Highway MPGs <40?

Purchased my first Prius a couple of months ago now (new to me '07) after having seen how great my parents Prius did on mileage, and because I recently got a job that includes a 90 mile roundtrip commute each day.

Despite my commute being >90% highway, I can't get the Prius to return greater than 40 MPGs. My speeds average between 70-80, as there is no traffic and the speed limit is 70, which I understand will cause a hit to fuel economy, but so much so that I can't even top 40?? Temperatures have been warming up (regularly in the 50s here in the Pac NW), but I didn't think that would have much of an effect since my driving is almost all highway. Am I missing something? Could there be something wrong with the HV battery even though no codes are being thrown? In the near future I plan on buying an SG2 to better track things. In the meantime, I've filled out the questionaire below, looking forward to others experiences and tips. I've also taken several trips that involved lower speed driving (driving to the coast on 2 lane... hilly roads) and still couldn't exceed 41-42 MPG at best.. calculated on an Excel sheet based on miles driven/gallons filled.

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)

36-40, comparing the Prius computer (about 2.5 mpg high) with manual fill up calculations tracked in Excel

- What fuel economy are you expecting and why?

43-45 highway, based on what I've heard and the fact that I was able to get mid 40s out of my families '07 Prius when they owned it (combination of around town and hwy driving)

- What are the approximate outside air temps?

mid 40s-low 60s

- How long are your trips?

45min-1hr all highway (about 45-50 miles each way)

- 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?

Almost all highway. First/last mile is city stop and go, the middle is wide-open highway (little/no traffic) avg 70-80 MPG

- What region/state are you in? (if you haven't set your location in your profile)

Portland, OR/Pac NW

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

gentle hills, mostly flat (I-5 along the Columbia River)

- Is your oil overfilled? (i.e. above the full mark on the dipstick)

nope, though I will change it myself next time

How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)

Previous owner said he replaced it about 2 years ago...

ACC mode - 12.4 V
2nd mode - 12.1 V
Ready mode - 14.1 V

Have you had your alignment checked? Any pulling or abnormal tire wear?

no abnormal tire wear, new tires I am assuming it had an alignment when the tires were replaced about 6 k but I'm not positive

- 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).

Bridgestone G019 Potenza's

- What are your tire pressures?

40 front/ 38 rear

- 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.)

'05 Outback XT (Turbo) about 21-22 MPG similar commute with premium fuel.

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?

Light braking, especially coasting/easy brakes coming off of the highway. Try to pulse and glide when possible.

- 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?
Auto, but A/C unchecked, temps no higher than 70. I will turn it off when I get into the city on cooler nights to get the ICE to shut off at stop lights (sometimes it won't)

- 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?

Nada.​

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Read more: http://priuschat.com/threads/fuel-economy-complaints-queries-please-copy-paste-answer-these-questions-esp-if-youre-new.77075/#ixzz2OfPAjAg7​

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Thanks in advance for your advice/experiences!​

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Ok did the battery test which I added to the original post... (cold after the car had sat since last night) and got the following:

ACC: 12.4 V
2nd mode: 12.1 V
Ready mode: 14.1 V

All seems normal to me?

An additional side note... I've read the posts about the non-linear tracking of the battery bars, and was wondering if people have experienced the following. On some occasions, usually after the previous trip ended in a lot of electric mode driving, I will start the car the following day and within the first 1/2 mile of driving the battery bars will drop 2, sometimes 3 bars in that short period of time. Sometimes it has gone from 6 bars to 3. It always comes back up within the next mile or two of driving after this. All other times (especially my 95% highway commute) it stays consistent at 7 green bars. During mountainous driving I've noticed it drop (as I would normally expect) to 4 or 3 bars over the entire climb... 2000+ vertical feet. The drop is generally consistent and does not appear abnormal... but I'm no expert.

 

Thanks everyone that's posted. It seems like 40 MPG is to be expected with average driving above 75. I guess my previous experiences driving a Prius were in more congested regions where it was difficult to exceed 70 for any extended period of time.

I'm bummed about the tires, they seem pretty good for traction especially in wet thus far. They've got maybe 6k on them, so I can't justify changing out almost new tires just because they aren't LRR. I guess if I keep up my driving, I'll be able to look forward to new LRR tires in <2 years! Although by then perhaps I will have upgraded to a Gen III.

Thanks for the link to the Gen III speed vs MPG graphs. It looked like there was a graph from a Gen II in that post as well. Has anyone done more extensive studying of Gen II MPG vs speed like Bob has? Or is there somewhere that people are posting their results for a larger database? I know there are so many variables to account for, especially rain/wind, hills, and even driving in a flow of traffic vs out on a road with no cars in front of you. I work the evening shift, so I've even found that driving to work (just keeping up with the flow of traffic in the high speed lane) that my readout will suggest 2-3 MPG more than when driving home at similar speeds with no cars to follow in line with.