If you are driving a regular car on a regular road for regular traffic purposes... don't even think about slipstreams

If we are talking racecars (e.g. NASCAR, Formula 1, etc), front car cuts the air/ air resistance, the car behind it is protected and can speed up without resistance. In a straight line or a long simple turn this is easy.

It gets more complicated when the car in front is going through a bunch turns back and forth at varying speeds, creating "dirty air" cuz that air immediately around that cut air is super turbulent and is going to collapse, creating fluctuating levels of resistance or downforce on the car behind. It's not like this turbulence doesn't exist on straight lines, but it's more of on obstacle on turns where you actually want a consistent weight of air across the car to push it down into the ground to keep grip. On a straight the air resistance is just going to hamper your top speed.