Ah. That will do it. I've seen cassettes last less than 1000 km because of that. It's also very hard on the knees. Learning to spin is one of the best things a cyclist can do. I typically ride in my 34 ring and spend time in the middle of my cassette. My small cog is 11 teeth and I've found that I can comfortably ride in 34-11 combination at about about 40 km/h although I don't ride in that combo very often because crosschaining is hard on the components.