I recently changed the timing belt, water pump, spark plug in a 2006 Honda Ridgeline. The MPG dropped from about 20 to about 18. Any ideas on the cause. Could the timing belt be slightly out of time and cause this drop?

 

If your timing was off, there would be a much more noticeable problem. Driveability would suffer considerably, even one tooth off would make it accelerate really poorly and you would know. Where did you get your timing belt done? At the dealer or at an independent shop? Or did you do it? Because most importantly, what sort of plugs were put in your car?

 

Also, where do you live? Did it get much hotter very suddenly in your town, recently enough to coincide with getting your timing belt replaced? Running your A/C more can very much affect your fuel economy, as can an under inflated tire (but you'd notice a TPMS light on). There are loads of things that can affect your fuel economy by a margin as small as 2mpg. So don't get worried yet.