Does anyone have a formula for the decrease in MPG due to increase in payload?

I'm thinking real hard about adding a Transferflow 70 gallon across the bed tank. This will save me money and time (San Antonio diesel is among the best price in Texas; and it takes time and mileage to go fill up). Neither will be a huge immediate savings, but I think that the savings will slowly add up. And the convenience seems like it ought to be nice.

I realize that a precise calculation will be complex if someone is going to consider the fixed weight addition of the tank (it appears the Transferflow tank and hardware will weigh about 190 pounds), and the constantly decreasing weight of the extra fuel (I believe that 70 gallons of diesel will weigh between 490 and 511 pounds, depending on the density).

So, for my purposes, a rule of thumb may be adequate

Or, if you have the "more complex calculation," then I can apply it to the "worst case scenario" of when the OEM tank and the Transferflow tank are both full.

It seems that the placement of the tank in the bed, up against the cab, would have little, if any effect on wind resistance (if anyone has info to the contrary, I'm interested in that, too.)

Also, if by chance you've actually calculated this with your '07 Duramax with a 70 gallon Transferflow tank, that info would also be fantastic to have.

Thanks for your help.

Wow, you sure are putting a lot of thought into a couple hundred extra pounds... When you have friends go with you to lunch, do you get worse fuel mileage? Would be the same thing right?
Regardless, I think the aux tank on my truck was one of the best additions. If you travel out of your area, the fact that you can fill up at truck stops and cruise past BFE fuel stops will save you money right there. I can choose where I get fuel. When I go to California, I get fuel at the last stop in Yuma (arizona) and I don't need fuel again until I get back to Arizona. Since Az. fuel is ~$.35 - $.50 less per gallon, it makes good sense to me. If you tow, I'd also recommend it.
If you are just looking to justify the cost of the tank by time savings... I don't know.

I seriously don't think you're going to notice the difference that adding an extra 500# wil make in your overall mpg. Think about what blksmok said... it would be no different than having 3 more people in your cab.