MYSQL most efficient / simplest way to count different values in same column [duplicate]

Customers rent cars, cars are grouped in different categories, 'A - D'

The following queries gets entered to display all vehicle groups rented throughout 2016.

SELECT car_group FROM rentals WHERE checkout_date BETWEEN '2016-01-01' AND '2016-12-30'

The output is as follows:

Now I could run the query and sort the groups via some PHP but I am sure it will be more efficient using a mysql statement...?

I know I can use SELECT count(car_group) as cars but that will return the total sum of cars NOT the amount of different groups.

Any advise appreciated.