Oh man, after all that, changing the refresh rate to 120 FPS didn't make any difference to the speed for me. I wonder if my ageing i5 just isn't up to it.
Also, I just could not get the pedals to behave with my DFGT in Rush 2049 either. I'm used to the business with MAME and wheels where you've got to keep your pedals semi-depressed when configuring the buttons and axes to stop stuff always registering as 'on' and everything looked like it was assigning the right way in the config, but the gas just wouldn't work in the actual game even though it was getting me through the menus with the car and course selections. Very odd. Eventually after much messing around I managed to get the gas to work, but only if I assigned no axis to the brake and kept it assigned to a button.
Mini edit: I know that this is probably an issue to do with combined Vs separate axis. It's weird though, I've never had to change any of the settings in the Logitech profiler for a game before. I had a quick glance at the profiler settings and the option to separate the axis wasn't jumping out at me. I suspect I probably am just being a bit blind, but can anyone tell me where to find it? Thanks.