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New Concrete Highway Means BIG MPG Boost!

Discussion in 'Gen 2 Prius Fuel Economy' started by a priori, Aug 6, 2008.

  1. a priori

    a priori Canonus Curiosus

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    I have been a big fan of state and county highways and roads for many reasons. I've driven interstates so often that I am sick of seeing the same thing everywhere I go -- the roads all look the same and the exits all have the same truck stops, fast food restaurants, etc. State roads take you through towns and you get to see and interact with real people.

    An extra benefit of traveling these roads (as opposed to interstates) is that mileage tends to be better. Much of this is simply from driving a bit slower, of course, and this is another great draw for me. I've not been able to get a tank of gas above 60 MPG by traveling on the interstate - until now!!

    For those of you who live in or travel around Chicago, you may have experienced the same thing of late as I've just found. When driving on I-294 (southern suburbs section) and I-80/94 into and through NW Indiana, I've seen VERY high mileage lately. When driving on the brand new concrete surfaces, I've seen steady, regular MPGs of about 70 when traveling 55 and 65MPG at about 60. My most recent trip from the western suburbs around Chicago and into Northern Indiana gave me a 200+ mile stretch of better than 65 MPG! I actually saw a drop in mileage when I exited and drove U.S. 30 and U.S. 35, even when I was driving steadily in the 45 to 55 mph range.

    Has anyone else witnessed this and recorded the difference road surfaces make to your MPGs?
     
  2. Sheepdog

    Sheepdog C'Mere Sheepie!

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    Rolling Resistance I guess. The wind here seems to kill any attempts at my breaking 55 mpg tho.
     
  3. a priori

    a priori Canonus Curiosus

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    I've assumed it is rolling resistance as you suggest. I am just surprised at the significant increase in MPGs at highway speeds.

    Wind resistance? It could be that you are still breaking in that great little car. You also may not be driving distances or for long periods of time. All of my high-mileage tanks have come when I've been on the road for long stretches of time (very few 5 or 10 minute drives and many 60 or 90 minute drives). Of course, you ARE in Florida, so it is hard to believe you are driving without A/C. I think the combo of a new car, A/C and short(er) trips would make it very difficult to break 50. And you are doing it!
     
  4. pdoege

    pdoege New Member

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    The different road surfaces here have different elevations, wind patterns, etc. Makes it hard to compare.

    I work just outside of Sanford FL, and I've been getting 50MPG tanks.

    Regards,
    Peter
     
  5. carz89

    carz89 I study nuclear science...

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    Went on a long trip recently (San Diego-Monterey) and took advantage of the opportunity to record my MPG over numerous stretches of highway at various speeds (using cruise control), flat terrain, 95-degree outside temperature, 100% sunny, AC on. In summary, on I-5 northbound (north of the mountains north of LA), I averaged 60MPG at 65MPH for 60 miles. When I got off the interstate, travelling SR 46 (Paso Robles Hwy), I dropped down to 55 mph and averaged 55 MPG for a good 30-40 miles. As far as I could tell, all conditions were the same except for the average speed and the road surface. Wind was negligible. I checked that there was minimal elevation gain on my GPS.

    Not being a "road surface expert", as far as I could tell, the interstate was concrete, and the SR-46 was some sort of cemented gravel (not smooth asphalt, not concrete).

    The results were a bit shocking. I was expecting my MPG to go up 5 to 10MPG after slowing down from 65 to 55mph, but just the opposite occurred. The only explanation I can think of was due to the difference in road surfaces, although a 10-15mpg difference from the expected mpg seems excessive to attribute to the road surface.
     
  6. Tripod137

    Tripod137 New Member

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    I have found, as have others on the forum, that there seem to be speed (MPH) sweet spots in the Prius that seem to depend on the temperature and maybe humidity. I have noticed that at certain temp & speed combinations, my MPG shoots way up to the 60's and 70's. 1 MPH more or less and the MPGs go down again. The trick is to find the right speed for the current temperature.

    Give it another try on those roads and drop/raise your MPH and see what the MPG does.
     
  7. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    never noticed any change in mpg's but i did notice the HUGE reduction in road noise on concrete...unfortunately, concrete is so expensive that its not used much around here.

    it is nice to drive on though... the lack of noise is eerily pleasant.
     
  8. Marlin

    Marlin New Member

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    I don't know about that. Every concrete highway I've every driven on feels and sounds like a horse galloping. Thump-thump... thump-thump...thump-thump...
     
  9. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    that is true, but better than a constant hum i think... plus, i have also noticed that the expansion joints dont always make a lot of noise... sometimes, its just the slight bump is all that i notice even with modest stereo volume
     
  10. Celtic Blue

    Celtic Blue New Member

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    A well made/well maintained concrete road can be silent. I drove over one a few weeks ago that had recently had work done to remove unevenness and it was one of the quietest roads I've ever been on. It was far from new looking but the patch work was incredibly level. IIRC correctly only the right lane had been finished, making the "truck lane" the preferable one to drive on, usually the passing lane is smoothest.

    "Thump-thump" roads make me think of Louisiana and Arkansas where highway maintenance has often been atrocious.
     
  11. carz89

    carz89 I study nuclear science...

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    I would beg to differ about the noise.

    I have found, without exception, that the asphalt highways in SoCal are significantly quieter than the concrete highways. And it doesn't seem to matter much what car I'm driving. Asphalt and concrete seem to be equally interspersed all around the region, so there's ample oppurtunity to sample each flavor.

    Each time I transition from asphalt to concrete, I cringe due to the extra noise and reluctantly turn up the volume on my radio just to achieve a similar level of SNR.
     
  12. Bob64

    Bob64 Sapphire of the Blue Sky

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    according to cummins truck mpg secrets or whatever its called... concrete has one of the SMALLEST rolling resistances out there.
     
  13. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    all these posts makes me thinks, all concrete is not created equally
     
  14. a priori

    a priori Canonus Curiosus

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    Even if all concrete were to be created equally, it certainly isn't laid equally. A bit of the performance of the roadway is derived from the handling and laying down of the concrete. A larger piece probably comes from the subsurface preparation, but the handling (or over-handling) is also a significant factor.
     
  15. PaulHS

    PaulHS Member

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    One more factor to consider which applies to both asphalt and concrete: the age of the road surface. From recent experience (nearly all the roads in the county have had sections resurfaced) a newly resurfaced asphalt road is much smoother and quieter than six year old concrete.
     
  16. b2j2

    b2j2 Member

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    If there is a regular "thump-thump" it's probably an old Portland cement concrete (pcc) road (even with an asphalt concrete overlay that has been there a while), with regularly spaced formed transverse joints. These were typically reinforced with relatively short steel rods intended to prevent vertical offsets in the surface. Short wheel bases (think tandem semis) resonate and increase effective loading on the edges of each slab at the transverse joints, with progressive deflection and failure. Vehicle speeds cause impact loading when there are surface irregularities. Improvements to that scheme include skewed sawcut joints at random spacing and continuous reinforcing. One reason for joints is to allow for temperature shrinkage of pcc --pcc is much better in compression than in tension. The latest is to post-tension slabs, permitting much longer segments, eliminating most transverse joints.

    As between pcc and asphaltic concrete (ac): Pcc is deliberately made rough in finishing (in the old days by "transverse brooming" and now by dragging a cylinder on the surface before it sets up. If the surface is thought too smooth for traction and/or resistance to hydroplaning you might see longitudinal saw cuts or grinding (also used to bring the profile into spec.). Pcc is hard, so think high pressure in your tires. There is the concept of "concrete wheels on rubber roads". Old tires are being recycled into a component of ac, with a quieter (for passengers and neighbors) and smoother ride. A new ac overlay on existing pavement will likely be much smoother than pcc because it can be laid in much longer stretches between construction joints and underlying irregularities will be (at least temporarily) patched over. Some jobs put down a structural plastic mesh which somewhat distributes longitudinal stresses.

    Costs of both materials are influenced by the oil market--asphalt is petroleum, and portland cement is the result of high temperature generated by one fuel or another (often natural gas). Both pcc and ac are recycled in one way or another. Ac pavement can be ground down and reused for base material or even as part of a new mix. Both material require good aggregate and good base material, not always handy. All the materials are heavy (finished weight about 150 pounds per cubic foot). In some areas one material or another may have a significant cost advantage. More thought is now being given to "life-cycle" costs with longer life a key factor. (Many components of the Interstate system were based on 50-year life-spans, and Ike was President when?)

    Structural deterioration is mostly related to loading cycles and then mostly as a function of vehicle weight. Light vehicle traffic is good for keeping weeds from growing in the cracks, while maximum weight or overloaded trucks will have serious effects over time.