Take it to the limit: On speed limits

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  Posted by ken in Concord Sat Apr 11, 2009 9:45am PDT

  OK, I'm going to bring up the point that every article I've EVER read about speed limits DOES NOT include in their article. The last paragraph of this article says that increasing your speed from 55 to 75 MPH will decrease your fuel efficiency by 20%. That's fair, and I will not argue that fact. It is physics. What they are leaving out of that equation though is that increasing your speed from 55 to 75 mph is also decreasing your travel time by 36.36% (20mph increase of speed vs original speed of 55mph). So you have your vehicle running and burning fuel 36.36% less time. So doing the math, if you're burning 20% more fuel while you are driving, but doing it for 36% less time you're actually using less fuel overall to get to your destination at the higher speed limit. Just my $.02

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  Posted by Andi G Sat Apr 11, 2009 10:07am PDT

  Good point K.holt, accept you for got one thing: Fuel efficiency is measured in Miles per gallon not Gallons per Hours. If you took at trip from point A to point B you would go 100% of the distance. If you took the same trip from point A to point B 36.36% faster but burned more fuel, you would still travel 100% of the distance.

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  Posted by the dude Sat Apr 11, 2009 6:54pm PDT

  because it is LAW!!!!!

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  Posted by Michael Wed Apr 22, 2009 6:39am PDT

  It because it Earth day each year of April 22th and that why I read the story what happening throught the recycling by green day and why that I was working through Wal-mart for 9th year of 2009 for long time and making money and get paid each 2 week savings as banking...And that time I was working as overnight as the stockers assiciate as use to do the box cutter and the plastics for trash but it not for trash it use to put it by the cardboard bale the greenbale for the green recycling bin only for truck to pick up. it easlier for them to work on for green earth day and recycling company to refresh to use it. What what my Employee At wal-mart Do for recycling Do to helping for the recycling for the earth day to help for the company to pick up as truck and the cardboard to make the cardborad bale pale... That what I work for every night and the daytime for alway we do for... that (RECYCLING REASON) 2) SPEED LIMIT is what we do is the magic is at is alway we do what we have to drive is limit that law is 55 to 75 MPH but it the law is 55 mph it in the cities in Orlando FL when we drive to work at wal-mart and to home at shortcut because it saving alot more and more gas than highway I know it more faster because sometime it take her most time gas burned beasuse at night when we go to to work at wal-mart it was so traffic and it was kill her car gas and her money to filled it up her gas money and we really want to saving our money and we did got it sucessful into the next 3th week into the next check onto April 29th 2009... but my mom want to go at the shortcut at the lights that me and my mom we saw alot police catch people alot ticket to person not watching the speeding limit and the addident the light triffic and the stop sign pass over hit each other that why people have to watch what we go to and insurance will increace high paid that not going to be easy for us to do this way so be careful and follow the law and trust on the green day earth as the beauiful day follow the speed limit and the laws alway the times and avoid speeding tickets... the REASON SPEED LIMITS GAS SAVINGS/THE MONEYAlso Our house We do help about the house water bills the the problem the how we need to help to saving the money to save the water and if we need to take the bathe and wash the dishes and wash the hand with soap that dont took too long no but take it to quick fast it will run and save it as long as save the water and as save the money for the earth/green day for our home for our house as our blessing by Michael by call me Miguel Michael_scarcelli@yahoo.com

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  Posted by j l Wed Apr 22, 2009 7:28am PDT

  um yeah, a comma or two would go a long way in the post above.

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  Posted by gmaynard_13 Wed Apr 22, 2009 7:54am PDT

  no wonder you work at walmart! Door greeter?

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  Posted by Junda C Wed Apr 22, 2009 9:12am PDT

  so why is this "magic number" at 55mph and not lower?

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  Posted by Ricus Wed Apr 22, 2009 9:28am PDT

  Every 4 cylinder car I have had for more than a decade, did not get the best gas mileage at 55MPH as the article suggests. The best mileage I have gotten is at 65MPH. I would agree that 55 is safer, however it false that my 4 cylinder engines do best at 55. I bet the 6 and 8 cylinders get their best at 55, but I never tested them.

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  Posted by Michael Wed Apr 22, 2009 10:01am PDT

  I agree with Ricus ... I get annoyed with this 55mph number because when I am stuck driving that speed I notice I get 1-2mpg LESS than I do at around 65mph. (Not to mention the frustrating boredom at 55mph that saps one's concentration.) I also have a 4-cylinder car (with a 6-speed manual), and the sweet spot seems to be around 65-70mph, if I drive that I will usually exceed the highway mpg that my car is rated at. Also, if one is driving among traffic, drag is reduced so the speed can be higher. The best I've ever gotten was following an empty flatbed 18-wheeler at 65mph - my fuel economy jumped 7mpg above the highway rating!

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  Posted by justin g Wed Apr 22, 2009 10:12am PDT

  55 is the magic number because of gear ratios in your car. Most automatic cars have 4 gears so your best gas mileage comes when the least amount of work is required to turn the gears. plus the work to speed ratio plays into effect. bigger gears means less work at faster speeds. check your rpm gauge higher it goes the more gas you burn. for each gear you have an optimal gas consumption this is where speed plays. going 30 in 1st gear burns alot of gas but going 30 in 2nd gear burns less. but your transmission cant handle 30 in 4th gear.

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  Posted by david a Wed Apr 22, 2009 10:33am PDT

  Engines run more efficiently at higher RPM. This counteracts the increased drag. Also cars need to go at least 50 mph to use the top gear. The magic number for my 90 Accord seems to be between 65 & 70. According to car magazines, engines run most efficiently when the throttle is depressed 3/4, so 6 & 8 cylinder engines might have higher magic numbers.

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  Posted by Michael Wed Apr 22, 2009 11:10am PDT

  I think if you check those car magazines again, higher engine RPM increases power efficiency, but also burns gas faster which does not result in fuel economy necessarily. I doubt any car gets its best MPG at 3/4 throttle... I barely have to touch mine in 6th gear to maintain 65 or 70, and even 75-80 is not much more unless it's windy... (I'm in an '08 Sentra SpecV)

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  Posted by Michael Wed Apr 22, 2009 12:06pm PDT

  Sorry, one more point ... notice the "55" number came from the 70's ... seems like we could update that for today's car technology and at least raise it to 60 (going by some lowest common denominator). I'm sure there are enough vehicles that get their fuel efficiency sweet spot at that speed and also the advances in safety should be taken into account (though one could argue that actual driving skill and attention have suffered ;). 55 on a four-lane divided highway is just ridiculous IMHO.

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  Posted by justin g Wed Apr 22, 2009 2:15pm PDT

  also on the drafting issue unless you are tailgating going 70 you really are not gonna improve your gas mileage. maybe behind a large truck but you still have to be really close if not your just gonna catch all the turbulance thus making your car shake and have spuratic velocity. aero-mechanical engineering

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  Posted by Disgruntled Customer Wed Apr 22, 2009 7:21pm PDT

  a good rule of thumb on any vehicle is to push your accelerator just enough to go forward, a passenger should hardly feel any head jerk motion when doing so, then as soon as you reach your last gear on your transmission that is the magic number for your best mpg in any vehicle (on a straight road of course)

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