Compression Ignition Addict
Join Date: Sep 2012
Location: The 'Sip
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Yep, you're fighting several things here in the MPG department.
Gearing is not going to be great for highway driving (unless for some reason you have 3.73s... which I highly doubt). Adding a taller would help, but that's an expensive proposition on a dually with 19.5s. If you have the 17s, there may be hope, but I don't know enough about tires for these load ratings to tell you anything useful.
A tuner, airbox (intake), and DPF delete will probably help. But, there are some serious issues that are going to keep this from being a Honda Civic.
The other big problem you face is mass. The weight that you're moving around is detrimental to MPGs. It's not terrible when on the highway, but when you're in stop-and-go, it has to be a serious negative. I don't think there's much you can do about that.
The next is drag. On the highway, I'm going to go ahead and assume this thing cuts through the air much like a house. Reduce speeds is all I can offer there.
Increase the tire pressure as much as you can and keep your foot out of the boost.
It may be your only purpose in life is to serve as a warning for others.