Keys for city economy versus freeway economy

Discussion in 'Fuel Economy' started by some_other_dave, May 20, 2014.

  1. some_other_dave

    some_other_dave Well-Known Member

    I've noticed a few things with Fenimore, which also appeared to ring true with Mango Bango. After the "basics" are dealt with (tire pressures, looking ahead, anticipating traffic, accelerating moderately, etc.) there is really one thing that seems to determine my fuel economy on the freeway, and a different one that determines it in the city.

    In the city, my fuel economy is directly related to how much of the trip the engine is not running. Other stuff certainly helps, but the main variable in my case seems to be how long the engine isn't on. If I can cover a good part of the distance with no fuel burned, that is a big boost. Similarly, if I have to sit at a light for some reason and I cannot turn the engine off, the economy numbers suffer significantly.

    Out on the freeway, the major influence is how fast I go. The slower I go, the better my MPG is. Pulse and glide can help, but if the pulses go up over 60 MPH or so the average starts dropping off. If I limit my pulses to 55, things improve. Or I can set my cruise control (don't hate me 'cause I'm lazy! ;) ) to 55 and get pretty good mileage--over 50 MPG on some of my daily commutes!

    Spending time on the freeway with the engine off doesn't seem to help. This is likely because the higher drag on the car from higher speeds doesn't let you coast for long enough to make up for the extra fuel used on restarts--even with bump starts. And slowing down in the city doesn't really help, because most of the city speeds aren't getting you into the top gear anyway, and lower gears are less efficient as we all know.

    So, a couple of thoughts that have been bouncing around my head lately.

  2. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    Agreed on both counts. In my old car which took a long time to warm up, you could measure the effect by comparing mpg vs final water temp. There was a strong correlation between low final water temperature and high mpg. Less engine run time = lower coolant temperature = higher mpg.

    It's less obvious on the new car, because it has an aggressive warmup program. Most trips over 5 miles and not in winter get up to 180+ degrees and stay there.

    Highway? I try not to do that. :p
  3. phoebeisis

    phoebeisis Well-Known Member

    Yep-that is about it.
    My ancient huge 1998 GM SUV 226,500 MILES- gets 21+mpg at 62mph- but closer to 17 mpg at 70 mph-
    In the city-just shutting down at lights boosts me from 16mpg to 18 mpg-
    I generally motor on Pulse and glide-constantly-in city driving.
    Egg under throttle-old style pre hypermiling driving-gets just 13.5mpg
    Normal-but not excessive driving-gets 12mpg city-
    Common USA driver driving-gas brake-gas brake repeat repeat...but -not fast-gets 10mpg city

    Stay off BOTH pedals as much as possible-and that key-goes both ways.
  4. jcp123

    jcp123 Caliente!

    Engine off coasting is a huge help, and one reason my Echo doesn't live up to its potential (non flat towable automatic - not advisable to coast with the engine off in that one!). But on the tanks where I am shutting it down at lights, I notice at least a half mpg improvement at the cost of extra starter and battery wear.

    I'll agree on warmup too. Although the Echo warms up fairly quickly, I notice a massive jump in mpg if I run the same roads on a warm engine vs cold. This happens even though the IAC is gone, so the high idling isn't as significant a contributing factor as I would have thought. More friction plus likely a richer mix at cooler engine temps take a big toll. An engine block heater might help, or the ol trick of running errands all at once to keep the engine warm. I ran a grille block until the temperatures hit 80* around here and I noticed the cooling fan kicking on now and then. Winter is tough on warmups simply because the heater in full blast will keep the engine under 170*, so I run the fan at half speed.

    The only other thing I would add is that picking your route carefully is vital. I tend to avoid big hills, stop signs, and places where I know there will be traffic buildups. I also try to avoid as much of the chip seal road surface as I can. It's very common here and really tanks mpg, not to mention it's noisy and hard on your tires.

    On the highway I try to keep it simple... Constant throttle and engine on coasting when I can. It's naturally talented and gets me solid 44-47mpg numbers, typically.
    Last edited: May 20, 2014
  5. some_other_dave

    some_other_dave Well-Known Member

    Good point about warm-up, that also seems to play a role. Mostly on the freeway, though. I do tend to see higher numbers when I start with a warmed-up engine.

    But that's not something I have as much control over as the speed or engine running state.

  6. phoebeisis

    phoebeisis Well-Known Member

    Right-the 1998 Suburban-in say cold early Feb-
    morning trip 6:30am-10 miles 15.0mpg-maybe 38 degrees
    Way home-2:40 pm-car having sat in the sun for 8 hours-16mpg- maybe 52 degrees-but cab and under hood and of course engine-much warmer-

    My peak mpg is probably late May- 6:30 morning temp 70-afternoon maybe 88
    and I use AC both ways-but the city mpg is better than say March when I don't use the AC but the engine is much colder at start up-and takes MUCH longer to warm up.

    In fact the AC in the Suburban-which must be very "big"-doesn't make too much of a dent in the MPG-
    slightly under .1 gph difference at idle -
    but I usually try to time it to kick it on when I am lifting off or even expecting to actually BRAKE pedal brake-on a terminal glide pulse.
    Can't do that with wife in car of course-divorce court etc.
    I also never motor off pulse and glide-too cumbersome to drive with no power boost to steering and brakes-
  7. RedylC94

    RedylC94 Well-Known Member

    The biggest efficiency killers in "city" conditions:
    1) Avoidable braking,
    2) Idling,
    3) Low engine efficiency due to throttling loss at low loads (unless you pulse).

    The biggest at "freeway" speeds:
    1) Aero drag,
    2) Aero drag.
  8. EdwinTheMagnificent

    EdwinTheMagnificent Legend In His Mind

    I think you forgot :
    3) Aero drag
    4) excessive speed
    5) Aero drag.
  9. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

  10. RedylC94

    RedylC94 Well-Known Member

    But #4 is essentially the same as the others, when you can't realistically do much about the other factors that determine drag (air density, frontal area, drag coefficient, etc.)
  11. RedylC94

    RedylC94 Well-Known Member

    That one, choosing appropriate ratios, avoiding unnecessary idling, and several other common bits of advice all boil down to simply minimizing the number of revolutions of the engine. Any time an engine is turning with light load or no load, whether the rpm is high or low, it's wasting fuel.
  12. waltermlee

    waltermlee Well-Known Member

    For the Prius - I found that ( the big differences between getting high fuel efficiency in the city vs) getting high fuel efficiency on the freeway is:

    1. Speed matters - a Prius needs to be driven at lower top speed. e.g. 50 to 65 mph.

    2. Avoid regenerative braking As long as it is safe to do so - it is more energy efficient to go at a higher speed than to allow regenerative braking to slow down the Prius because regenerative braking is less energy efficient than the losing energy from aerodynamic drag due to the extra speed - especially at speeds under 65 mph.

    3. when using a Scangauge II - keeping the ICE running at 60% to 80% LOAD for longer time periods ( accelerating earlier) is more fuel efficient than allowing the ICE to turn off and accelerating harder later at a higher LOAD than 90% for any length of time. As the ICE approaches a LOAD higher than 90% it starts to be less energy efficient... not sure why though.
    Last edited: May 24, 2014
  13. 08EscapeHybrid

    08EscapeHybrid Moderator

    I've noticed that my FEH behaves in a very similar manner on the highway.
  14. jcp123

    jcp123 Caliente!

    I say the route is more important than anything. I have picked up 1.5-2mpg, or roughly 5% just using a different way to work and home. And as a side bonus it is faster for the same miles driven. It not only bypasses a rather wicked stoplight, it avoids some of the chip seal road, lets me glide more, and lets me catch the main highway my work sits on closer to a down hill stretch for easier acceleration.
  15. ALS

    ALS Super Moderator Staff Member

    When I run down to Naples I always get off at the first exit after crossing into Florida.
    It is route 200 that merges into 301 and I follow it to the Baldwin 301/I-10 interchange.

    Two lane, 55-60 mph, five miles shorter distance than taking I-95 to and around Jacksonville using the 295 bypass to I-10. The other upside on this stretch you almost never see any traffic on the road.

    Great road to pick up a few more MPGs with out loosing any time on my trip. :)
  16. 08EscapeHybrid

    08EscapeHybrid Moderator

    I'll have to try that. I drive down to Florida at least once or twice a year to visit my uncle in Daytona. I've never been pleased with Jacksonville traffic, and often get caught with an internal debate over which is best, 295 East, or 295 West. I've settled on East, but now I think you have a better route there. Plus, I'll be back on 95 in time to fillup with ethanol free at the Gate station in St. Augustine.
  17. ALS

    ALS Super Moderator Staff Member

    It wouldn't work for Daytona, this route only works great it your heading towards the west coast of the state.

    You come out on I-10 fourteen miles west of I-95.
  18. some_other_dave

    some_other_dave Well-Known Member

    Another significant effect on freeway mileage:
    -Running the air conditioning. With temps ranging up to 110F on the way down and 105F on the way back, I had to run it. That was on the order of a 10% hit in steady-state cruising MPG.

    BTW, I ran the speed/economy tradeoff in the other direction for this trip to L.A. and back. The trip would have been longer than I liked if I were to stick to 50 or even 55 MPH. So I ran my speed up to 60 MPH for much of the I-5 cruise. Even though the limit for trucks and anyone towing anything is 55, I was still passed by more trucks than I passed.

    With three days of commute, the trip to L.A., and part of the trip back, I got 44 MPG. Not too bad. I stopped pretty regularly (about every 90-120 minutes) on each of the long trips, which meant I had to get back up to speed several times. And of course, the Grapevine put a hurt on the average. ;)

    Not too bad, though!!

  19. PaleMelanesian

    PaleMelanesian Beat the System Staff Member

    I tested my AC to be 8% at 60 mph. Not far off your number.
  20. ALS

    ALS Super Moderator Staff Member

    1. Aero drag
    2. Mechanical drag, drive train.
    3. Road friction on the tires.

    Slower the speed the greater the savings from all of the above.

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