Outside of large cities with functional mass transit, most low income people do own (or finance) cars. Otherwise they would be no-income. You can find a lot of low end job listings that say “reliable transportation” and they don’t mean a bus ride sandwiched between two several-mile hikes. Their cars will be perhaps 10-15 years old, depending on whether their state has winters or emissions laws. They can’t afford the latest fuel efficient vehicles and might be getting 18-25 miles per gallon. Fuel will be a significant fraction of their transportation expense compared to anyone with a higher income, so a fuel tax will necessarily be regressive.