[ddmcse]

i think the tank holds 18 gallons

sounds like you used about 8-9 gallons
lets call it 8 gallons used thats like 11 mpg
you have any black or dark grey smoke when you punch it ? you must be running rich

my mileage dropped real bad a few years ago , i was down to about 13 mpg , but thats mostly boosting around not too much highway . I was running rich and i could see it on my air/fuel meter somehow during that summer i slipped into rich i changed plugs checked timing, kept going over and over the syptoms in the manual all that stuff.
what finally worked was replacing the vsv switches on the intake side once i got the egr vsv replaced i was back in the zone . i don't really care too much about the mpg number as much as seeing the a/f meter in right mix zone .
running rich is a waste of gas and horsepower and makes a black mess everywhere , running lean will kill the motor ..
do you know where the leak is ?
Benesesso just found a leak in his downpipe it's common and it's right near the turbo . it blows a ton of wicked hot air right into the back side of the engine and the firewall . another hot spot could be a crack in the manifold and you won't see that because it's hidden behind heat shields
i don't think the leak is causing such a drop in mpg , i'd say it's your mixture