The distance from my home to work is about 7 miles. Every day I do ride my bike to work, I do not drive my car there or back.
So, I do not use 1/50th of a gallon of gas every time I do not drive a mile.
Over the last few months since we moved our office to Cambridge, MA (a city openly hostile to cars) I have not driven about 50 times.
So I calculate that I have not used 7 * 2 * 1/50th * 50 = 14 gallons of gas. My only regret is that I was not driving my Prius, since if I were not driving a Jeep Grand Cherokee 4WD, I would have not used 5 times as much gasoline, or 70 gallons!
Conclusion, I should be not driving a Grand Cherokee!