better fuel economy

I have a 2007 Santa Fe with only 4600 miles on it. I was getting around 19.5 in the city before I started using Synthetics oil in it. I now get around 15.9 can any one HELP me out with this

you're getting a lot better mileage than I am. I only get about 11-15 city driving; 19to 24 hwy. what kind of gas are you using??

I use 87 thats whats been in it cents I got it.

I have the 07 3.3 litre also with about 5k on it too. I changed over to amsoil full synthetic after the break-in period. I haven't noticed a change in gas mileage. What part of the country do you live? If its cold, the gas has additives to prevent gel-ing, that has caused gas mileage to be worse, i.e. less actual gas per unit. I live in kansas city and notice worse gas mileage in cold weather, not as much of a drop as you've noticed. Even my little sentra gets noticeably worse gas mileage.

I also have an un-proven idea that cold weather in general will create worse gas mileage. Since O2 sensors want to keep the gas to fuel ratio optimal, cold air is denser and contains more oxygen; therefore more gas to keep up. Just an idea.

I live in South Carolina. An I will check on that O2 sensors also thanks for getting back 2 me on this