We are using a program called CarBuilder in my eighth grade class. It allows them to change parts of a car using different choices (normally between 6 - 15 choices per item. They can change the chassis (which determines the seats), the engine, transmission, fuel tank, shocks (both front and rear), brakes, steering, tires, paint, wheels, body shape, window shape. By experimenting with changes they learn how those changes affect the car's performance in aerodynamics, road handling, and quarter mile drag acceleration.
Unfortunately most of my students are stuck on trying to make their cars "look good" instead of perform well. They would rather have a design they like than get the mileage over 8 miles to the gallon. They care more about the rims than the type of tire they use and how it transfers the power into motion. They haven't formed the linkage between one and the other, but it's very hard to get them to think about it. They would rather get an F and have a car that "looks good" than make changes to make it fuel efficient.
I find it very frustrating that they won't make the changes and that even though I made a rubric that gave them specific changes to make, half of them just won't try to make those changes.