When installing my new tanks (just before Christmas) I had a little over three gallons left over from the stock tanks that I put in a fuel tank. Of that, I used compressed air and a couple pints to wash out the new tank's possible shavings. Once installed, I dumped the rest back into the tanks to check for leaks and then rode it for one short afternoon ride... then it started to snow.
Last week was the first time I was able to ride to work and on Saturday the fuel light came on. (My light seems to come on with only a half-gallon left?) I rode about four miles with the fuel light on and then filled them up... I put 11.7 gallons
in those tanks with the fuel just at the bottom of the raised threaded tops. Not really sure what was left in them before I tried to fill them, but I am thinking these will easily hold 12 gallons conservatively!
I am one happy camper (once the snows melt).