Declining mpg on my Prius

DUH- The only way the car generates heat is to run the engine. Running heat or A/C significantly reduces fuel economy on a hybrid. Your situation is apposite do to the climate.

Just guessing, but I would think that the on-board computer is calculating your mpg as an average based on current information. In other words, the mpg reading might be higher when you are driving under ideal conditions and lower when you aren’t.

The difference between 48 and 46.1 also needs to be put into perspective in order to be analyzed. And the way I see it, there’s very little difference.

If you were saying there is a 5 or more mpg difference, I’d think a lot of suggestions already given should be taken into consideration. Seeing such a small difference is normal and not worth worring about, in my opinion.

Agreed, not much of a decline but…

Lenjack mentioned tire pressure. Higher than Toyota’s 35/33 recommendation will actually help in terms of fuel economy and tire lifespan.

Overfilled engine oil is a common shop error.

The parking brake might be dragging.

Chamging fuel composition (the ethanol thing) ccould cost you 10%.

Probably too soon for your engine air filter to be clogged, but one would have to check it to know for sure.

Combine short trips whever possible because each restart costs a bit of fuel.


“…the hot weather is causing the OP to use the A/C, the gas engine is operating 100% of the time when he/she is driving. It is unfortunate that Toyota (and the other hybrid mfrs.) are not more open with information about the use of the A/C causing a hybrid vehicle to essentially bypass the electric motor function.”

This is not how Prius works, 2004 onwards. The A/C compressor is electrical and independent of the engine until the HV battery state of charge falls to about 45%. The owners manual supplied with the car is nearly adequate to understand the system ops. It is indeed unfortunate that these systems are not better understood, by both their operators and by the others ‘with opinions’.


I’m sorry that my opinion offended you, but most of us on this board are, ultimately, giving opinions.

On the other hand, I do enjoy learning something new every day, and I think that it is safe to say that most of us have learned something new about this changing and frequently misunderstood transitional technology.

So your getting 46 MPG and you’re complaining? Wow.

It gets better than OZ per mile… The readout is taking its cue from the ECM, which measures pulse width of the injectors. Pulse width is the amount of time (nanoseconds?) the injector nozzles are open. This reading, with other factors such as speed are used to calculate fuel economy indicated on the dash display.

Update: I’m not exactly sure if this method applies to a hybrid - but it does to most gassers with OBD-II systems.

Better mileage in hot weather is usually an indication that they oxygenate the fuel only in winter where you live. Fuel oxygenates (such as ethanol) have less Btus per gallon than dinosaur juice.

I learned something new from VDCdriver’s post. I thought that the only alcohol made from wood was methanol. A Google search led me to tech article describing the processes for making both ethanol and methanol from wood. It is not particularly efficient, but it has been done.

You need to read a similiar question posted in August 2008 the 6th to be exact.

ssshhhhhhh everyone bought the prius because it’s a “green” car and to save them from their SUVs they bought because it was the “in” thing to do at the time. Now that the hybrid trend is wearing off, they’re just now discovering the problems with their hybrids.

The law of averages. As the miles add up in the computer, the mpg math is spread out over many more miles. The display is a grand total AVERAGE. manualy do the math for each tank to know your actual this week. My 06 hybid Escape has 21.000 miles now and is virtually impossible to change the number on the sceen with out driving a massive number of slow speed miles. And the way I drive, that won’t happen any time soon >:0. I keep my display on “miles to empty” for a clearer picture of what’s going on right now.

We have a 2008 Prius, Touring edition. It has more normal tires. We get 44 MPG, and Consumer Reports agrees with 44 on the Touring and 46 MPG on the Base models. The fuel is in a collapsable bag, so we got many variations on what is “full.” It is supposed to take 11.9 gal., but we can’t get more than 10 gal. on “empty.” Doing your own calculations will get different mileages from the car’s computer (which is slightly optimistic). If your are showing 46 MPG (which is probably 44-45 MPG), enjoy your efficiency, and look at the long term savings.