The joystick's circular dead-zone, which works well for a mouse, does not work well to two unrelated functions assigned to the axes. Steering and gas/brakes are not related to each other the same way up/down & left/right are on a mouse.
TL;DR: Add a row to your Drive sheet to change the joystick dead-zone from circular to square:
joystick_dead_zone_shape, normal, 0
If that is not enough, increase the size of the dead-zone
joystick_deflection_minimum, normal, 5
Play with the size of the dead-zone until it feels right.
Alternatively, you can use greater_than functions on the keys and not use the preferences:
| kb_up_arrow | greater_than 5
| up |
| kb_down_arrow | greater_than 5
| down |
| kb_right_arrow | greater_than 5
| right |
| kb_left_arrow | greater_than 5
| left |