Just one thing to add how are you calculating your gas mileage. You said you are getting 135 miles to a tank, but how much gas did you put in? Calcualte gas mileage by dividing the miles driven by the amount of fuel put in. Over several tanks you will get an accurate indication of acutal mileage. I often fill my tank when I have gone around 150 miles and put in about 3.8 gallons for a mileage of 39 mpg. That is the number I get around town and on the highway at steady cruise of 70 mph I get about 44-46 mpg. Several factors can affect mileage like loading, wind, throttle hand, alltitude, humidity, just to name a few. If you are putting in 4 gallons (the point at which many low fuel lights come on) at 135, you are at 33.75 mpg. That would indicate a fairly rich setting in the carb.
I have an 06 Silverado, with Cobra Speedster Slash downs, Barons BAK, 170 main, needle 4th groove, PMS at 2.5 turns out, and AIS removed operating at sea level.
Capture some mileage and fuel numbers and we can see where you are.