If his problem was really in the vapor tank system (or even partially), the tank will "top off" at different levels from fill-up to fill-up (BTDT), making it impossible to get anything like a consistent mileage calculation. I could see one or more of those lines getting pinched if the tank was hit hard enough from the bottom to dislodge or deform it. 
If that was the case you'd think he'd be getting varying mileage figures. If it's consistently down 15% and doesn't vary much I'd be looking for a leak or opening, as you said, possibly in the top of the tank (like one of those large grommets) that's either actually leaking and then vaporizing before he can see the gas, or it's just allowing a bunch of his gas to escape the tank as vapor. 
He probably needed a new tank by now anyway? : /

Cya,
Robert

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of Dave Mcneely
Sent: Wednesday, June 23, 2010 2:23 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: squished gas tank?

Why would the tank being bent affect the mileage?  The gas mileage is the miles driven divided by the volume of fuel used.  If one fills up, drives 100 miles, and filling up again takes five gallons, the mileage is 100 miles divided by 5 gallons, or 20 miles per gallon.  A dent has no affect on that.  It just affects how much fuel the tank will hold.  Now, if the tank is not only bent, but has a lead, that would reduce mileage, as well as be dangerous.

DMc

---- Jeff Palmer <jpalmer@MTS.NET> wrote:
> So when filling my tank the gauge never reads "Full" anymore, and I'm getting about 15% worse mileage.  Does that sound consistent with others who've had their tank squished when up on the lift?
>
> Thanks
> Jeff
> 85 Westfalia GL

--
David McNeely