There is another post on this topic a few months earlier. since I just read it yesterday, here I start a new post.

I drove a testing vehicle during vehicle development back to yr 2003. I liked it very much for its good looking, plenty space and good mpg (19 city/26 highway mpg for new car). for these reasons I bought a used 2006 Rendezvous (3.5 liter engine, FWD) 3 yrs ago with 96k miles. The vehicle was in good condition, but very soon I feel the mpg is lower than I expected: 16 mpg in town and 22 on highway. I found the thermalstat didn't work well, as the pointer did not go beyond the first mark. I changed the thermalstat, then the pointer stay around 1.5 mark when engine is hot. but this doesn't improve its MPG. I also changed the 3 front spark plugs. because the existing plug are in very good condition, I have not changed the other 3 plugs in the back. I also bought a good OBDII reader to monitor Oxygen sensor's voltage, they are file.

So this is my current MPG: 16 city and 22 highway, I am disappointed. but reading other posts here, it seems mine is not too bad.