In that one-less-car year I put about 800 miles on my bike (900 the year before the car loss) but my spouse put an additional 500 miles or so on her bike and the kid trailer.
In that year, we have used some public transit, lots of bikes, carpooled rides and a couple of times calling cabs. But we havn't really missed the car terribly.
We were working on driving bikes more anyway. I put 900 miles on my bike two, three and five miles at a time around town the year before we ditched one car. We increased that to about 1200 miles post-car, but and this is key we rode in an auto nearly 10,000 miles fewer. The annual mileage on the missing vehicle mostly went missing.
Oh a couple thousand miles just transferred to the remaining car -- but in the end, by going cold-turkey and losing one car, we saved thousands of car-riding-hours that we would not have saved by just trying to drive bikes more.
Where We Live
Where we live is important. Pasadena is relatively bike friendly; lower speed limits, smaller streets, share the road signs, everything we really need is available locally (within, say, five miles). Last week, for example, I drove my bike (including trailer) to my school site for work 2.5 miles from my home. My son and I rode our bikes to the dentist on Friday; on Saturday my wife and I took the 5 and 12 year olds to do our grocery shopping by bike. (She towed the 5 year-old's trailer, I towed the empty one for the week's groceries.) The type of community we live in has been important in facilitating our one-less-car year.
So, the short report on reducing car miles: (1) Live near your work and a complete community (not a bedroom suburb outside of a "real" city); (2) Insist that your community by bike and pedestrian (transit user) friendly; and (3) Kill a car cold turkey.