Just FYI: It's helpful to be specific about model years when asking and answering questions about RR/Xtrainer gearing. (Both share the same gearbox for their respective model years.) Using 13/51 on a 2016 will yield different results than the same sprocket on a 2020 bike.
2015-2017 models were built with 28/70 primary drive gear ratio and delivered with 13/51 sprockets.
2018-2019 were built with 27/72 primary drive and delivered with 13/48 sprockets.
2020-2022 are built with 27/72 primary drive and delivered with 14/49 sprockets.
See the specific model year owner's manuals for gearing details.
The
Gearing Commander website is a great tool to compare the effects of various gearing combinations on RPM vs. groundspeed etc. If the factory configurations aren't already available for your bike on the website you can add them using the info in your owner's manual.