How could friction affect the speed of your car?

Answer 1

Friction is basically pushing back against your car. If you were running, you could go pretty fast by yourself. But if you had someone in front of you trying to hold you back, you would run much slower. This is the same concept. There is rolling resistance (friction) of the tires on the ground. However, the biggest force of friction acting on a car is the air. Aerodynamics play a huge part above speeds of 35mph. The vehicle is moving through air, and it must push that air aside as it moves through it. This causes friction and can slow the vehicle down. There is also friction in the engine and drivetrain as well. It requires horsepower to overcome that friction. That's why an engine rated at 55HP may only actually have 48HP when you measure it at the wheels, because some is lost to friction.

Additional comments.

Friction is not "pushing back", since this would cause the car to run in reverse if you shifted to neutral.

Friction opposes your forward motion.

The air resistance is, correctly speaking, not entirely friction. If we take friction to be the action between the car body's surface and the flowing air, this is a small part of the total resistance. Friction can be reduced by a "slick" surface, such as the "Cottonising" polish applied to WWII Spitfire reconnaisance aircraft that improved airspeed by some 50 mph.

Much of the total resistance (correctly noted above) is caused by the need to displace air. It's the same problem you face falling from a great height into water - the injury (or death) is not caused by water's "surface tension", but by having to displace a large mass of water instantaneously.

Answer 2

Friction between your car's tires and the road surface can either slow down or speed up your car. Decreased friction, such as on icy roads, can reduce traction and cause your car to slow down. However, optimal friction can help your tires grip the road better, enabling faster acceleration and braking.