Carbon Offsets for Work Vehicles?

In March 2010 I drove my work truck (2004 F250 diesel) 943.5 miles
Using the EPA's website:

CO2 emissions from a gallon of diesel = 2,778 grams x 0.99 x (44/12) = 10,084 grams = 10.1 kg/gallon = 22.2 pounds/gallon

My trucks gets about 16mpg so 943.5 miles gives me about 59 gallons of gas or 59x22.2 =

1,309 pounds of CO2

The cost per ton at Carbonfund.org is $10 so about $6 to offset the work truck.  Or either about 4 pine trees or about 21 maple trees (once they are 25 years old) according to www.erasecarbonfootprint.com

I also drove 341 miles in my work car

CO2 emissions from a gallon of gasoline = 2,421 grams x 0.99 x (44/12) = 8,788 grams = 8.8 kg/gallon = 19.4 pounds/gallon

The work car gets about 18mpg so 341/18= 19 gallons or 19x19.4 =

367.5 pounds of CO2

At $10 a ton that should be about $1.80 or 1 pine tree or 6 maple trees.

I used 25 gallons of gas in the VW so

25x19.4 = 485 pounds of CO2

which is $2.40 or 1.44 pine trees or 8.64 maple trees
Total for the month of March is:

Work
$7.80
5 pine trees or
27 maple trees

VW
$2.40
1.44 pine trees
8.64 maple trees