If you weren't regenerating at first then you would see outstanding MPG as you were but you also clog the DPF as you did. I drive all highway and the best I can get unloaded is 17MPG which I would think in town would be around 13 or 14 MPG. The only thing you can do to increase MPG is gut the emissions. Interestingly, 17 MPG is the best I could get in my 1/2 ton Nissan Titan so I'm happy.

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2007.5 Dodge Ram 3500 HD,QC, 6.7L Cummins, 6speed Auto, 4X4, Bighorn Edition.
Click for installs: Carr Steps PML Diff Cover Edge Juice with Attitude Volant Intake Hankook ATM RF10