Remember when 42t Cassette extender cogs were considered big? The 1x evolution continues now with the even bigger GC45 for 11 speed Shimano mountain bike cassettes. With the GC 45 on your Shimano cassette you will have an overall range that is very similar to the 10-42 from SRAM but given the same front chainring you get an easier climbing gear. An 18t cog and spacer are included with the GC45 to eliminate the large tooth gap in the smaller cogs.
Optimized for use with Shimano 11 speed 11-40 cassettes
Also compatible with Shimano 11 speed 11-42 cassettes
Can be run in 2x configuration with a long cage (SGS) rear derailleur (max front difference is 13t with a double and 11-45 cassette)