I'm new to speedrunning Sonic R, and I am unsure as to how I should be getting my in-game time for my runs. Can anyone help me out?

I recommend this site: http://www.unitarium.com/time-calculator Just add up all of your final track times. If you need to see what I mean, I add up my IGT at the end of every PB in my submission. Watch my run here and at the ~10min mark I use that site to calculate my final time: https://www.speedrun.com/sonicr/run/mexg4p9z

Or just use autosplitter and set the timer on livesplit to count in-game time. That's how I do it.

Depends if he’s running PC or GC