Click Edit Mapping, then click on your RPM bar. For Predefined Role, make sure you select Engine RPMs. Some dashes have it pre-set, some not.

That will sort it out.

By default, most of them work well enough. However I noticed when trying to drive an F1 car in Assetto Corsa, the RPMs that are shown are just way off, causing me to shift too early which ruins the experience for cars I'm not familiar with. It probably doesn't help that the car is modded into the game, but having the option to manually adjust this would fix the issue. Is this feature available anywhere?