Re: 5 miles to a gallon
Regardless if the petcock is the problem or not, throw out the stock petcock. It's an embarassingly poor design and it actually limits the flow of the fuel. If you bested it open and looked inside, the holes that are controlled by the valve are HALF the size of the nozzle! The stock petcock doesn't really have hard on and off positions and it's prone to leaks.
I made a petcock out of a barbed brass spout and a plastic fuel valve from Orchard Supply Hardware. The brass spout wasn't metric but it fits well enough into the stock tank that it stays in and doesn't leak at all. The plastic valve works really well, and I use E85, so I've found that it's extremely resistant to wear. And it has hard on and off positions!