MPG drops after oil change

Discussion in 'Gen 2 Prius Fuel Economy' started by wfz, May 23, 2013.

  1. wfz

    wfz Junior Member

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    I changed the oil on my 2008 Prius Touring in the weekend. I consulted the manual after the oil was changed. One mistake I made was to use 10W-30 instead 5W-30 recommended. The manual mentioned that it affects MPG with 10W-30. For the 270 miles trip I took right after, my average MPG dropped from ~46 to 45 highway. My short commute driving dropped from ~37 to 34. Comparison are made on the same roads with similar temperature and driving speed.

    Is that normal? Will it go back up after a while?

    Thanks,
     
  2. jdenenberg

    jdenenberg EE Professor

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    It will recover after you change the oil again, but with 5W30.

    JeffD
     
  3. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Also, be sure you are staying under the fill mark on the dipstick.
     
  4. wfz

    wfz Junior Member

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    Thank you all for the feedback.
     
  5. uart

    uart Senior Member

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    On the highway once the engine is warmed up, there's no real difference between a 5W30 and a 10W30. So it's likely that your very small change in highway MPG is either a random variation of a minor difference in oil formulations that's unrelated to the grade.

    For short trips however, where the oil often doesn't properly warm, then yes it's likely that 5W30 will give a small boost in fuel economy over 10W30. How much depends on just how cold it is where you live and how short are your trips.

    As pointed out above, double check to make sure you're slightly under the full dimple on the dipstick for best fuel economy. Overfilling the oil can add extra drag.

    BTW. What brand/product oil did you use, and what was your previous fill?
     
  6. xs650

    xs650 Senior Member

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    I agree completely except for your first sentence. There is a wide variation of the allowable viscosity of xW-30 oil at operating temperature. It can run from 9.3 to 12.5 cSt at 100C (212F). 12.5 cSt is 34% more viscous that 9.3 cSt. For a comparison of a random pair of 5W-30 and 10W-30 oils, either one could give better fuel economy at operating temperature.
     
  7. uart

    uart Senior Member

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    Yeah I didn't explain it very well because I didn't want to get too technical, but that's what I meant when said "minor difference in oil formulations". That includes other things as well, like different additive pack and friction modifiers, but I was thinking primarily of within grade viscosity variations when I said that. :)
     
  8. HaroldW

    HaroldW Active Member

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    I would be using 0-20. That is all my cars have ever used. H
     
  9. wfz

    wfz Junior Member

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    Thanks for all the info. I used Valvoline full synthetic previously and switched to Mobile 1 synthetic this time.
     
  10. HaroldW

    HaroldW Active Member

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    I really like Toyota brand 0-20 synthetic and it is reasonably priced. Here in Canada around 6.50 per ltr. .H
     
  11. uart

    uart Senior Member

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    Ok, thanks for the info. Both Valvoline and the Mobil-1 line include several different full synthetics, but the regular Mobil-1 10W30 should be pretty similar to the regular Valvoline Synpower 5W30 at normal operating temperature.

    From their relevant technical data sheets:

    Synpower 5W30 viscosity: 10.7 cSt @ 100C and 59.5 cSt @ 40C

    Mobil-1 10W30 viscosity: 10.1 cSt @ 100C and 63.2 cSt @ 40C

    So Mobil-1 10W30 is actually slightly thinner at 100C than the Valvoline 5W30 and so theoretically (all other things equal) should give slightly better highway mileage (if viscosity were the only factor).

    Are you sure you didn't use the high mileage version of the Mobil-1? That one's quite noticeably thicker, especially when cold.

    Mobil1-HM 10W30 viscosity: 12.0 cSt @ 100C and 78.1 cSt @ 40C
     
  12. wfz

    wfz Junior Member

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    uart, thanks for the technical info. I had another round trip and finally found the reason for the dropped MPG. I didn't have the car long and I had defrost on a lot of times and my temperature was set to ~ 70 F for the whole winter and early spring. The temperature increased to 90 F since the last couple of weeks. With defrost on, the air coming out was cool. Even my AC was not on, the air coming out of was way lower than outside temperature. I guess it burned gas to keep the temperature low. After turning off the defrost and set the temporature to 76 F, my last trip was as good or even better than the previous records (probably due to higher temperature). So overall, the engine oil has little to do with the MPG.

    Thanks you all for the advice,
     
  13. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    It is my belief that defrost always runs the A/C to dehumidify the air.
     
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