20. I am getting about 12-14mpg

 

ha 6.4, our 8.1 avalanche we use at work to haul bikes get 8.8 with a 28' 12k pound trailer! check her over good, definitely not healthy. But i hear you on the winter gas here in mn, i lose 2-3 mpg during the winter in all my vehicles