Twenty6 Products is well known for making some killer looking parts that perform really well. They recently posted some great new shots of 40T and 42T rear cogs they are making. This increases your rear range a lot. We’ve seen similar products from Wolf Tooth and others but these are made out of Kaiser 7075 aluminum.
“Hot off the Mill 40t and 42t rear cogs. We start with some of the best grade aluminum out there, Kaiser brand aluminum 7075 plate. 7075 grade aluminum is a superior grade aluminum to use for a bicycle cog due to its strength, stiffness, and wear resistance. Each cog is machined out on one of our Haas VF-2 super speed mills. We optimized each cut that the cnc makes to keep the cog stiff and light. The cogs weight in at 68g for the 40t and 80g for the 42t.” – Twenty6 Products