If you haven’t I would upgrade to Logitech’s latest interface software. Then if your wheel has the capability, you can try some of the settings neurokinetik mentioned.
Reverse feedback and centering are probably your main issues.
I have everything turned up full, with a 2% deadzone. Some people like things softer, so you will need to play with different settings to see what you like.
As for the wheel diameter of the G25, not sure of the exact spec, all I know is that it’s larger than the Driving Force Pro.