In the back

Putting your bikes in the back of the car is the value choice. There is no added drag with the associated increased fuel consumption. Bikes stay safe when you stop for food. You don't have to pay for a rack or a roof setup. But you do need to have a van, wagon or hatchback to make it work. And if there are more than two of you it gets tricky.

Of course, if you have a people mover it can work out ok.

How to make it all work

You still need to be pretty good at Tetris to make it work, unless the car is massive. Use some padding like old towels or foam camping mats between the bikes. You might need to push dropper posts down and remove front, if not rear wheels too. Chock the brake calipers with the travel chocks, or a 20c piece or some cardboard works as well. Make sure you don't have pedals crushing downtubes, and rotors aren't being bent. Treat your rear derailleur very gently too!

Pros:

- This is as cheap as it gets

- It's pretty quick, and secure

- highly cost effective for fuel consumption

Cons:

- Mountain bikes get dirty and that comes into your car

- Very limited capacity for mates and their bikes

- Potential damage from one bike to the other: rotors, frames etc

- Did you put all the wheels in? And the through-axles?

There are plenty more options, but hopefully this gives you some ideas on how you want to transport your bike. If you're taking it places a lot, a rack that's easy to use will be a good investment. If it's a once off trip - maybe a simpler rack that takes a little more time to setp is a good choice. If you're not sure - ask your local bike shop or rack specialist.