2007 Duramax Diesel

Hello guys well many of you may have seen my previous thread about the duramax that I was purchasing and the good news is i went ahead and bought it!!!!

No pics yet because it is at the shop till Monday getting e-brake fixed.

I have some questions for anyone who is willing to help?

Specs:
-2007 LBZ with allison
-standard cab, 8' bed
-4" Rancho suspension lift
-315/75/16 Yokahoma AT's
-4x4 Automatic

The gas mileage on the dash says average is 14.3.

Do you guys think that I need a tuner to recalibrate the speedo and that might improve the gpms? Or do you think the gas mileage decreased this much do to lift and tires? If so will a tuner with economy tune improve mileage?

What tuner do you recommend?

Is the Edge any good?

Any info welcome...

Thanks again for help...