everything I've read says it's 35 ga.
With an 1/8 tank the most I can get is is 20 ga.
Do they have different size tanks or is the fuel gauge off ?

 

Sounds like your fuel gauge is incorrect. I've rcently owned a 2003 E250 and now own a 2007 E350 and fill well over 30gallons when very low.
The most I've added is probably 33.xx gallons, but that's with the needle pointing below the empty line.

When you fill it, does it indicate Full?

I'd suggest keeping a 5gallon can full of gas and go run the thing til it dies.

 

yes , it says full and I was thinking the same thing about running it dry

 

An off-topic bit of advice: don't consistently run the tank below 1/4---its premature death to the fuel pump.

I believe I too can crowd about 34 gallons into an empty tank. The gauge will read full and not move for about 80 miles which would equal about 6 gallons.

 

No , I wouldn't do that.
As far as I can tell , I wasn't even close to getting under a half
The gauge was reading 1/4 and all I could get in was 17 gallons.

 

At a 1/4 tank 17 gallons might trip the gas pump's auto shut-off on the handle, that so-called "first click". While its not the best practice I tend to stuff as much in as I can because I fuel up at an outlet the gives me a discount according to number of points I can accrue.

 

That is not the case in this instance.It's full as it can be.
My gauge has got to be whacked