Suddenly getting 18-20mpg in my 2008 Prius

- Have you read This Thead Yet?
Yes

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)
I'm getting about 18 or less mpg. The bar graph and my average (reset 150 miles ago) show 40mpg average. However, I drop a gas bar every 15-18 miles and am only getting less than 200 miles to a full tank of gas meaning that can't be correct. 

- What fuel economy are you expecting and why?
For the last 9 months, except the first two or three tanks I ever drove while learning how to drive a Prius, I've gotten a very consistent average of 40 mpg or 400 per full tank to one bar (not flashing) so this is what I have come to expect. 

- What are the approximate outside air temps?
88-90 degrees Fahrenheit. 

- How long are your trips?
30-40 minutes, 15 miles, is my commute to work daily. 

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?
It's all suburb driving, average speed 40-50 mph and not very many lights or stop signs... Maybe 4 traffic lights the whole way. Not stop and go. It is 90% flat road. 

- What region/state are you in? (if you haven't set your location in your profile)
I live in Baltimore (county) Maryland. 

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)
Mostly flat, a few very small hills. 

- Is your oil overfilled? (i.e. above the full mark on the dipstick)
No. I had it changed about 1,000 miles ago. 

How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)
.... No idea. 

Have you had your alignment checked? Any pulling or abnormal tire wear?
Hasn't been checked, but no pulling and just had one tire replaced (drove over a screw) and all are in excellent shape. 

- Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).
Using factory tires. 

- What are your tire pressures?
35 front, 33 back. Checked today. 

- Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)
I used to drive a 2004 Saturn ION. I didn't drive this route but drove one similar, and got 16-20 mpg. 

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?
Braking slowly and gently, accelerating and gliding. 

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?
No. 

- Are you driving using D or B mode?
D. 

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?
Using AC, about 70 degrees. It's been much cooler this week than previously (when I had better gas mileage!)

- If reporting a mileage drop, did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?

Oil change 1,000 miles ago. Not sure if the problem was happening before then, but I had it changed at the same place I always have it changed. 

I also had a tire replaced a week ago (drove over a screw) with a new factory tire and all tire pressure is normal. Problem was happening before the tire though. 

Commute has been the same for 4+ months, bought the car December 5, 2010. 

************************************

Long story short, I'm getting about 18-20 mpg and have been for a few weeks. I have no idea why. Nothing at all has changed, not commute not where I get my oil changed, not even where I get my gas. What's going on? I'm extremely frustrated.

Thanks in advance guys. And sorry for the lack of HT
L, I typed this on my iPhone.

 

Okay so, I just checked my battery and oil. (Already? Yes. Possessions I love more than my car = ... nothing.)

I checked my battery using the URL listed in the questionnaire I posted earlier.

And it read:
12.2 off in maintenance mode 

11.8 when on

Charged up to 14.2 then drops to 13.9 in about 2 seconds 

Battery at 5 bars when turned on not in maintenance mode.

This seems to be in the normal range except 11.8 is a little low, the guide said 12.0 is the lowest it should go. Should I have this looked at? I hadn't driven the car in about 2 hours.

Oil level was normal, about 2/3 below the line.

Here's the thing about my mpg, it MAY be a wonky gas gauge:
I was on one bar (not blinking) and filled up to full. So full the pump won't go anymore. Prices here are about 3.79 a gallon. I filled up $22 worth which equals out to about 5.9 gallons. So... That seems a bit off. Usually I drive about 100 miles before the first gas bar drops off then the others drop off around 40-50 mpg. This time I lost one almost immediately and each other after 20 or less miles. I filled up full, and have driven 159 miles and have 3 bars left. I will fill up completely tomorrow and see how many gallons I get to see if that's the issue.

Something odd:
In February of this year I ran out of gas. Car manual says on one blinking bar you have 3 gallons left, and full tank is 11.9 gallons. I just went on blinking bar. Drove 30 ish miles. Bam. out of gas. What gives? Now I go to the nearest gas station when I hit one bar, never blinking, to ensure that never happens again. Makes me kinda miss the ole red "E"! Then I at least knew how far I could go!

Also, my car door says 35/33 for tire pressure. So that would be right... They were not cold though. I'd been driving about 1 hour right before I checked. Cant check now cause I dont have a gauge, used my bosses. Should I inflate em a little?

Thanks again so much.

 

Oops. So I was supposed to check the battery after sitting overnight not just 2 hours. Will update you all again at the buttcrack of dawn - to me - 630am.