Burning more oil on highway than around town, why?

Discussion in 'Gen 2 Prius Technical Discussion' started by pasadena_commut, Mar 9, 2026 at 8:33 PM.

  1. pasadena_commut

    pasadena_commut Senior Member

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    It is my impression that our 2007 burns more oil at highway speeds than around town. Admittedly this may be observational bias, because going 700 miles on a trip on the highway only takes a couple of days, while the way we drive in town, it would be two months. So the same amount of oil burning per mile may seem like a higher rate because of the shortened time frame.

    Anyway, for the sake of discussion, if "my Prius burns more oil on the highway than around town" is actually true, why would that be the case? The only thing that I can think of is that for some reason more oil is pushed through the PCV valve under those operating conditions, where the ICE motor is on and at a higher RPM than in more sedate driving. Some oil definitely follows that path under all conditions, since looking past the throttle plate there is always some oil down at the bottom (not a pool of oil, just signs of oil.)
     
    #1 pasadena_commut, Mar 9, 2026 at 8:33 PM
  2. ChapmanF

    ChapmanF Senior Member

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    What is your average engine RPM in highway driving, compared to around town?

    I've never had a car that didn't burn oil faster at higher RPM.

    I've never thought the PCV circuit had to be the only path or most important path. The cylinder walls are a path. Piston rings can only be so close to perfect.
     
    #2 ChapmanF, Mar 9, 2026 at 8:38 PM
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  3. BiomedO1

    BiomedO1 Senior Member

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    If you know you have issues; you should be checking at every fill-up. That's a good rule that seems to have gotten lost by the computer generation.
    I was helping my friends daughter move to the east coast for her graduate program. The first fill-up; I didn't say anything. The second fill-up I mentioned it; still nothing. On the third fill-up; I insisted. It was 1.5 quarts low, not touching the dip stick. We were only about 1K miles into the trip; so I had to ask - "Is this how you were planning on getting another car?". We had a good laugh over that; because she knows better. Blasting down the highway averaging 80+ mph will do that to an old Lexus engine with 200K+ miles on the clock. It had a 5-6 quart sump.
     
    #3 BiomedO1, Mar 9, 2026 at 8:56 PM
  4. PriusCamper

    PriusCamper Senior Member

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    It's interesting how we call it burning oil when there's no leaks on the outside because the loss of oil isn't as straighforward as it just burning, but how it circulates in the engine. Would be great if someone like you or someone else who knows alot could help explain the finer points of this subject.
     
    #4 PriusCamper, Mar 9, 2026 at 9:16 PM
  5. pasadena_commut

    pasadena_commut Senior Member

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    On a Prius unless there is an OBD2 reader plugged in you would never know the RPM. On our 1998 Accord there are spikes to 3K on any acceleration (not foot to the floor, just speeding up normally), cruises at around 2K on the highway, spins only a little lower at 45 or 30 mph, idles at 600 (ish). The average RPM will be higher on the highway because there won't be nearly as many periods of deceleration or sitting still idling, not because the RPM range is much different between the two conditions.

    My son thinks that one reason the Prius would burn more on the highway (per mile) is that the ICE would be on most of the time and the oil would be at an average temperature which is much hotter than when driving around town, especially in any sort of stop and go. Since the oil viscosity falls with increasing temperature, and the thinner the oil, the more that will get by the rings, it will burn more on the highway. I think by the same logic on the highway the lower viscosity oil will more easily form small droplets, and those will hang in the air long enough to be sucked out through the PCV valve. That would be testable if I had an oil catch can. Unfortunately there does not seem to be a CARB approved one, so installing one is likely to result in a failed smog test at the inspection stage.
     
    #5 pasadena_commut, Mar 10, 2026 at 1:33 AM
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