Thanks for the responses!

To respond to some of the points: I've got a cassette that has 34 as the biggest cog. Speed is less of a concern to me. What is a concern is a 4% to 6% grade (or more sometimes) for 16 to 18 miles and me blowing up trying to tote my fat ass and 45 pounds of stuff over a couple of mountain passes on a self-supported tour. Knowing my body and what hasn't worked in the past, the low I've got isn't low enough and I'm just trying to figure out the most cost-effective way to go about rectifying that dilemma.