Every car commuter that becomes a car-pool passenger reduces CO2 emissions.
To make our calculation we used https://cncf.com.au/carbon-calculator/
That calculated 2.41T of CO2 was produced by a medium sized petrol car (with a 7.5L / 100km fuel efficiency) over the average Australian km travelled per annum: 13,200.
The US EPA calculates that 1 metric ton of greenhouse gas emissions is the equivalent of 25 tree seedlings grown for 10 years.
We then entered the 2.41T into https://www.epa.gov/energy/greenhouse-gas-equivalencies-calculator to calculate the equivalency.
This calculated the equivalent to 62.5 seedlings grown for 10 years.
The calculation would be affected by different fuel consumption and different numbers of Kms travelled.
In the USA the average car consumes more fuel per Km (21.6 miles per gallon or 10.8 L / 100Km), and drives further average miles (11,300 miles or 18,100 Km) to produce 4.73 tons of CO2.
That’s the equivalent of 123 seedlings grown for 10 years.(https://www.epa.gov/energy/greenhouse-gases-equivalencies-calculator-calculations-and-references)
You can use these calculators to approximate your carbon footprint.