Wow, here's an old can o worms reopened!!
I have one and compared to the stock petcock it's like night and day. No idea on gas mileage, because frankly, I don't care any more.... 120-130 miles and I look for a station.
There should be a slew of articles in the archives about these - the only problem mentioned was the installation. A gasket and some blue hypergoo was furnished at one time and the gasket would tear and smush out from under the mount plate. I fixed mine by making a thinner gasket out of Garlock paper and doping it up real good with Seal-All, a gas resistant sealer made by Goop. Hasn't leaked a drop since. I think it's a worthwhile mod......
