My car is strange, it does this all the time. On average my car gets about 190 or so miles to the tank. I am usually pretty happy if I manage to hit 200 miles. Well, the last tank I managed to get 240 miles out of it. I scored a perfect 20MPG after I ran the calculations.

I watch my mileage like a hawk and the car does this fairly often. I will go a couple months averaging 190 miles, and then out of nowhere I get another 50 miles out of the tank and I will usually be able to fill up once or twice more with this higher mileage and then it goes back down to 190 miles.

My driving habits never change either, you can damn near set a clock to it. 99% highway miles, no major speeding (maybe like 5-10mph over) and the same route every single day.

I wish I knew what the hell happened to the car to get me higher mileage.. if I knew, I would positively do more of it.

 

Could it be the cold weather? If you were normally running rich the denser air might give you a better mixture and result in better mileage.

 

Sounds like you've got the 12 gallon tank deal like I do. Could be that you haven't been using the A/C as much this last little while. Less traffic? Who knows.

 

colder day + defroster = AC running and worse AVG MI. just a thought

 

I don't have A/C.. and secondly it happens in the summer months as well as winter months. This has been going on and off now for a couple of years.

 

A/C has little effect on my mileage.

Sonar, do you have a data log?

 

Maybe a weak O2 sensor? Working right sometimes...making it rich others?

A data log is probably your best bet for figuring it out. Especially if you can get one while you still have good mileage and compare it to when you don't.

 

I know i've logged it before and didn't see that much of a difference as far as the computer is concerned. I would have to take a look again, but unfortunately I have no idea when the car is getting good mileage vs when it isn't. The car drives exactly the same either way.

I think the only way to find out for sure would be to hook it up to a wideband and do it from there.