[bestclimb]

Aux ank would be above cars tank, let gravity do the work for you. Make sure the car's fuel tank is nearly empty then crack a valve to let the fuel flow into the main tank. could be electric valve, or mechanical with a push pull choke cable affair to move it. If the tank is insulated from physical damage during an accident, and securely mounted it would be safe enough.

I have thought about doing a mod such as this to one of my vehicles for a couple reasons.
1 winter road trips extra gas can be helpful. Some places aint opened year round and I figure I could get most the way through Canada without stopping.

2 while stopping at night in winter can be an issue in regards to preheating not being available (at 50 below you gonna count on being able to restart with out plugging in?) so extra idle time could be handy. A portable generator to run heaters (or gas stove under the oil pan [I have done this]), and bringing in the car's battery would eliminate the need to idle all night.

3 Fuel may be hard to get during the zombie apocalypse....or what ever, so added range may be handy.

4 For highway travel being able to have more range may put you into territory where fuel is cheaper.

As far as mileage goes, on the highway extra weight won't hurt much as you are not on and off the binders much.