MPG on New Prius

[stuay]

A recently got a new Prius and now have about 1500 miles on it. When I first got the car about a month ago I was averaging about 42-45 MPG (I reset the monitor everytime I fill up the tank). A week ago I filled up the tank, reset the monitor and have driven about 200 miles on the new tank and noticed that the average MPG is now only about 34 MPG. I was in bumper to bumper traffic right after I filled up and it took me 3 hours to go about 20 miles, plus the ac was on? Would that have contributed to the big hit in milage? I also noticed that it's tough for me to get the car to run solely on battery power. I notice that when I lightly step on the gas as soon as I get to about 8 or 10 MPH the engine kicks in. Is this normal?

[JimboK]

Originally Posted by stuay

Would that have contributed to the big hit in milage?

Yes. The AC runs off the battery, but the engine has to run to replenish the charge. Yours was a worst-case scenario for summertime fuel economy. And you still got 34 MPG. Pretty sweet, huh?

Originally Posted by stuay

Is this normal?

Yes. I wouldn't worry about trying to get the car to run on the battery. The battery's value is not for sustained propulsion, and to try to force that actually is less efficient than letting the car do its thing. There are ways to manipulate the car's systems to improve fuel economy. Do a search on the subject and browse the fuel economy forum to find out more.

[Rokeby]

stuay,

Welcome to PriusChat.

Everything you have reported sounds pretty much normal. Yes, when you
run the airco at low speeds/stop-and-go,you really take an MPG hit.

As to running solely on battery power, thats not a very good idea. Power
coming out of the battery is very inefficient. Let me try to explain using very
approximate, generalized numbers.

Lets say you have 100 units of energy in your gas tank. When you burn that
fuel, you get something like 30% efficiency. So in the best of all worlds, you
car is now moving with 30 units of kenetic energy. If you were to go
immediately into a blue arrow regen mode, you would only recover about
33% of the kenetic energy as chemical energy in the battery. So, .33 x 30 =
~10 units of energy in the battery. So now you use just the battery to
accelerate/move the car. Again the conversion of energy from chemical
energy in the battery to energy at the wheel is incomplete, about .33 of
those 10 units actually move the car. 10 units x .33 = ~3 units of energy.

The 100 units of energy in the gas tank becomes only 3 units of energy by
the time it is passed through the engine to moving the car, then to the
battery, and finally to the wheels! This is not a good way to spend your
hard earned money/gas.

As you read the many threads here about how to get the most efficient
operation of the Hybrid Synergy drive, you will come to recognize that there
are some times, some very short in duration times, when running solely on
battery power is the right thing to do. But, these times are rare indeed. 'Tis
far better to be running at the posted speed limit in an engine off, 'no
arrows glide,' the Prius driver's nirvana.

Hope this helps.

[richard schumacher]

Just in case: don't use "B".

[a priori]

Originally Posted by Rokeby

The 100 units of energy in the gas tank becomes only 3 units of energy by the time it is passed through the engine to moving the car, then to the battery, and finally to the wheels! This is not a good way to spend your hard earned money/gas.

Well . . . In your scenario, even with inefficient use of the HSD, you still get 30 + 10 + 3 units of energy, or 43 total, as opposed to simply 30 if you had only the gasoline engine.

Still, your point is quite valid. It is hard for new users of the car to get past the notion that using all battery is the best way to save gas. Perhaps the only way to gain that knowledge is through experience. I don't know how many of us started with MPGs in the 50s and 60s.

To the OP: I thought I was doing very well one year ago. I was very happy getting more than 50 MPG on a whole tank of gas. Now, in the same time of year but with another year's worth of experience, I am highly disappointed when I have a tank of gas give me only 50 MPG. My most recent tank was in the mid-60s after about 350 miles. Unfortunately, I ran right into a wild thunderstorm and drove against heavy winds and rain for about 60 miles. It absolutely killed my mileage, and when I filled up I was disappointed to show only 61.1 MPG. What a change in perspective!

To start out in the 40s while learning about the car is not uncommon and nothing to be worried about. If, after a few months, you don't see it head into the lower 50s in decent warm weather (no heavy A/C in stop-and-go traffic, though), then you need to take stock of your driving style and patterns.