The wrong ramp gets abandoned in a garage. The right one gets used every day.
Three things matter. Incline: gentler is always better, and a longer ramp at the same height is a gentler climb. Surface: a dog that slips once will distrust the ramp for weeks, so look for real grip rather than a printed texture. Width: your dog needs room to place all four feet without aiming.
Introduce it flat on the ground first, reward every crossing, and only then raise it. Patience in week one buys you years of easy car trips.