Get a foolish type friend to follow you.

Get on it. Give the guy following you a signal that you are getting it on!

See if you are blowing black smoke out the exhaust. This may give you some idea of where the gas is going.

Take a look under the car. Look for stains etc where you might have some leaks.

These kinds of things.

David Duffield

I was getting 5 mph. On my 89 930 it turned out to be the WUR. The exhaust was black, and there were black deposits inside the exhaust pipe and on my bumper. I diagnoised it with a CIS fuel pressure tester.

...and once you diagnosed it was the WUR,

Did you fix the WUR (If so how?)
-or-
Replace it? (If so where?)

I bought a rebuilt unit from someone in the Dallas area. It cost $190. I just did a google search on the part number and got exactly one hit.

Here's the local link

http://forums.pelicanparts.com/porsche-911-technical-forum/148707-89-911-930-running-poorly-5mpg.html

Wow...

I can't imagine my 930 running remotely decent if it were rich enough to burn 5MPG. As you're checking the components others suggested, keep a sharp eye for fuel leaks. I had a similar problem and it was a under-torqued banjo bolt on the fuel distributor.

Gas was leaking down on to the case where it evaporated without a trace. Let me tell you how fast I went and got a fire extinguisher for all my cars after I found THAT problem!

I'm not an expert on how the spark advance is set on these cars (yet) but I'd suggest that you not forget the spark. If your static timing is set low and you have no vacuum advance, you could really kill your combusition efficiency, off boost power, and mileage. Because the car is so dead off-boost, you might not notice the problem and the car could otherwise run normally on-boost. Especially given the twin-plugs and aftermarket ignition setup, I'd take a look at the spark advance at idle.