HEY another 'Nati guy.
I hate to tell ya, but your SOL. A tuner will be your best bet, but on a budget your gonna hate the price.
Highest I've EVER got was a 100% all highway 20mpg.
Originally Posted by sam0330
...with the tailgate down...
You get worse mileage with the gate down. The gate helps create a high pressure area behind the cab that allows air to just flow over the bed.