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Sudden Drop in Gas Mileage

I have a 2001 Honda Odyssey with about 55,000 miles on it. Have always kept up maintenance. Have noticed in my last three fillups that my gas mileage has dropped from about 25 mpg (local, around town driving) to 20 mpg. New air filter recently installed. Vehicle passed inspection about a month ago with no problems. Any ideas? Thanks!

Check your tire pressure. Also, have you out any heavy things in the vehicle more often over the past little while? Has your route changed (as a result of collisions, construction, etc)? Is there a gas tank leak? Have you been idling the vehicle more lately? Have your child(ren) been driving the van?

When was the last time the spark-plugs were changed??

How about the fuel filter???

Typically if there is a sudden change in fuel mileage, there is a cause. lprocter has some good suggestions. We are assuming you have not changed your driving route or have not started to sit in a parking lot with the engine running. With those assumptions, there should be a cause like a low tire or a fuel filter needing changing. It also could be you need new spark plugs. Is the engine still running like before? Meaning it’s not running rough, poor accelerations, etc. Have you changed fuel suppliers to a poorer quality of fuel?

That is great mileage anyways for that van. I have one and we get about 16-17 in local/city and 25, at best, on the highway. You are getting substantially better than what that minivan would normally get:

I had a 1998 Honda Civic and experienced the same problem. The solution was to (sorry about vagueness) replace a “plug” in the front of the engine. I think it might have been an oxygen sensor. Maybe someone can step in here and set this straight if I’m mistaken on what that part was. I’m virtually certain my memory is good on this.

Bobbyandberyl has a great point. It could be an oxygen sensor. Today’s cars run a leaner mixture of fuel to air to meet the emissions standards. If the oxygen sensor went or is out/not reading, then the engine most likely will default to a richer fuel condition resulting in a noticeable drop in fuel mileage. It does that to “safe” the engine and prevent damage from running too lean. If it is an oxygen sensor, it should come up on a code that can be read. I’ve heard, but never verified, that many auto parts stores will read codes for you. So you may want to check this out.