Can I travel to Canada by car with my green card if my passport expired?

A green card is only a US document, it has no bearing.
You'll need a valid passport or 2 photo-id cards.

definetely.. since you don't need a visa from USA to canada a passport is not needed.. you need an ID (and green card is one, you need to be from a country that does not requires visa to enter in canada, and whereve ryou are originally, as long as you have a valid green card you are considered legal in the USA even you ar enot officially USA citizen.. double check, before you cross
valid passport to canada was discussed, but the law did not applied yet.. you do not need other ID

Canada will accept a US issued green card for entry to Canada.

"Citizens of the United States should carry proof of their citizenship. A passport is ideal, however, a birth certificate, certificate of citizenship, or naturalization certificate, when presented with valid photo identification such as a drivers licence will also be accepted. Permanent residents of the U.S. should bring their green card."

Yes, you may...for now. After SUMMER 2008, a passport will be required for land travel to and from Canada. See website below and read top half of the page carefully for current info. You will need to use a passport from your HOME country. Since you are a green card holder in the U.S. you are responsible to maintain a current passport from your home country. You need to contact the embassy of your home country in the U.S. to get a new passport .

"Citizens of the United States should carry proof of their citizenship. A passport is ideal, however, a birth certificate, certificate of citizenship, or naturalization certificate, when presented with valid photo identification such as a drivers licence will also be accepted. Permanent residents of the U.S. should bring their green card."

Read the above paragraph carefully ... it means you need proof of citizenship, photo ID, AND your green card. The green card is NOT proof of citizenship by itself. As a citizen of a country other than US, Canada, or Mexico you are REQUIRED to have a valid passport to enter Canada by ANY means. The PR status with the US *may* exempt you from having to have an additional visitor's visa issued in your name. (See first link)

International Travel
A Permanent Resident of the United States can travel freely outside of the US. A (valid) passport from the country of citizenship is normally all that is needed. To reenter the US a Permanent Resident normally needs to present the green card (Permanent Resident Card, Form I-551) for readmission. A reentry permit is needed for reentry for trips greater than one year but less than two years in duration. (See second link)