Avg. MPG gave DROPPED!!!

Discussion in 'Gen 2 Prius Fuel Economy' started by jbean, Jul 13, 2016.

  1. fuzzy1

    fuzzy1 Senior Member

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    As far as I'm concerned, gas brand should be mouse nuts, a tiny factor compared to the MPG shortfall you are trying to diagnose. If there is a problem with Costco's product, it must be specific to your region.

    The upcoming trip should greatly help in determining how much of the problem is related to the short trips (initial engine warm-up and cabin cool-down), and how much is elsewhere. The longer trip ought to produce much better MPG than the short trips in town. For even better separation, consider resetting the Trip MGP meter (or fill up on gas, if that is the only way to get daily MPG on a Gen2) after the car has driven 3 to 5 miles, so the engine warm-up and cabin cool-down fuel penalties are excluded from that trip's MPG measure.
    If that is about 20F cooler than when you measured tire pressure, then expect about a 2 psi drop. That would leave your front tires 3 psi short of the 35 psi door label. Rears should be 33. (I'm assuming Gen2 numbers are the same as my Gen3s.}
    They might give some blowback about anything about 40. If so, just take the 40.

    The pressure tuning you need is well within the range of floor-stand bicycle pumps with ordinary muscle power. If you have one, or can easily borrow one, that may be easier than dealing with the shop. Or even use the compressed air at some filling stations, though some folks don't care for the included moisture.
    When you are ready to get a new 12V battery, just tell them to replace it. Their hands won't be tied.
    It should be significantly better. But that doesn't mean it will be as good as you want, or can get. Please report back to us regardless of the outcome.

    This longer trip should also help charge the 12V battery somewhat, if the short trips have been depleting it. So, measure the 12V level again after the trip, and after the car has been shut off 4 hours to overnight. Don't bother during the trip, the voltage will be whatever the charging system is normally putting out, high-13s to low 14s. And don't bother immediately after shutdown, it takes hours for the raw battery voltage to 'relax' and stabilize.
     
    #41 fuzzy1, Aug 12, 2016
    Last edited: Aug 13, 2016
  2. jbean

    jbean Junior Member

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    I appreciate the information. When I get air in the tires today, should I ask for 40 in the front and 38 in the back? I'll report back after the trip.
     
  3. bisco

    bisco cookie crumbler

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    actually, if you're trying to figure out what's wrong with your car, you should try to keep everything the same. otherwise, you're just masking the problem.
     
  4. jbean

    jbean Junior Member

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    I don't think I'm masking the problem. I'm trying to troubleshoot the problem. If I kept everything the same, where would that get me? I don't understand.
     
  5. jbean

    jbean Junior Member

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  6. bisco

    bisco cookie crumbler

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    if you want to bump up the tyres, any combination of 2 pounds more in the front is fine. some people even run them all the same. you can go as high as the max sidewall rating, usually around 50 psi. i run 42/40 f/r.

    what i'm saying is, if you pump up your tyres higher than what they were when you were getting good mileage, your not solving your problem. you're just adding another factor, the will raise your mpg slightly.
    if your tyres were around 40 back when you were getting good mileage, then yes, you should correct them. otherwise, you should be looking for what else has changed. tyres pressure is not going to fix your car.
     
  7. jbean

    jbean Junior Member

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    Here's my report from taking a trip today:
    (Yesterday I increased the air pressure in the tires to 40 in the front and 38 in the rear).

    It was 108F driving to our destination. I had the a/c on HIGH at 73F. I don't have tinted windows and I'm planning to get them done soon. On the return trip it was 106F and the a/c was the same setting.

    The AVERAGE mpg was 45.0 and I traveled 92 miles. That's a more normal reading, I think, that I've gotten for trips like this.

    I ran the DIY test and got these readings:
    Battery Voltage 12.4 (compared to 12.2 before the trip).
    With a load on the battery it was 12.1V (compared to 11.9V before the trip)
    The last reading was 13.9V, the same as before the trip.

    Is it normal before turning the car off that the engine or battery is really revved up? I had the same experience with the previous test.

    I ran the second test 5.5 hours after arriving home.

    I'm looking forward to hearing what everyone thinks. Obviously it's an improvement, but what do the test results mean?

    Thanks so much.
     
  8. jbean

    jbean Junior Member

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    Just wondering if anyone has had a chance to read my report from Sunday, 8/14/16. Thanks.
     
  9. bisco

    bisco cookie crumbler

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    your 12 volt sounds weak.
     
  10. jbean

    jbean Junior Member

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    Do you think it needs to be replaced now, soon??
     
  11. dolj

    dolj Senior Member

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    Possibly not, if you have a 12V battery charger (preferably one with an AGM setting) and you can charge it up for 12-18 hours.

    If you can't do that and you do not drive enough to keep it in-car charged (2x 10 mi trips daily) then you may continue to get niggly problems. If you drive less frequently than that, it may be worth buying a battery charger if you don't have one. You can then get into a monthly overnight charge routine which will help you get much longer life (than the 3-4 years that people here seem to think is acceptable) out of your 12 V battery/batteries.

    All that said, your battery is a separate (and coincidental) issue that, IMO, does not have a large bearing on your MPG issue.

    Your drive showed that your car is performing averagely, although 45 MPG is not great, and I think you have addressed your main problem by inflating your tires to 40F/38R PSI. Having to cool the car to 73℉ with the OAT at 106-108℉ is certainly dragging down the economy from the 50 MPG that it should be. I'd try 76℉ and run it on auto. Make sure all vents are open full and not pointing directly at any person. Pointing slightly up and straight back is best. You are trying to cool the cabin, not people. Adjust it down 1℉ at a time if it is not cool enough, though I don't think it would make much difference.

    If you want to get better MPG, read the link in my sig called "Hypermiling technique: Pulse & Glide in Gen II", which will help when the speed limit is 40-45 or less. There are other techniques that can help such as DWB (driving without brakes) which does not mean driving dangerously fast because you don't touch your brakes, but rather look far ahead and anticipating your speed so that you slow the car only with re-gen from lifting you foot off the go-pedal (or a very light braking–Prius only technique), such as slowing for corners, slowing/stopping for traffic lights, etc. All of these techniques take patience and practice (plus the associated attitude adjustment) to perfect them to the point where they make a real difference.

    One last thing, stop focussing on short journey MPG runs. When you are getting it right, you should get good MPG reading on 300 mile - 1 tank distances. Bear in mind that the (USA only) bladder makes it difficult to calculate accurate tank to tank MPGs in the USA Gen II. So trust the on board a bit more. Your calculated average MPG will level out over multiple tanks, so don't sweat it.

    That was a bit more than I intended to say, but hope that helps.
     
  12. bisco

    bisco cookie crumbler

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    how old is the 12 volt?
     
  13. Sondre

    Sondre New Member

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    I had similar experiences with my Prius. I made sure that 12v battery and the tire pressure was within acceptable limits. The only culprit I could think of was the AC. It felt like the car was struggling to cool down the cabin, and the air from the vents never felt cold enough, so I took the car to Toyota for an AC checkup. Sure enough, the AC system had only one third of the required pressure, and they refilled the system. After that, the avg MPG immediately improved. Might be worth a try...

    Posted via the PriusChat mobile app.
     
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  14. CrazyLee

    CrazyLee Member

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    jbean said:
    If you want to get better MPG, read the link in my sig called "Hypermiling technique: Pulse & Glide in Gen II", which will help when the speed limit is 40-45 or less.

    I have found that you can hypermile at about any speed. 40-45 is great, but how about 70-75? It works. Keep pulsing +5 mph and gliding down to the original speed (70) That speed keeps you legal and safe. In states that have a 75 mph speed limit you may get more mileage than at 70 mph.

    Overpasses and rolling hills work great for hypermiling. The secret is to hold the throttle steady going uphill,letting the speed droop (this fools the ECU to keep the current fuel map and the fuel mileage stays higher) . once you crest the rise, release the throttle to slow down the engine and press down slowly to gain speed down the hill .
     
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