Prius milage reversal: better hwy mileage

Discussion in 'Gen 2 Prius Fuel Economy' started by THF, Mar 6, 2013.

  1. THF

    THF Junior Member

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    We have a 2004 prius. (I'm assuming it's a v, because it's relatively large.) I'm rather intrigued by the fact that it is getting lower milage in town than on the highway.
    We are in MO ("We drive SUVs"), and the terrain is more up-and-down than flat. Frequently, when in stop-and-go traffic, the car if often accelerated uphills. Only sometimes, however. I'm getting 52 MPG hwy and only about 40 in town, for 46.5 average. This is still a good average, but I am baffled by the reversal.
     
  2. fuzzy1

    fuzzy1 Senior Member

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    There is far far more to MPG than just the EPA numbers on the window sticker, which are computed and de-rated from some lab tests with a fixed driving profile that represents almost no one's actual use. Real world results vary enormously by specific trip profile, driver style, weather, and other factors.

    Prius has lower Hiway than City MPG ratings because the EPA formulas had different fudge factors, marking down the highway number much more than the city number. If the same fudge factor was used for both, Highway would still be higher than City. Don't worry about it.

    If the original formula from the early 1980s was used, without fudge factors, both your EPA ratings would be in the 60s, and those us with Gen3s would see ratings above 70 MPG. But public outcry about unrealistic ratings prompted the EPA to lower the scale decades ago, and again in 2008.

    Are your city trips relatively short? If so, engine warmup likely keeps MPG low. Your driving style might also be causing low MPG, especially if you are relatively new to hybrids. Stop-and-go can often be finessed to improve MPG if done just right, but having to include accelerating up hills may make it impossible.

    See also: Fuel economy complaints/queries? Please copy, paste & answer these questions, esp. if you're new
     
  3. HTMLSpinnr

    HTMLSpinnr Moderator
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    ** Moved to 2nd gen forum **

    Hills do a number on fuel economy. 40mpg is still great compared to any other car around.
     
  4. cwerdna

    cwerdna Senior Member

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  5. uart

    uart Senior Member

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    Highway versus city MPG is an interplay between lower speeds (which is good for MPG) and more stop starts (which is bad for MPG). Most normal cars suffer so much fuel consumption penalty from stop start driving that they always get worse fuel economy (FE) in city driving despite the lower speed. With the Prius being so much more efficient in stop-start situations however, it means that the situation can go either way.

    Typically to get really good "around town" FE you need to at least have a moderate amount of opportunities to "glide" on relatively flat or fairly slight downhill terrain. I can never get very good FE when it's all up and down and relatively steep either.
     
  6. nai1ed

    nai1ed 2006 Prius

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    I experience the same thing here in Maine. Roads here are worse than those in third world countries. We have lots of hills, curves, and cold weather. Lots of one-ways and dead-end streets, and traffic lights that don't make sense. It's a big cluster fk in the city. Back roads is like a Safari, which I had concerns about my Prius breaking in half some day. I drive slower than most, but I do notice worse millage in the city. My average over 200 miles is 42 MPG.
     
  7. xs650

    xs650 Senior Member

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    That is likely the answer, IIRC, the official city mileage ratings are based on about 30 minute driving cycles.
     
  8. THF

    THF Junior Member

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    Our average trip into town is 27 miles of up-and-down rolling hills into town, which is then still rolling up and down combined with stop-and-go.
    Most are not too bad, but for the most part it's all crummy highway. If there's no one behind me, and I don't have to be anywhere really quickly, I can drive like an old lady (no offense to any old ladies on here) and get 52 MPG, which I think it pretty good. (45-50 MPH.) Last time I filled up (I keep a log book), it was 350 miles with 46 MPG average. Currently I'm at 45 average at 215 miles.
    Do the tires have anything to do with milage? They are a little worn, not too bad, but they do not appear to be low rolling resistance tires and they do not match the numbers or PSI on the door sticker.
     
  9. fuzzy1

    fuzzy1 Senior Member

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    Yes, tires have something to do with MPG. Worn tires will actually help. Most of this is real (less stiff and sticky), a small portion is imaginary (worn tires have smaller diameter, so odometer rolls up slightly fast, but this effect is smaller than you'd expect from ordinary geometry).

    The PSI numbers on the tire sidewall and door sticker will never match. The sidewall numbers are a maximum tire rating, the door sticker is a car maker recommendation (including ride comfort and suspension tuning) that many of us take as a minimum.

    The size numbers on the tire and door sticker ought to match if you are concerned about MPG. Some owners will up-size tires for others reasons (handling, performance, appearance, etc.) that are perfectly legitimate, but this usually cuts MPG (both real mpg, and possibly apparent mpg if different diameter throws off the odometer).
     
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  10. bisco

    bisco cookie crumbler

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    how many miles on her? you'll get better mpg's when the weather is nice.
     
  11. THF

    THF Junior Member

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    Thanks! I'm learning a lot on here.
    It was a very nice day today, and I was getting about 51 MPG. The other day, however, it was cold and I drove a little faster than normal and only got about 38, even though the previous evening I'd gotten 53.4.
    So, should I air the tires up? They are at 21 PSI, and the sticker says 35 PSI.

    ETA: There are 165k miles on this 2004.
     
  12. nai1ed

    nai1ed 2006 Prius

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    At 21 PSI, I'm surprised the Tire Warning Light hasn't come on. Mine comes on when it hits below 30 PSI.
     
  13. fuzzy1

    fuzzy1 Senior Member

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    :eek: Never mind MPG, air them up immediately for the sake of safety and tire life.

    And check with some other pressure gauges, in case the one you used is bad.
     
  14. jdenenberg

    jdenenberg EE Professor

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    2004 Prii didn't come with PSI monitoring.

    JeffD
     
  15. uart

    uart Senior Member

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    Yes 21 psi is way too low. Both MPG and handling will improve a lot if they're at the correct pressure. Also the tires will wear much more evenly, you'll scrub off the tread from the tire edges very quickly cornering at such a low pressure.

    You can even go a little bit over 35 if you want. A lot of people here run about 38 to 42 psi. Handling is very good at these pressures, but ride a little harsher.
     
  16. cwerdna

    cwerdna Senior Member

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    21 psi is dangerously low.

    As for when a Gen 2 Prius equipped w/TPMS might warn you, well, it depends on the pressures they were set at. It only warns if it's a certain percentage (IIRC) below the point at which it was reset. If you reset it at 20 psi... well, it sure won't warn you at 21.

    As for the OP, make sure that the tires pressures are checked cold, after the car has sat MANY hours after it's been driven or, better yet, after it sat overnight. If it were at 21 psi hot, they are REALY REALLY low.
     
  17. THF

    THF Junior Member

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    Thanks for the warning everyone. I find it odd that they were all exactly that low, however. But I aired them to 35 psi and they're holding. Perhaps I'll get better mileage now!
     
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