Prius milage - why higher in city?

Discussion in 'Gen 2 Prius Technical Discussion' started by daveleeprius, Sep 25, 2007.

  1. daveleeprius

    daveleeprius Heh heh heh you think so?

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    I've owned our Prius since last October, and I've never observed the milage to go up in the "city" verses driving on the highway. I have observed milage on the freeway, traveling 65-70mph between 50-80mpg, while in the city milage is between 12-45mpg. I've never recorded higher milage in the city than on the highway and wonder how they came up with the idea that the car gets better milage in the city?

    If it is because the engine shuts off when the car is coasting or at a stop, I would argue that a normal engine at idle uses very little fuel and this shouldn't push the curve much at all mpg wise, but this does make the car a lot more green.

    In the city, you have a lot of stops and starts, which is the most inefficient type of driving. Driving at a steady speed on the highway is a lot more efficient with my experience, and my data with the Prius has proven that.

    So I guess what I want to know, is who here on the board has data that shows consistantly higher milage in the city vs. on the highway?

    Thanks
     
  2. efusco

    efusco Moderator Emeritus
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    This isn't the easiest question to answer in a few simple bullet points.
    First, recall that the EPA numbers were based on some very specific settings...very unrealistic settings as well.

    Next, a "city" can be many different things. In stop and start traffic every few blocks for many miles in heavy congestion mpg will suck...no doubt about it. In a 'city' like where I live it's often 1/2-2 miles b/w lights that I need to stop at, I can time my traffic signals and often glide as much as half the distance I actually travel burning zero fuel.

    Likewise, the highway test for the EPA was pretty silly compared to real world (note that both the updated city and highway test cycles are much more realistic, I'm talking about the old numbers for the sake of emphesis). Someone going a nice steady 60mph on a flat road in good weather will blow away the EPA estimates.

    So yea, my "rural city" I often get 75+mpg on my usual commute, but have gotten some pretty crumby 45-50mpg tanks under less ideal circumstances as well.
     
  3. David Beale

    David Beale Senior Member

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    I'd have to do a long term study to give concrete results, but Pearl gets around 47 MPG on the highway so far and closer to 54 in city only driving. It's improving in both as she breaks in. Both will tank with winter here in a few weeks.
     
  4. subarutoo

    subarutoo New Member

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    I do almost no city driving, 99% freeway, and my overall mileage is now at 51.5. On a recent traffic crawl across Las Vegas, I was able to max out the yellow bars on the MFD, though. Kinda surprising.
     
  5. daveleeprius

    daveleeprius Heh heh heh you think so?

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    Well to me "city" driving means a lot of traffic lights, starts and stops. No way will this kind of driving ever get better milage than sustained driving at speed.

    If you mean "rural city" where there is a lot of sustained driving at lower speeds, like 35-50mph, then yes this would get better milage than sustained driving at speeds of 65-70mph.

    I've also noticed in city driving, where there are a lot of lights, etc, that the battery usually goes down quite a bit, which would also lower milage as it would tax the ICE more and keep it on more to charge the battery.

    So not to diss the Toyota engineers, who I say a silent prayer of thanks to every time I get into the car, I say no way does the Prius get better milage in the "city" vs. the highway/freeway and would welcome anyone to show concrete data proving that this is in fact the truth.
     
  6. Tideland Prius

    Tideland Prius Moderator of the North
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    Most of my driving is city (> 95%). The rest is highway and highway speeds (60mph and above) kill my mileage.

    Don't forget that city driving usually means hilly areas in addition to stop and go. The ideal "city" would've been flat and with long blocks and moderate traffic. This allows you to glide more often. Also, crawling in traffic gives you an advantage of not idling while you creep along.
     
  7. Trevor

    Trevor Member

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    <div class='quotetop'>QUOTE(DaveLeePrius @ Sep 25 2007, 03:36 PM) [snapback]517527[/snapback]</div>
    The following is from the EPA web site.
    The city test is approximately 11 miles long and simulates a stop and go trip with an average speed of about 20 miles per hour (mph). The trip lasts 31 minutes and has 23 stops. About 18 percent of the time is spent idling (as in waiting for traffic lights). A short freeway driving segment is included in the test. The engine is initially started after being parked overnight.

    The highway test simulates a 10 mile trip with an average speed of 48 mph. The vehicle is started "hot" and there is very little idling and no stops.


    If you mean "rural city" where there is a lot of sustained driving at lower speeds, like 35-50mph, then yes this would get better milage than sustained driving at speeds of 65-70mph.

    <div class='quotetop'>QUOTE(DaveLeePrius @ Sep 25 2007, 03:36 PM) [snapback]517527[/snapback]</div>
    The Toyota engineers didn't say that the Prius gets better gas mileage. The EPA did.

    I find that when I'm driving around NYC my "city" MPG is not that great, but if I'm on road trip and hit a less populated city where the speed limit drops to 20 and traffic lights are at least half a mile apart then my MPG shoots through the roof.

    The EPA are changed the testing methods for the 2008 model year so I think the Prius now says 48 city, 45 highway.
     
  8. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(DaveLeePrius @ Sep 25 2007, 02:36 PM) [snapback]517527[/snapback]</div>
    It's not the Toyota Engineers that say that the Prius gets better milage in the "City" than on the "Highway".

    Our government has an agency, the Environmental Protection Agency (EPA), that tests all models under identical conditions and then reports the number of miles per gallon they determine the model can travel under the test conditions.

    If you drive your car in an environment exactly the same as the EPA's test conditions, and drive the car in exactly the same way, you would get the MPG numbers reported by the EPA. The fact that your particular city and your particular driving style doesn't exactly match the conditions that our government uses for their test explains why your milage may vary (YMMV). Personally, I find that when I drive on the expressway, I get between 50 and 60 mpg. In the occasional instance when I get to drive in the city, I get between 60 and 70 mpg. I frequently get nearly 70 mpg as I drive from the south side of Chicago to the north side on Lake Shore Drive.

    What kind of "concrete data" are you looking for? If the EPA tests don't satisfy you, what would?
     
  9. daveleeprius

    daveleeprius Heh heh heh you think so?

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    <div class='quotetop'>QUOTE(Tideland Prius @ Sep 25 2007, 03:15 PM) [snapback]517553[/snapback]</div>
    But still I notice the car gets very low milage at slow speeds and moderate gas pedal, around 12-30mpg. This still doesn't beat the 58mpg I got on the freeway.

    On the highway I use cruise control 90% of the time, and the car is amazingly efficient when it is at cruise. I drove from Bellingham to Bellevue and got 58mpg. From the exit to home, about three miles, the milage dropped to 52mpg because I have to climb some hills.

    Overall that tank got 50mpg.
     
  10. Tideland Prius

    Tideland Prius Moderator of the North
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    yeah.. what did I say? Hills lol.

    Your best bet is to talk with the other Seattle area owners (daveinoly is one) and ask them about their mileage.
     
  11. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(Tideland Prius @ Sep 25 2007, 04:05 PM) [snapback]517579[/snapback]</div>
    Unless I'm misunderstanding the original posters comments, he isn't unhappy with the mileage he's getting, and he doesn't want to know why his mileage is less in the city, nor is he looking for verification from other Seattle area owners that his mileage is appropriate.

    The comments seem to indicate wanting to know if the car can actually get better mileage in the "city" than the "highway" as he defines it, or else confirmation of his belief that it can't. Furthermore, there seems to be a bit of confusion on his part as to where the "city" and "highway" mpg numbers come from with a tendency to blame Toyota for the results of our government's testing.
     
  12. douglas001001

    douglas001001 smug doug

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    <div class='quotetop'>QUOTE(DaveLeePrius @ Sep 25 2007, 02:36 PM) [snapback]517527[/snapback]</div>
    Have you tried to find a route with a speed limit at or below 40 and done multiple tanks on this route practicing the techniques suggested on this site?
     
  13. IsrAmeriPrius

    IsrAmeriPrius Progressive Member

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    <div class='quotetop'>QUOTE(Trevor @ Sep 25 2007, 01:45 PM) [snapback]517566[/snapback]</div>
    Here is what Csaba Csere had to say about the EPA tests in Car and Driver:

     
  14. Danny Hamilton

    Danny Hamilton Active Member

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    <div class='quotetop'>QUOTE(douglas001001 @ Sep 25 2007, 04:33 PM) [snapback]517598[/snapback]</div>
    And how exactly would this satisfy his query as to whether the Prius can get better "city" mpg than "highway" as he defines the terms? The driving you are describing sounds more like "rural city" as he defines it.
     
  15. efusco

    efusco Moderator Emeritus
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    <div class='quotetop'>QUOTE(DaveLeePrius @ Sep 25 2007, 02:36 PM) [snapback]517527[/snapback]</div>
    Showing you proof is very easy...or impossible...depending upon how you define "city", I tried to explain that in my first post, apparently w/o much success. As the EPA defines a city both before and after then new standards clearly the Prius does do better in the City. It may not do so in YOUR city...but I still bet you many of us could get better MPG in your city than on your highway...technique is a huge factor.



    The reason the battery gets lower in the city is b/c the ICE is NOT running...while it may tax it slightly more when the SOC gets low the overall MPG benefit is decidely in favor of better FE than were it to idle and keep the SOC high. The whole point of the battery is to act as a buffer to power the car when the ICE is not on and to store excess power when it is.
     
  16. bwilson4web

    bwilson4web i3 and Prime

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    <div class='quotetop'>QUOTE(DaveLeePrius @ Sep 25 2007, 02:36 PM) [snapback]517527[/snapback]</div>
    Early on, I found that there is one speed range in the city that is pretty bad:
    [​IMG]
    The critical speed is 42 mph, which I later learned is the boundary between two Prius control laws:
    • <42 mph - hybrid operation with mixed ICE and electric operation, VERY efficient
    • >42 mph - always has gas engine running even if gas power is not needed
    The problem is if you transit 42 mph, you can often trigger either engine start or stop and this takes energy. But if you keep a guard-band so you only transit 42 mph as infrequently as possible, your mileage goes up. The key is to keep at least 4 mph below 42 mph or 4 mph above 42 mph:
    1. 0-38 mph - very efficient city speed, slower gets better and better
    2. 46-68 mph - efficient engine powered mode if you have at least 1 mile of travel above 42 mph
    If you are on a street posted at 40 mph, use 38 mph in the right lane and have a happy. If you are on a street posted at 45 mph and the traffic supports that speed, use 46 mph and have a happy. But if the street is posted at 45 mph but the traffic keeps you cycling through 42 mph, get in the right lane or behind the slowest truck and hold to 38 mph. If a gap develops, be courteous and let them in. Better still, take the next exit and take a slower speed route so no one cares.

    If you avoid TRANSITING 42 mph, your city mileage should improve. Go above it only when you know you'll have at least a mile of +42 mph speed and the control law boundary won't bite you in the tank.

    GOOD LUCK!
    Bob Wilson

    ps. I forgot to mention that I drive as much as possible using Cruise Control and use the accelerator only to get back up to speed or deal with traffic. Cruise control ensures smooth power transfers.

    pps. Also, a lot of city driving includes dealing with short trips and warm-up problems. My commute is about 20 minutes, which is as short as you want it to be. If you have a fairly short commute, you may want to consider an engine block heater or changing your first mile to be at 25 mph through the neighborhood instead of going to a higher speed, cross-town freeway. Our Prius like to be well warmed before they start sipping gas.
     
  17. madler

    madler Member

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    The simple physics answer is that if you have a vehicle that uses just the energy it takes to accelerate and maintain speed against rolling friction, air resistance, and elevation change, and recovers all of the energy from braking and elevation change, and you end at the same elevation you started, then you will get lower energy usage per mile at low (city) speeds, e.g. 35 mph and higher energy usage per mile at high (highway) speeds, e.g. 65 mph.

    The reason is that it takes more power to maintain higher speeds than lower ones, due to increased friction and drag at high speeds. Especially the drag, since for that component the expended energy per mile to fight the drag goes as the speed squared. Stop and go is free if you can recover all of the kinetic energy of the vehicle.

    So a perfect vehicle should always get better mileage in the city than on the highway (assuming that highway is faster than city).

    The Prius is not perfect in this regard, but it does address the key areas, which is not running the engine when it's not needed, and recovering at least some of the energy from braking and going downhill. It goes far enough that it is possible, even with stop and go conditions, to get better mileage in the city than on the highway. It will do particularly well if you can always brake gently to maximize the regeneration.

    In practice, I seem to always get the danged red light at the bottom of the hill, so a lot of energy gets thrown away when the brake pads kick in. There are lots of hills where I live -- my main street is called "Foothill" -- so I end up getting lower round trip mileage in the city than on the highway. ("Round trip" means ending at the starting elevation.)
     
  18. jiepsie

    jiepsie New Member

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    <div class='quotetop'>QUOTE(madler @ Sep 27 2007, 09:48 AM) [snapback]518367[/snapback]</div>
    This is the best explanation I've seen so far. It's 'simple' physics! Go faster, burn more. It just shows how inefficient regular cars are at slow speeds and in stop and go traffic, they completely negate the advantage having less drag because of their inefficient brakes and engines.
     
  19. daveleeprius

    daveleeprius Heh heh heh you think so?

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    <div class='quotetop'>QUOTE(madler @ Sep 27 2007, 02:48 AM) [snapback]518367[/snapback]</div>
    I also live in an extremely hilly area, so my milage is always better on the freeway than it is in the city. I envy people who live in flat areas and can get 70mpg on their commute.

    I have observed that my old 1988 Civic sedan, as well as our 1991 Corolla and my former 1991 Civic wagon, all get 40mpg on the trip from our home to Mt.Ranier and back. Because the route I take the speeds are usually between 40-50mph. I agree, 48mph is the optimum speed for most cars to get the best mpg.
     
  20. usbseawolf2000

    usbseawolf2000 HSD PhD

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    Simple. It is a matter of avoiding energy losses. In the city, you can recover the energy with the regen brakes. On the highway, you can not recover energy from the wind resistence.

    Ideal situation would be to go slow without the need to brake. 30-40 mph is where you can achieve 100+ MPG with P&G technique.
     
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