Another good reason to carry a spare tire, even on shorter rides than extended loaded tours, is in case you cannot find what caused the puncture. On more than one occassion I had a slow leak and changed the tube. I tried to find what caused the leak but could not. 30 mph wind, cars, etc. all make finding the tiny hole in the tube difficult since you literally cannot hear anything. And running the fingers over the tire does not find the tiny glass or thorn tips imbedded in the tire. So I've put a new tube in and pumped the tire up. And 10 miles later, or less, its going flat too. I usually eventually find what was causing the problem. Sometimes I get back home and just toss the tire and start fresh because I cannot find the glass, thorn, wire, etc. causing the flats. With a spare tire, you can put it on and a new tube and you will not have to worry about anything being imbedded in the tire causing a second flat. I plan to use this idea on next year's brevets. Probably go with a Continental Supersonic, 160 grams. Lightest clincher around. Good enough to finish a ride with some peace of mind after a mysterious flat.