Kei cars can comfortably go up to about 60mph. Not great for an 8 hour drive, but I regularly do 3-hour-drives with a loaded kei-van. Yep, same 660cc three-cylinder engine that those kei-trucks have. It'll hit 70mph, but not comfortably. 50mph is easy.

And on the highway, I see tons of other kei cars, too. Light weight class cars even get a toll discount!