Here are little cars that may some day tak

问题：

Here are little cars that may some day take the place of today’s cars. If everyone drives such a car in the future, there will be less pollution in the air. There will also be more parking spaces in cities, and the streets will be less crowded. Three such Cars fit in the space now needed for one Car of the usual size. The little cars will cost much less to own and to drive. Driving will be safer, too, as these little cars can go only 55 miles per hour. The Cars of the future will be fine for getting around a city, but they will not be useful for long trips.

What’s the speed of little cars
These little cars can only go（）per hour.