Ok so Dirt 5 has got two modes by default "prioritise image quality" and "prioritise framerate". If you turn the video settings to 120Hz in the console settings you open up a 120fps mode.
Having recorded and counted three clips:
1. the "quality" mode is 1440p but doesn't always lock 60. You do feel dips especially if you move to the front bumper view, down to what feels like 50fps.
2. "prioritise framerate" performance is better but the graphics settings are really turned down. If you pay attention you an also see where it switches to lower resolution targets also, though 3 out of 4 counts were still at 1440p.
3. "120hz" mode is a bit of a disaster. I had 3 counts at 908p and 3 at 762p, (25% of the resolution!!). I dont think the settings were dropped noticeably further than on framerate mode. 120hz seems great though. I think it looked OK on the snowy track but the blurriness on the "dirt" track made it look really ugly. I never counted it above 908p, so maybe that's the effective cap on DRS.
TLDR; 1440p/60fps or 762~900p/120fps