One of the easiest ways to show appreciation is to offer to contribute toward fuel or other expenses. You could suggest a weekly or monthly amount that feels fair for both of you. The goal isn’t necessarily to pay for every expense, but to recognize the effort they’re making.
If your coworker refuses money, there are plenty of other ways to show gratitude. You could occasionally buy them coffee or breakfast, give them a fuel gift card, or return the favor in another practical way.