The extra capacity gasoline tank from Clarke Manufacturing had some growing pains when it was new, but it seems that they have worked out the problems. They were having some issues with the bond between the plastic and the brass fittings.

I'm on the third tank now there hasn't been any leaking problems for six months or more.

I got mine directly from Clarke, it's item number 1480 for $180.00

The extra capacity is wonderful. The stock capacity is something like 1.9 gallons and the Clarke can hold 2.7 gallons - a bit over 40% more.

Installation is easy, here are the instructions in PDF: ClarkeInstructions.pdf

The brass fittings (#49019 and #490191A which also operate as the reserve function) are removed from the stock tank and installed on the new tank. I hear that Clarke now supplies new o-rings as well.

I haven't measured it, but I estimate that there's around 0.5 gallon left when you hit reserve.

The fit isn't as good as it could be, the tank doesn't seem to rest squarely on the frame mounts. Additionally, the "peg" for the seat mount is poorly designed, I fixed it with a collar.

With the extra capacity, there's a gap now between the seat and the radiator shrouds. I covered up the flanges on the shrouds with some electrical tape as seen in the above photo.

Photos of the early leaking problem: