What am I doing wrong?

This seems like the most likely culprit to me at this time. Considering the OP purchased the car in November, this is the first summer and "a few months ago" is when this was noticed/started. That's basically the summertime, and thus with the A/C set to 73 seems likely to be the cause.

Let's hypothetically say you were putting 10 gallons in each fill for that 400 miles. Now you put that same 10 gallons for 370 miles. You lost 30 miles per tank, or 3 mpg. That's easily the hit you'll take running the A/C, especially at a setting of 73. I notice when I keep mine around 78 normally, the hit isn't bad but if it's a rainy or humid day, I might bump it to 76 and the hit is more noticeable. My drive is shorter, so the hit for you might not be as bad, but the hit is still there in the beginning while the car is being cooled down.

 

First of all....you need to measure your MPG by the tank. If you want to use the MDF (Multi-Function Display) that's fine....just remember that you're really getting about 5-percent less that the display is saying that you are.

OK....so based on that I'm reading, you need to set your A/C to a value that's comfy to you between 75 and 80.
Check your oil level every tank.It's not that hard to do at the filling station, and if your dealer changed your oil at some point in your dark and hazy past, and you don't already know if they overfilled or not......well, you need to be checking your fluids more often. Just sayin.

You're in SOCAL traffic, which probably means that you're using your accelerator like an on/off switch. Brakes too. I would say "stop doing that"...but you're driving almost 80 miles per day just to get back and forth to work, so I'm not going to bother.

If you make all of these changes and you're not happy with your mileage,
Rent an SUV for a week.

Good Luck!

 

- What are your tire pressures?

I'm not sure
Get Them UP 42 front 40 rear.

 

At high speed like that, if you eliminate drafting and
partial drafting - the only hypermiling techniques that
I can think of is Driving with Load (DWL). DWL takes
advantage of continously hilly terrain and is often used
by diesel trucks to maximize their fuel efficiency. When
a vehicle is going downhill from the top of one hill and
then must continue without stopping uphill to the top
of another hill of equal height, the driver apply energy
as the vehicle moves downward from the top of the hill
so that the vehicle gains a signficant additional velocity
by the time it is at the bottom of the hills. As the vehicle
climbs up the next hill, the driver, reduces power gradually
and allows the vehicle to gradually lose the additional
velocity that it had gained going downhill as it climbs uphill.

For example, a Prius at the top of the hill would start
off at 65 mph, but by the time it was at the bottom of
the hill it would be running at 75 mph. As the Prius climbs
up the next hill it gradually loses velocity until it is
at running at 65 mph at the peak of the next hill.
This technique build momentum when its cheaper to
gain speed (downhill) and loses momentum when its
more expensive to gain or maintain speed(uphill).
This method is more energy efficient because the load
on the ICE and electric motors is less when the car
is going downhill than when the car is going uphill.
For a given velocity V, when the peaks of two hills
are of equal height then the downhill energy savings
at velocity V, E1(V), should equal the uphill energy
cost of maintain velocity V, E2(V) when compared
to the cost of of driving the car on a flat surface
at velocity V, E(V). Supposing

E1(V) = .80 * E(V)
E2(V) = 1.20 * E(V)

if you are using cruise control then to go downhill and then
uphill at a constant speed would equate to

ET(V) = E1(V)+E2(V) = E(V)(.80+1.20) = 2* E(V)

Given that it takes more energy to move
a vehicle at a higher velocity, we create a simplified
energy model, where if the velocity V increases
by 10% then cost to push the vehicle forward,
E(V), increased By 10% and if the velocity
V decreased by 10% then E(V) decreased by 10%.
so

E(.90 * V) = .90 * E(V)
E(1.10 * V) = 1.10 * E(V)

given that a DWL cycle has a top speed = 75 mph,
an average speed =70mph, and a low speed = 65 mph
and the grade cost is 20% more going uphill and
20% less going downhill then...

DWL energy cost =

ET(V=70 mph) = E1(1.10 * V) +E2(.90*V)
= .80 * E(1.10*V) + 1.20 * E(.90*V)
= (.80*1.10 * E(V)) + (1.20 * .90 * E (V))
= .88 * E(V) + 1.08 * E(V)
= 1.96 * E(V)

cruise control energy cost =
ET(V=70 mph) = 2 * E(V)

DWL improvement over cruise control in Energy cost
= 2 * E(V) - 1.96 * E(V) = 0.04 * E(V)
DWL percentage improvement = 0.04/2.00 * 100% = 2%

estimated improvement (70mph,cc) = 51 mpg * .02 = 1.0 mpg

 

I don't think one reduces power when DWL uphill, it's more like a constant load (or constant mpg) via keeping the gas pedal locked. From the cleanmpg definition of DWL:

"DWL: Driving w/ Load: Instead of relying on Cc (Cruise Control) to maintain speed, you rely on your iFCD (Instantaneous Fuel Consumption Display) and accelerator pedal for those automobiles that have them to stay locked in at a given fuel economy. One example would be when climbing an overpass. Instead of holding a steady speed up, over, and down the other side, you allow speed to droop as you climb while maintaining load or FE on the ICE and climb back to initial target after the decline on the backside. Begin the overpass climb at 65 mph, drop off speed as you climb, reach 62 mph at the crest, increase speed on the decline back to 65 mph. The technique depends on elevation deltas and traffic conditions. This can be simulated in a non iFCD equipped Accord or other automobile by locking in the accelerator pedal when approaching the overpass. Just hold the accelerator steady into, up, over, and down the back side at the same exact angle while arriving at the same initial target speed after the overpass has been cleared. There are slight accelerator pedal changes that can maximize the technique for those with iFCD’s but the locked down accelerator will work well for those just starting out and with a lack of an iFCD . An even easier way to understand the technique is to drive like a roller coaster coasts over the peaks and through the troughs. Pros are increased FE over any small terrain delta with a minimum of work. Cons are that there is thought and user input involved as well as slightly lowering your overall average speed to a given Point B."

copied from Beating the EPA - The Why’s and How to Hypermile - CleanMPG Forums

 

And this is why I became a writer. Math equations...

 

Whoops! - I stand corrected.

"reduce power" should have read "maintains a fixed
power level" because the power output of the ICE/MG is
fixed/limited ("constant load") during the DWL
uphill climb session-section of the entire DWL cycle
which inturn causes the vehicle's speed to drop as it
fights the additional gravitational forces.

 

I live in Los Angeles County and drivers here are too impatient and drive higher than the posted speed limit. Most will try to bully you for following the posted speed limit. For the first 5 months of my prius III ownership, I've felt intimidated by tailgaters causing me to drive inefficiently.

I have a thule t2 hitch rack installed and I leave it on the down position. I was advised that in the upright / stowed position I may likely get a citation for blocking the rear plate. In the down position, drivers have stay waay behind my vehicle. I am so happy now!!!!

Within the last few tanks, my Prius III has been averaging 500 miles to 9.80 gallons of gas.

 

I'm another Southern California driver. I've long since become immune to the bullies who try to intimidate me into driving faster than 65 or 70 at the most. But I'm also considerate and make a point of not driving in the #1 lane or keeping the impatient person blocked in. It doesn't bother me in the least to have most people pass me by.

The pulse and glide technique doesn't work all that well in slow/stop and go traffic in L.A. I used to use a technique of trying to maintain a continuous crawling speed, leaving enough room in front of me so that the speeding up then stopping for an instant didn't affect me (was driving a vehicle with a heavy-duty clutch). That would work fine when I was in the carpool lane, but not at all in the regular lanes. Too many drivers think that if you leave defensive space in front of your car, you are driving too slow. So they pass you, cutting you off so you have to stop. It'll be interesting to see how I do in the regular lanes tomorrow, the first time I'll have to face rush-hour since I got the Prius. I'll try to use the pulse and glide, but have doubts I can make it work right though.

 

My simple solution to your problem is to drive 400 miles and then fill up, everytime. Your problem is that you are stopping to fill up 25 miles too soon. Problem solved.

OR

At some point in this thread you should acknowledge that you need to or will start collecting data that includes the amount of gas you are putting into your vehicle AND the total miles you drove. Divide total mile driven by gallons filled = Miles per gallon.

 

I would dispute the car taking a different amount to fill up everytime. Our Prius is always 10-10.2 gallons on the nose to fill up. Granted my drive is always the same, and always with mild traffic.. I think either you have a lot of people cutting you off forcing you to brake heavily/lose regen braking. Also leaving the AC on 73 will deplete the HV battery fairly quickly. Especially when creeping in heavy traffic/stopped.. My best advice to you is if you know you are going to be stopped in traffic, and the weather allows to turn the AC off & roll down your driver window till you get going again so you can use your HV battery juice to creep through traffic. My biggest MPG loss personally is running AC, creeping in traffic, and then once the battery depletes the engine continously runs & never recharges due to the constant stop/go.

I will also agree with the others that your tire pressures & oil level are very important for obtaining optimum mileage. Also using the sun shade reflector in your windows helps keep the cabin cool for your drive home. If you have lots of pot holes though be careful with over inflating as you do not want a blow out. To deservice the oil level you can get a pump for $9.99 at Advance Auto and put the hose down the dipstick tube to pump it out. I do this at 5000 miles and change the oil out in the pan. I service it to about 1/4" to 1/2" below the full mark.

 

I drive the 405 and 5 (Sepulveda Pass, Newhall Pass, Grapevine). On the 405 I would leave lots of extra room in front of me to absorb most of the accordion effect, slowly catching up with the car in front of me just about the time that person started up again, often just about coasting to a stop. I made no effort to keep up with traffic at all, dropping back and catching up as needed, coasting most of the time. I'm sure I frustrated the people behind me, but only one or two got obnoxious about it. That worked well with the Jeep (put it in 2nd gear and let it coast, wouldn't have to use the clutch) and the Honda Fit. It won't work in regular lanes because too many people will cut in front of you, filling up the space you need to make this work.

I don't think that's necessarily the best way to drive the Prius though. I was stuck today in the back-up from a cleared accident, coasting between 4-15 mph (rarely coming to a complete halt) and couldn't get enough regeneration breaking to keep the battery charged - the engine came on before I managed to get clear of the back-up. Before the engine came on the car said my average was 57.1 mpg - by the time I got some speed up (from the I-118 exit to the merge with the I-5, 8-10 miles?) it was down to 56.4 mpg (no air conditioning, should have thought about turning off the heater fan earlier, I didn't need it). I'm not sure that it would have been possible to keep the battery charged enough to keep the engine from starting, regardless of how I drove because the traffic was barely moving and moving pretty much at one speed (no accordion effect). Of course, it wasn't that high by the time I reached the top of the Grapevine.

 

In your first example (reducing the accordian effect) I think is one of the ideal ways the Prius shines. Slow and go traffic. People behind you have to work past the psychology that the big gap you leave doesn't mean they will be trapped on the freeway forever. It is funny how people get all bent up over 30 feet.

In your second example, I don't know how to keep the charge up either. Despite the fact that you didn't stop, when you are below 8 mph, you are using the traction battery to move your car (on a flat surface). At 8 mph, you finally get the "no arrow" glide. Above it, you'll regenerate. Still, if it is any consequence, you were still getting the best gas mileage of any gas powered car out there.

 

Thanks - good to know that 8 mpg seems to be the speed to attain for regenerating the battery. That's extremely useful to know for slow and go traffic. Knowing that helped me get a bit better gas mileage in more normal slow and go traffic today (filled up and the average for both days, plus some from the day before was 50.83 calculated).

 

Oh, the 8 mph won't show any bars going to or coming from the battery, I'm pretty sure all the car power sources will drain from the traction battery (lights, wipers, displays, radio, nav, etc) so over time, even if you were at 8 mph, you will still see a drain on the battery. But for short term use, it will be negligible (this is based off of my observations, rather than anything I've read).

 

It really is. Miles driven / gasoline pumped = MPG

Any other measure (in the US) is not helping us help you.

So far this is a 'story problem' with not enough information.

 

Is the 389/9.6 = 40.52 actually what you put in? Because now we can start talking numbers (40.5 mpg).

You do realize that you have a 11.9 gallon tank, so you could drive 400 miles and only take 10 gallons or so. The last pip starts flashing around 9 gallons or so, and I think that when the Miles to Zero hits zero, you've got about 2.1 gallons of gas left (9.8 gallons to fill up), which is close to what you are reporting.

Heavy A/C use will lower MPG's significantly. F8L did a thread (I think in the Gen II fuel mileage Forum) regarding his observations of A/C usage and MPG's with his 2005 (nearly a 10 mpg drop while the A/C was running, something like 56 mpg to 46 mpg on his scan gauge). Search that one out, if you can.

Keep tracking your Miles per Gallon (keep a log, spreadsheet, or join Fuelly and use their spreadsheet). You see interesting variations throughout the year and will get a much better understanding of your driving behavior and its effect on MPG as well as weather's impact on MPG's.

It's fascinating stuff.....