Seems like Forza 5 is not holding 60fps constants too... some guy in B3D get two random samples in the gameplay video and get 57 and 54 fps.
http://forum.beyond3d.com/showpost.php?p=1765069&postcount=151
He did get a bigger sample after (5 seconds) and got these resultads.
300 frames, 17 duplicates
283 distinct frames, with a result of ~ 56.6 frames per second over this 5 second period
http://forum.beyond3d.com/showpost.php?p=1765290&postcount=161
It is not a big deal... just different from Forza 4 that have a fixed framerate... here it is more dynamic in this alpha code.







