MPG on highway

Hello all,

I'm a new Prius driver and as many have concerns about the fuel economy. I got a 2008 prius touring from a garage, recently serviced and warrantied, but after reading a lot on this site I do not fine the same figures.

What concerns me the most is the highway driving, where I get 35 MPG (33 calculated after filling the tank). After reading some advices I inflated my tires to 42 PSI front 39 rear (was around 28), cut heating or AC, and try to apply different driving habits, but got it up to 37 MPG only on the highway (see pictures)

I usually set the speed at 72 mph through the columbia gorge (portland-hood river). It's moderately hilly and usually windy. In both ways the fuel consumption at stabilized speeds seems to stay around this value.

The second concern I have is that the ICE always turns on over 15MPH. I can't stay in electric mode over it.

So the question I have: is this something I have to worry about? or should I investigate (battery, spark plugs,...).

Here's the questionnaire

- 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)
35 by computer, 33 manually

- What fuel economy are you expecting and why?
Expect 40mpg stabilized on highway based on readings

- What are the approximate outside air temps?
50F

- How long are your trips?
1h15min

- 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?
10% city, 90% highway

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

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

- Is your oil overfilled? (i.e. above the full mark on the dipstick)
No

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

Have you had your alignment checked? Any pulling or abnormal tire wear?
Alignment done, drive perfectly straight

- 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).
Falken sincera 250 AS 195 65 r16

- What are your tire pressures?
42f/39r


- 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.)
2004 Pontiac vibe GT manual. 25 mpg winter (studded tires), 28 mpg summer

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

- 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?
AC off, 68F

- 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?
new car


Thanks!

 

Thank for your answers!

I'll try to not use cruise control on hills, I feel sometimes it's revving a lot. I always drove manual cars and never used cruise control, but read it was recommended for the prius.

The tires are not LRR, being rated M+S i guess they're not designed for this, so they are probably not helping for the mileage.

When I start the car, the HV battery is always in the 80% range, goes to the green bars quickly. I haven't seen it drop below 40/50%.

So I guess the combination of new driver + not efficient tires + cold weather + hilly terrain + wind explains the low mileage. I'll try to run the 12V battery test to be sure.

The car has 112k miles. I'm the second owner, but it's a rebuilt title, it went through an insurance auction.

 

The data is really interesting, that confirms my doubts about the mileage I get. I am not really careful about how I drive the vibe, knowing the GT version has a really high revving engine. I tried to be more careful with the prius. I still save around 15$ per fill up for the same distance, but I hoped I could get better.

The car was in a involved in a front crash, they replaced the left fender, hood and headlight. The bumper and radiator were not touched. It seems it was pretty minor and only visible on close inspection. I'll try to see if the wheels are rubbing anything but I doubt it.

The car was auctioned in november so maybe the 12V battery got hurt by sitting still. But besides that the seller inspected everything and warranties it, so I think if there's something wrong it's not obvious.

 

So here are the result of the diagnostic. The car sat there in the cold since last sunday (it's been snowing in Portland and less than 30F).

Yesterday I moved the car to get another parking spot, less than 1 min. Upon starting, the battery was 80%. When I stopped it, two bars were left. This morning, the HV battery was at 0%.

I ran the diagnostic, and it showed the 12v battery was sitting at 11.4V, with HV at 0. When putting the engine on ready, it jumped to14.1 V.

Is all of that normal, or does that mean my 12V battery is dying or beyond recovery?

 

I tried the 12V again this afternoon, after my 60 miles commute this morning. The battery sits now at 12V, and goes down to 11.7V with accessories on. That's better than friday but still seems under the threshold for a good battery.

I tried to talk to the garage that sold me the car, as I have a warranty on it, but I believe until I have an error message they won't do anything about it.

Beside low mileage what is the risk with a low battery? Unable to start? Would it throw codes before reaching this point?

And as a side question, is there a way to push start a prius? I've always had manual gearbox cars, I wouldn't know how to deal with an automatic. I remember I couldn't start a manual car with a push start button either (renault laguna)

As for the HV battery, I mistakenly thought it read 0% when I put it in diagnostic mode, I don't think there's a problem with it. Thank you for the help anyway.

 

I'll get a jump starter then. I'd prefer not to invest in a new 12V battery, if there's a chance to get it changed by the shop I got it from.

As for fuel economy, here's a really approximate reading based on the yellow bar reading, for what it's worth

Short trip in city (5-10 min) <25 mpg
urban highway @ 65 mph > 40 mpg
Flat highway at 75 mph = 35-37 mpg
country road at 55-60 mph = 44 mpg
city when engine is hot, after highway = 45-50 mpg

 

Thanks! that's a good advice. I really enjoy the car, more than I thought I would. I'm mostly worried because I had a lot of horror stories with used cars.

I filled it back up, 33.8 mpg calculated vs 34 indicated in the computer, seems precise. I repeated the measurement of the battery value at various times, it seems that it's always just under 11.9V without load. I'll try to check with a multimeter to confirm. It seems that the consequence is having the ICE turn on more often to load it even if not needed. It doesn't really stay off for long at stoplights.

After talking with the person that sold me the car, it doesn't seem that he will do anything unless the car throws a code. Even then I don't even know if the battery would be covered under the warranty. And I'm not sure if I should change it myself. So far the car behaves correctly, no weird window or radio behaviour as other persons reported. If a new battery would get me up to 40 MPG, it would take a year to get my money back, assuming it would cost 200$. So far I just keep driving it...