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Interesing PSI MPG Observation

Discussion in 'Gen 3 Prius Fuel Economy' started by Hank101, Mar 22, 2012.

  1. walter Lee

    walter Lee Hypermiling Padawan

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    50 psi front/ 48 psi rear... I have a SG2... I'm grill blocking too.

    I can normally achieve from 55 mpg to 75 mpg (starting with a cold engine for going back and forth to work) in my 16 mile one way commute(32 miles round trip). While my commute peaks at about 50 mph, during most of my commute's my median top speed somewhere closer to 35 mph and my average speed is about 18 mph. When I have the engine warmed up I can get between 75mpg to 90 mpg on my 16 mile commute when the temperatures is over 60F and the traffic is light and if I'm a bit lucky. Most of my miles are from my mainly city traffic commute and my overall average is 60 mpg. My low speeds FE is normally between 60mpg to 70mpg.

    I only change my tire pressure setting when I go on day long interstate superhighway trips(+500 miles/day). I rarely drive on the highways...but when I do the occasional beltway highway driving I normally don't change the tire pressure settings because i'm still trying to see if I can make overinflated tires achieve hi MPGs at high speeds....

    For example, yesterday I was on Md 32 from Clarksville,md to I95 exi with my cc set at 55 mph - hoping to get over 60 mpg with my tire pressure still set to 50/48 psi with a fully warmed up engine ... no dice... I was averaging about 55 mpg. MD32 is pretty flat and straight as local highways go (55 mph is pretty pokey so I was on the right lane ridge riding). The only way I know of to get over 60 mpg on MD 32 is to use DWL -- varying the speed from 53 mph to 63 mph.
     
  2. SageBrush

    SageBrush Senior Member

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    Well written! Your experience mirrors mine to a T, except that I have never managed 90+ mpg on a round trip.

    Care to opine on the use of MAP ? This past month I have been trying to avoid if at all possible ICE use with MAP < 10. In practice this often means accelerating into the 2000 - 2200 rpm range for a few seconds, then backing off the fuel pedal to whatever steady state rpm I need. Keep in mind the MAP numbers are for mile+ high elevation driving. I think this works out to < 2.4 pressure units below ambient.
     
  3. revhigh

    revhigh MPG Enthusiast

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    Hmmmm .... I regularly make trips from PA to OC MD and back, which is right about 220 miles ... driving almost the entire way at 65 MPH with cruise control, usually only stopping at 2 or 3 lights, and I can EASILY get 57.2 +/- calculated MPG for the trip. I'm very surprised you can't hit 55 MPG at 55 MPH on a long straight road with those tire pressures. At 55 MPH, I'm closer to 60 MPG on the highway.

    My tires are at 42/40, with grill blocking.

    You've obviously done a lot of research, and a lot of studying on this matter, so it surprises me that several of your assumptions seem contrary to standard thinking (like your tire slippage/higher PSI theories), and in my case ... my own observations. I've never heard of a GEN III that can't easily crack 55 MPG at 55 MPH. That's one of the things that surprised me about my Prius ... that it got such good highway mileage ... I wasn't expecting my Prius to do better on the highway ... I guess I just don't have the right city type roads, because I can't come near 50 MPG in my environment without a healthy dose of highway miles in the mix.

    REV
     
  4. Former Member 68813

    Former Member 68813 Senior Member

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    so, what is your result with 37.5 psi?

    FYI, my hwy driving gives me 57mpg@60mph, 52mpg@70mpg, 48mpg@75mpg, and 43@80mpg (indicated, real mpg is ~5% lower). All with tires (Ecopia) inflated to 40-42 psi.
     
  5. ghosteh

    ghosteh Member

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    After bumping up my pressure to 45/40, I agree. I didn't see any significant difference in mpg.

    A much bigger variable to my personal mpg is the wind direction and speed. At 65mph, I can see a HUGE difference in my mileage when the wind changes. As a result, I'm always praying for strong tailwinds!
     
  6. walter Lee

    walter Lee Hypermiling Padawan

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    I know I can get better fuel efficiency on my 2010 Prius III
    on the superhighway (speed 50mph to 70 mph) by dropping
    my tire pressure from 50psi front 48psi rear to 40psi front 38psi
    rear because I've done it before via one of my 600 mile trips
    from Maryland to Michigan. I know I can at least get atleast
    58 mpg (calculated) via 55mph-cc and 63 mpg (calculated)
    via DWL with the 40psi/38psi settings . So my test runs
    on MD32 of 55 mpg(est scangaugeII) via 55mph-cc and
    60 mpg(est scangaugeII) via DWL with 50psi/48psi
    setting has been dissappointing since I know that even
    my scangaugeII tends to be a bit optimistic by 1 or 2 mpg
    when compared to the calculated at the pump MPG
    results.

    My hypothesis is that increasing the air pressureabove the manufacturer's recommended tire pressures settings does decrease the tire's rolling resistant but that as aerodynamic drag increases the value of lowering a tire's rolling resistance to the overall energy efficiency of the vehicle decreases. That is to say there is a sliding scale of energy efficiency diminishing returns with respect to upping your tire pressure and a vehicle's top speed. My guess is that the manufacturer's recommended tire pressure represent the optimum air pressure for the best fuel efficiency at 65 to 75 mph but as your speed drops you can raise the air pressure to increase your fuel efficiency depending on the type of tire and road surface you are dealing with..

    This is rather inconvenient since I know I can get better fuel efficiency on my 2010 Prius III on my normal city-suburban commute(speeds 25mph to 45mph) by upping my tire pressure to 50psi front 48 psi rear.For example, yesterday the scangaugeII registered about 69 mpg (scangaugeII AVG est for 16 miles downhill +300ft) and and 65 mpg coming back (ScangaugeII AVG est 16 miles uphill +300ft) for my regular commute using 50/48 psi setting. Changing tire pressures just to do highway driving is inconvenient since I only have a manual air pump. I am now seriously thinking of getting a heavy duty 12VDC automotive electric air compressor.
     
  7. walter Lee

    walter Lee Hypermiling Padawan

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    I've only achieved +90 mpg a few times (to work - 16 miles, 60 minutes, +65F degrees, dropping 300 feet) and normally on the returning commute trip I am achieving a less stellar result of 78 to 83 mpg (from work ,16 miles, 60 minutes, +80F degrees, going up 300 feet). The Prius engine must be pre warmed to about 100F to make this happen -- I did ths by going on a 1 mile round trip to the grocery store before that (1 mile round trip, 5 minutes, 65F degrees, no elevation change, overall 25 mpg) about 5 minutes before starting my commute to work. The way I did this going home from work is that I parked my Prius in sun on a warm day and that kept the engine warmer than normal (but the cabin was really hot after work which was a problem since I also had to make sure to avoid getting the HV battery too hot at the same time). My elevation change is from 500ft to 300ft and then back up from 300ft to 500ft. The only time I drive higher than that is on my trips from MD to MI when I use the PA and Oh turnpikes. I've never used the MAP gauge before...while I have some skill -- I'm still a novice "hypermiler"
     
  8. B. Roberts

    B. Roberts Hypah Milah! Ayuh.

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    Interesting thread and posts. These are great MPG results for a Gen III. Stellar, even. I'm fairly new to the III and was not the main Gen II driver while we owned it 7 years. I did put serious seat time in it during long trips, and a fair dose of local 15 to 25 mile (one way distances) errand type drives.

    My best results driving the Gen II, with 2 aboard, in 60 to 65 degree temperatures and light or calm winds were on a couple of trips of 25 to 50 miles. On the way out I managed 60-61 MPG, and then on return about 55 MPG. We came back with a load of groceries and some other supplies but probably no more than 50 to 60 pounds total. Unfortunately, I don't live in an area where there are many flat road sections, although these hills are not severe or very long.

    Road trips in the Gen II were usually pegged in the 51 to 55 MPG category... depending on weather conditions and driving speeds. On one trip, I decided to keep up with the fastest Interstate traffic, 2 aboard and heavily loaded with luggage (all inside the car) and managed to lower the trip MPG to 47! But we're talking about maintaining speeds in the 75 to 85+ MPH for extended periods. That particular trip took place in mild dry Spring weather. MPGs all calculated at fill ups, not from display.

    My Gen II tire pressures were generally maintained as close to 42/40 PSI as possible. I live in an area where I can test the tire pressure changes, during cool weather, say 45 to 50 degrees ambient, and good cloud cover that keeps the road surfaces about the same temperature. Significant tire heat can still be generated under these conditions by the flexing of the tire components/materials as they rotate at steady 55 MPH for about an hour. I measured 46/43 PSI at the end of a drive in this kind of weather, after a start with 42/40 PSI. My accelerations are very moderate and I tend to drive as steadily as possible under all circumstances.

    It will be interesting to see how the Gen III performs on an upcoming long trip, compared to the Gen II's MPG abilities. I might toy with the pressures a bit, depending on weather conditions to see if I can duplicate some of the outcomes that we have read about in this set of posts. I carry a small tire inflator and an accurate pressure guage, so it will be easy to do.

    Fun stuff!
     
  9. Former Member 68813

    Former Member 68813 Senior Member

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    I researched the possibility of increased RR with higher inflation pressure and found this bicycle study:

    [​IMG]

    So, Walters point is correct, but it should not apply in a hardened road condition.

    Too bad the OP never came back with a follow up to this issue.
     
  10. Econ

    Econ Member

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    Now that is interesting. Measurable .. that is the key
    Seems to me that it would encompass a lot of ##/##### ths to gain even 1 MPG
    [FONT=&quot] I believe that the construction and design of most tires today, more so in Michelin tire, use a BAZ application that will prevent the tire from responding to a circumvenial increase by air pressure or speed.that would measure up to any significant measurable increase MPG[/FONT]
     
  11. walter Lee

    walter Lee Hypermiling Padawan

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    I've lowered my Yokohama Avid S33 tire pressure to 50 psi front and 48 psi rear to 38 psi front and 35 psi rear for an +400 miles interstate superhighway trip and I will take fuel efficiency readings at fixed speeds using cruise control (cc) using Prius MFD Trip computer and scangaugeII (AVG). Will test out DWL techniques. I've now got a high volume 12V air compressor to vary tire pressure(psi) against speed(mph) using cc. Will try to use similar road conditions/elevations when doing the comparisons or atleast document stretch of road segments - will post result in different thread.
     
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  12. Former Member 68813

    Former Member 68813 Senior Member

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    OP: any follow ups on this "interesting observation"?
     
  13. Former Member 68813

    Former Member 68813 Senior Member

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    No, I didn't think so. I guess that retest of 37.5PSI didn't work as planned.
     
  14. cinder

    cinder Member

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    +1 . Hopefully the air won't run out of this post . :clock: