Mileage dropped

[steved]

I have an '03 SE v6 with 60K miles. I drive about 100 miles per day back and forth to work. 95% is highway driving. I was getting close to 400 miles per tank. This past July I bought a set of Revos, same size as stock. I didn't notice a difference in mileage then. Around October my mileage has dropped 25%.
I have asked the dealer when it was in for routine maintenance to check this out. Of course they found nothing and blamed the tires.
I know that hear in the Northeast they use oxygenated-fuel for the winter months and you lose performance. But never has it been this bad. I have tried to watch my driving habits as well and still the mileage suffers.
Any ideas what could be going on? I will be back at the dealers next week for the 60K miles check up.

 

[boston_kevin]

Not sure about NJ, but in MA with the temps bouncing between 50 degrees during the day and 11 degrees at night, my tire pressure fluctuates alot. I put them at 38 on a day when it was 50, then checked them last night and they were at 29-30. Not enough for the sensors to fuss over, but enough to drop my gas mileage. I don't really feel like filling up the tires on a daily basis, so I leave them until the weekend and I will top them off then.

 

[Bluto]

Spark Plugs were done at 30k, correct?

You are due for a new set at 60k. See if the new plugs make a difference. Make sure the dealer is using the correct plugs.

Replace the air filter.

Change the rear diff gear oil. Go synthetic if you can.