I have a 100 Gallon Transfer tank/toolbox combo in the back of my Dodge (total onboard fuel = 135 Gallons) and can get to Yuma easily from OKC without having to stop and buy fuel but I have to stop and pump the fuel, I have an external electric pump instead of the gravity feed. What I have heard is if you have a fuel mileage readout in your truck it could create a check engine light due to the fact that the trucks computer sees that for a long time your tank remains full and confuses the computer and throws a check engine light thinking something is wrong.