[Coroner]

I run a Cargo Van and buy lots of gas all the time. My truck too, is capable of running on high octane or 87, but not E85.

My experience with testing the performance of the van (60K miles a year) with different octanes has shown stark differences in mpg's when using the lower octanes.

Winds, traffic and other factors weigh heavily on performance. While in Chicago for instance, the faster moving traffic can make a difference of 2-3 mpg for that leg of a journey, plus the drive through the corridor like highways will knock out most of the winds except for the traffic wind. That being said...

Your new car is also breaking in, take note of the mileage markers as thing change. What mileage did the sudden 10mpg loss occur? Make a note of the mileage and anything that you may have added, or taken away to the car.

My van started out at about 15-17, then it dropped to 13, and slowly crept up to the current avg of 18-19. When I hyper mile, I can usually maintain 20+ in calm conditions, but as the weather has been of late, the 20-30 mph head winds have made that almost un-obtainable. When I have 2K of freight onboard, I can really take advantage of the down hills and even push the mpg's to 23 for a peroid of time, of which without the weight, the downhills aren't as productive, go figure!

The point is, every little thing that can help in one direction may hinder in the other, AND, a 1 time loss is nothing to get too worked up about. Continued losses however, are cause for investigation.

If you continue to have losses or poor mileage, I would look at all the things you changed, like the amount of stuff packed in the car, the octane of fuel you're using and so on. From your post it looks like you're going to go back to the higher octane gas, and if you regain the mileage, inquiry solved, if not, then with all the other things considered, it may be time to head back to the dealer and have them check all the sensors and ECM. If a sensor went goofy, the ECM may have gone into limp mode which runs the fuel mixture rich all the time.

Are the roads you travel in dis-repair, bumpy, potholes and so on?

Although when we buy a new car, one would expect things to work properly for at least a good and reasonable amount of time, we unfortunately must realize that in today's manufacturing environment, the assembly line and the quality of materials are not always perfect.

I look forward to hearing what the fix for this sudden loss in mpg's is.