In my case, the car without ACC is an RV and since driving it is like trying to push a small house through the air, it responds pretty quickly to speed reductions. Plus I tend to keep a pretty long distance from the car in front of me, so I have more time to respond. I like to pick a truck to pace behind since a lot of people don't want to get stuck behind a truck so they don't pass me then pull in behind the truck, even with 5 or 10 seconds of following distance. Another advantage of pacing a truck is that they usually keep a pretty steady speed unless they are going up a hill, but then on the hill I can usually find a truck that's light enough to keep a pretty steady speed uphill.