I am simply amazed at the length this thread has gone. When in truth we are all driving a brick that is not made for speed OR mpg but please keep it going humor is something I love to read......

 

Here is the solution - aerodynamics - plus power .... but not much off road...

 

That poor Defender......

 

As a rule of thumb, on a semi, the drag is 25% frontal, 25% rear of trailer, 30% under truck and trailer, and 20% the spaced between trailer and truck. So, for the home DIY, a few dozen cases of great stuff foam, and some sculpting, ....

 

Since you're obviously not interested in wheeling, go with some ULRR tires. They have great results.