Hope it’s ok to announce this here.
I released an app about a month back originally for GT6 but I’ve since made the leap to XBox One and Forza 5, and still found myself craving a way to track and compare all my laptimes. I added multi-game support a couple weeks ago to include Forza 5, and just tonight uploaded a version with the cars released yesterday. (may take a couple hours for version 1.3 to appear on the Play Store)
You guys seem to have much better hotlapping support in the game (love that rivals mode) but I’ve been still finding it fun to keep a running tally of all my laptimes, especially when I want to run a car stock or compare with other cars.
App is pretty simple, just aims to let you quickly log your laptimes and the performance you were tuned to. It’s currently a free app, contains no ads, just a donation option if you are so inclined. (I’m going to keep it that way provided it doesn’t grow to a size that requires more expensive hosting) You need to make an account, privacy policy link is included on the registration screen.
App is available here: https://play.google.com/store/apps/details?id=ca.rmrf.hotlaps
Just head to the main menu and hit “Change Game” to get off GT6. 
Hope someone finds it useful!