I've got it turned on and turned down to 30. If I raise it higher than 30 it just makes the wheel "heavier". I still don't feel any lateral grip or connected to the road feeling.
If you're coming from real-life racing, you might want to read this thread. Other than tips on how to correctly setup your FFB settings, there's also explanation why FFB will never feel like it does in a real race car.
After some testing, I've found out, for the G25, 26-27 in-game FFB setting with 101% strength in the wheel profile allows the wheel to reach near (what feels like) full wheel strength and provide feedback in the XRT. (attached is a generalized graph of the force cutoff)
This setting is car specific. Generally it can be used across the board for good feel, but in certain cars, the LX6,or FXO for example, the wheel feels "light" at peak G on the skid pad and must be readjusted to use the maximum strength of the wheel, while others' need to be turned down.
To adjust FFB by feel, first test what full wheel strength feels like. When steering force feels the same at any steer angle, that is the wheel's maximum output. Find a smaller range to adjust between by lowering the FFB slider until the steering force feels less than maximum at any steer angle, then move the slider up to a number less than 200 (most likely less than 50). Working with a smaller range will speed up the process. Split the range in half and test (for best results, test at race speed). There is now a low range and a high range to be tested. If the initial half is too strong, then the upper half can be discarded and vice versa. Repeat the process until the desired feel is reached.
Does anybody know if there is a way to detect the output of the wheel motor(s)?