MPG decrease with OLM decrease. Any one else notice this?

Discussion in 'Maintenance & Upkeep' started by Pikey, Oct 30, 2013.

  1. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    I noticed over my last two oil changes that when the oil life monitor gets down around 25% my mpg begins to decrease. From averaging 17.4 in the city to around 16.4. On the highway it drops from 21 to 18mpg. I changed my oil today (olm at 30%, I changed it because I wanted to see if mpg increased like last time) and my mpg went back up to 17.5mpg city and 22mpg highway. I use Mobile 1 5w-30 high millage oil and ac delco pf 46 filters. Yes, the oil does look dirty when it comes out. The millage is from the DIC and hand calculated. I don't think that the oil would be broken down that much considering that the OLM is set for conventional not synthetic. But, I have noticed the change over the last 2 oil changes. When I change it, it goes up instantly. Any ideas? Anyone else notice this trend?
  2. BRB46

    BRB46 Rockstar 100 Posts

    That's quite interesting. I haven't noticed that but wasn't looking either. I am at 83% now and will monitor it as it gets lower. Will report my findings later.
  3. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Thanks! I am just interested to see if others notice the same thing. Like I said around 30%-25% I notice the loss in mpg. What oil are you using?
  4. BRB46

    BRB46 Rockstar 100 Posts

    I use Pennzoil Platinum 5W30 full synthetic. I had previously used Mobile 1 but for some reason i had to add a quart every 2500 miles. Now I add one every 5500 or so.
  5. dpeter

    dpeter Epic Member 5+ Years 500 Posts

    Not intending to hi jack the thread but.... I change my oil when the DIC says it's time and that is normally around 5,800 to 6,500 miles. So the question is ; What is this Oil Life Monitor you all speak of and does my 2000 Yukon have it? How does one access it?
  6. BRB46

    BRB46 Rockstar 100 Posts

    It is part of your DIC. Mine shows oil life in %, hence the life of the oil. I keep track of the mileage with my tripometer A. It starts at 100% and as you drive the % drops based on driving habits and outside air temp I believe has an effect on it as well. In the summer months I can go 8000 miles before it hits zero percent. In the colder months 5500 is all it gets.
  7. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Mine is the same and I do the same thing as described above. My 2002 just had a light that would come on and say change oil, no percentage.
  8. buckmeister2

    buckmeister2 Rockstar 4 Years 100 Posts

    Pikey, you are a great addition to this forum, and you really have me stumped here. There is nothing in the breakdown of lubricity or viscosity that could have that much impact on your MPG. Obviously, as oil gets dirty and thickens, it loses some of it's lubricity, which creates a bit more friction (or "drag") on internal components. But, I think we are talking about an effect well below 1/10 of a mpg, versus one or two full mpg. What I would rather suspect is that the bypass of dirty oil through the pcv and back into the intake may affect combustion, hence MPG. As the oil gets older/dirtier/contaminated, it is still passed back through the intake. Even clean oil injected into a fuel mixture reduces octane, and dirty oil would be that much worse. Only reason I can think of. Does this make any sense?
  9. Pikey

    Pikey Moderator Staff Member 5+ Years ROTM Winner 5000 Posts

    Thank you for the compliment.
    that does make sense. I am considering making a catch can. I think that for under $25 I can get a high quality watts filter
    Last edited: Jan 3, 2014
  10. 99'HEARTBEAT

    99'HEARTBEAT MODERATOR Staff Member 5+ Years ROTM Winner 1000 Posts Platinum Contributor

    Mike....just as the other Member's have mentioned, that's interesting......:neutral:

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