Here was my ****** post on the subject:

Speaking as a cyclist, as long as it's in proportion to the amount of CO2 released per mile, fine.
An average human releases about 2 pounds of CO2 per day. If a cyclist is moving 18 MPH (that's my average) it takes about 3.4 minutes to go a mile. Assuming the repiration doubles during aerobic exercise, heck, let's be generous and say it triples, that's 0.014 pounds per mile.
Burning a gallon of gasoline release 24 pounds of CO2, so you're looking at roughly a pound per mile. While driving that mile, the human inside (assuming there's only one in there) releases 0.0046 pounds in that time.
So say you want to tax a cyclist $1 per 100 miles. I'd be totally behind that if you put the same $/pound CO2 tax onto gasoline. $1 per 100 miles would be $1 per 1.4 pounds of CO2 released. That's the price you just put onto creating CO2.
Since burning a gallon of gasoline produces 24 pounds, that means you're saying that there should be a tax of $33.60 per gallon of gasoline.
It's only fair.
To be realistic, say you want to put a tax of $1 per gallon on gasoline. That sets the price at 4 cents per pound of CO2 released, which means you collect $1 from a cyclist per 1800 miles travelled.
You would get about $3 from a very dedicated bicycle commuter. I'm not quite that dedicated, I only do about 3600 miles a year, so $2 from me per year. The average car getting 24 MPG and going 12,000 miles, collect $460.80 from them per year.
Oh yeah, and don't forget to collect $29 from every human for breathing 24 hours a day.