2007 Sierra Dash Lights, Voltage Guage, & Mpg-fuel Used Computatio

[jmshan]

Two and possibly a third anomaly with my 2007 Sierra SLT Extended Cab 4X4. First, the fuel mileage and fuel used computations are off. The mpg will vary by .3 to .7 mpg more than the vehicle actually gets and the fuel used is .5 to .9 gallons less than it takes to fill the tank. I have drives a '07 Impala and the mpg and gallons used were dead on and I would have expected GMC to be the same. Is there an upgrade to the computer software to solve these differences? Secondly, when I first start the truck in the am (about 7 CST), all the lights are on even though it is not dark. All the dash lights and displays are nice and bright but as I drive to work over a period of 10-20 minutes, the lights get dimmer and the radio display is completely unreadable due to it being so dim. Then about 15-20 minutes into the morning drive, the display lights come on nice and bright again. Have not gone to the dealer yet as I need the truck on a daily basis and most likely will have to leave it overnite. Lastly, I have driven GM trucks for the past 17 years and I have never had a voltage guage that didn't stay rock solid at 13-14 volts. This one did for the first week or so but now starts at 14 volts and will slowly sink to 12 volts. Has the charging system been changed or do I have the start of an issue.

 

Any thoughts will be greatly appreciated.