In the general case I would say "yes" to this question. But more and more, engine control parameters are being kept in non-volatile memory for emissions control reasons.

I suspect that it will always be true that an MIL will be set when you do this, but the EPA is apparently insisting that the vehicle run in the most optimal manner for emissions control reasons.

On the other hand an OBDII reader, an OFFICIAL one, not "aftermarket", will likely always be able to reset parameters.

 

I'm here in the SF Bay Area and I have a 2008 FEH AWD. I find it tough to get more than 25-26 MPG in the colder months. In the Summer, I'm fortunate to get more than 30... but that is nursing it ( and not annoying the drivers behind me).

I'm not a speeder and am an average driver that is contientious of my hybrid feedback. I find it almost impossible to get the numbers I keep reading about here.

So I wonder how many of the stats here are for 2WD vehicles vs AWD? I would think that should account for a mile or 2 per gallon.

 

That varies from state to state and sometimes season to season. Minneapolis/St. Paul used to mandate 10% during the winter to minimize smog and later decided to mandate it year round. Now there's a law on the books mandating 20% in 2013 (unless ethanol has already replaced 20 percent of the state's motor vehicle fuel by 2010).

 

I'm down south of you, and I have an 07 AWD and drive with traffic like yourself. I've been able to get pretty consistent 28mpg in the winter and 30 in the summer. I've removed the roof rack, used low gear, a K&N air filter, and tire pressures just over 40psi all around.