I had an opportunity today to use my Genuine Innovations Ultraflate Plus for the first time since I bought it a year and a half ago. It was my first flat since getting it though I'm not sure how I've managed the streak of fortune. It worked like a champ and I didn't have any issues except for (sort of) one thing. When I got home, I gave my bike the once-over and put a new tube and 12 gram cartridge back in the seat pack. Just for a wag, I checked the pressure on the tire I repaired and was quite suprised to see that it was at 60 psi. I run 700C x 25 front and back.

According to the G.I. website, a 12 gram cartridge will inflate a "27 inch road tire to 90 psi". 90 psi is about where I keep tire pressure so it seemed reasonable that this was the cartridge size of choice for my seat pack. Just to make sure I hadn't used the inflator incorrectly, I repeated the process in my garage, with the same results.

While I realize calling something a "27 inch road tire" is pretty broad and covers a lot of possible sizes, I didn't think a reasonably equivilant 700C x 25 was so much removed in size that I only got 2/3 the "rated" fill.

Where did I screw up my math?