Me and a friend got into an argument about whether or not cruise control reduces MPG's or not. I said that it could help MPGs but not reduce them. Because it keeps the engine at a constant speed and not flunctuates. I am sure that it doesn't reduce MPG's but could help them.

 

If anything, it helps MPG

 

For the vast majority of drivers the cruise control wil give 4-7% better gas mileage depending on which report you read. The exceptions seem to be in hilly country or for those of us that use hyper-miling techniques. I always use the cruise feature on the interstates, less often on other roads. My overall average for 22K miles on my '06 LX manual is about 36 MPG. Your mileage may vary.

http://fueleconomy.gov/will give you more info and tips on getting better mileage.

 

if you leave the cruise in, sometimes going up hill the throttle can be almost nearly fully depressed if you're in your top gear. Throttle position can have a big effect on mileage, sometimes moreso than rpm's. Sometimes, downshifting would have been a better option to save gas.