I totally forgot that Steam had that option!If you use steam...
You can...
Steps...
In Steam (while no games are running), just head to Steam > Settings > In-Game and then pick a position for the FPS display from the “In-game FPS counter” dropdown. Look at the corner of the screen you chose while playing a game and you'll see the FPS counter.
Maybe Steam fps displays average frame rate over a much longer time than the in-game fps counter does? As the Steam fps counter is much smoother in its change. (Steam fps changes 1 or 2 fps up or down at a time, while in-game fps changes 50 or more at a time.)I'm not sure why it jumps up and down those big numbers. I'm sure Steam's FPS system is accurate.