I got just over 1k miles on my 4.0L. Been checking the avg MPG in the dash and it's been as low as 15mpg just driving around the city and I got up to 26mpg on the highway when I took a 4 hour trip for Christmas.

 

Although an older thread, I needed to add clarity here.
Increasing the octane has NO effect on the BTU/gal of fuel. It is just a measure of the resistance to knock. Modern engines can detect the pinging/knock and adjust the timing, which may effect performance slightly.

The manual states:

FUEL REQUIREMENTS
3.3L and 3.8L Engine

All engines are designed to meet all emissions regulations and provide excellent fuel economy and performance when using high-quality unleaded “regular” gasoline
having an octane rating of 87. The use of premium gasoline is not recommended,
as it will not provide any benefit over regular gasoline in these engines.

The 4.0L engine is designed to meet all emissions regulations and provide satisfactory
fuel economy and performance when using high-quality unleaded gasoline having
an octane range of 87 to 89. The manufacturer recommends the use of 89 octane
for optimum performance. The use of premium gasoline is not recommended, as it will not provide any benefit over regular gasoline in these engines. Light spark knock at low engine speeds is not harmful to your engine. However, continued heavy spark knock at high speeds can cause damage and immediate service is required.
Poor quality gasoline can cause problems such as hard starting, stalling and esitations. If you experience these symptoms, try another brand of “regular” gasoline before considering service for the vehicle.

 

I am only getting 12-14 mpg in town and 17-22 mpg hwy with my 2010 grand caravan sxt. I've had mine since September 2010... still can't figure out what's going on.

 

I have the 2010sxt as well (bought used, it has 12k miles on it). Its an awesome vehicle! The 3.8L engine gets almost exactly what you are getting, about 14city and 19-20 highway... thats way off from what the van should be getting. I am not sure what is going on, I will have to be looking into it. Otherwise its just the usual list of things to do, check the tire pressure, drive slower, siphon the neighbors gas. Good think its not my commuter car.

 

Winter time may not be the best time to complain about auto manufacturers falsifying fuel economy stats. The vehicles computer monitors intake air temp and causes the injectors to supply more fuel during colder temps. Colder air is denser, therefore the computer supplies more fuel to maintain the "ideal" air/fuel (oxygen/fuel) ratio. Some HOD (hydrgen on demand) guys and fuel economy guys suggest ways of fooling the vehicle's computer into thinking the intake air temp is higher so the computer will supply less fuel. Theory is a couple of degrees may effect a couple mpg. Best not to try this at home unless you know what you are doing.

There are other things that will reduce fuel economy in cold weather. I live in a four seasons climate. ALL of my vehicle get worse fuel economy in the winter.

 

DING!!