About the same as if you got 7000 people to substitute a 1 mile car drive with walking, once.
> The average passenger vehicle emits about 411 grams of CO2 per mile
Now that's only tailpipe emissions, so once you include the emissions from producing the gasoline/diesel, it's more than a pound per mile.
[1] https://www.epa.gov/greenvehicles/greenhouse-gas-emissions-t...
The answer is that only the C in CO2 comes from gas, the O2 comes from the air.