Hello I will help you with your question,
You might try last weeks episode of "Car Talk" , This months edition of Popular Mechanics or Snopes.com for more on this urban legend.
Let me know how I can help you
Yes it is a well know urban legend and well documented at the sites I referred you to.
I am not a kid either being 57 and have heard the legends about the 200 MPG carburetors since I was ten.
Sorry if this offends you but do the research at the sites I posted and you will see the legend unfold.
Sorry I can't help you Detroit never produced a 3 ton vehicle that got 50MPG. That is the stuff of legends.
I will OPT OUT and wish you luck with your chase
If you could post a picture of this it would help- just click on the "paperclip" icon above the dialogue box.
BTW, HERE is a a link to the Snopes.com story