I just thought I'd share how I calculated the cost (in gasoline, not wear and tear or anything like that) of common errands we run.
I looked up the average city MPG for our car (city since we rarely travel by highway). Of course it is possible to calculate your actual MPG by tracking your fillups and mileage, but I was content to just grab an average. I found a range and I used the low end (22mpg) because we live in a hilly region.
To calculate the cost per mile, I took the average gas price these days - well, I just used $4/gallon even. 4 divided by 22 is about 18 cents a mile.
Then I used Google maps to estimate the distance to a bunch of common trips - the grocery store, the park, the library, the YMCA, etc. I multiplied them all by 2 (for a round trip) and then by .18. So now I know one grocery store costs 33 cents a trip, and the other one in town $1.45 a trip. The park is 36 cents. The library 18 cents. And so on.
As you can guess, one of the grocery stores (unfortunately, the more expensive of the two) is walkable, as is the library. The park is sort of walkable except it involves a MAJOR hill (mountain, even).
Knowing the cost of the trip makes us more aware of our gasoline usage. Even the little stuff adds up. If we drive to the park every day, that's over 10 bucks a month right there. The grocery store that is further away from us saves us more than the $1.12 extra it costs to drive it, but only if we plan well and shop only weekly or less. If we're running there for just a gallon of milk, it's a huge waste - and now we know that if we really need just one thing, it's better to walk to the closer, but more expensive, grocery (it's more expensive but we're paying less than $1.12 premium on a gallon of milk).
Just thought I'd throw this out there in case it helps anyone else conceptualize their gas usage, and motivate them to plan trips better and walk/cycle more if possible.