Can you put a ten speed on a fixie?

You cannot directly put a ten-speed gear system on a fixie, as a fixie bike is designed with a single gear and no derailleur setup. However, you could theoretically modify a fixie to accommodate a ten-speed gear system by replacing the rear hub and adding appropriate components like a derailleur and shifters. This would change the bike's fundamental characteristics, making it no longer a true fixie. It's generally more practical to build or buy a bike designed for multiple speeds.