Check the pressure and state of your tyres every month.
When the pressure of a tyre is wrong, it does increase the resistance on the road and the fuel consumption. It does also impair the life expectancy of the tyre itself. On a global scale, this participates to the increase of tyres used and thrown away earlier than they should.
Take a step towards improvement here, have a look at the recommended pressure for your tyres (in the user manual of your car) and do a monthly check.
As an example of the costs involved in not taking care of tyres, this represents a total cost of 500 million Dollars a year for Canada (!).
So, check and save!