{"id":1503,"date":"2008-10-17T22:57:48","date_gmt":"2008-10-18T05:57:48","guid":{"rendered":"http:\/\/www.myrideisme.com\/Blog\/?p=1503"},"modified":"2009-09-13T22:09:16","modified_gmt":"2009-09-14T05:09:16","slug":"speed-to-drive-for-best-fuel-economy","status":"publish","type":"post","link":"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/","title":{"rendered":"Speed to drive for best Fuel Economy"},"content":{"rendered":"<div class=\"mr_social_sharing_wrapper\">\n\t\t\t\t<!-- Social Sharing Toolkit v2.2 --><span class=\"mr_social_sharing\"><a href=\"http:\/\/pinterest.com\/pin\/create\/button\/?url=https%3A%2F%2Fwww.myrideisme.com%2FBlog%2Fspeed-to-drive-for-best-fuel-economy%2F&amp;media=&amp;description=Speed+to+drive+for+best+Fuel+Economy\" class=\"pin-it-button\" count-layout=\"none\"><img border=\"0\" src=\"\/\/assets.pinterest.com\/images\/PinExt.png\" title=\"Pin It\" \/><\/a><\/span><span class=\"mr_social_sharing\"><fb:share-button type=\"button_count\" href=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\"><\/fb:share-button><\/span><span class=\"mr_social_sharing\"><a href=\"https:\/\/twitter.com\/share\" class=\"twitter-share-button\" data-count=\"horizontal\" data-url=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\" data-via=\"myrideisme\" data-text=\"Speed to drive for best Fuel Economy\"><\/a><\/span><span class=\"mr_social_sharing\"><div class=\"g-plusone\" data-size=\"medium\" data-annotation=\"none\" data-href=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\"><\/div><\/span><span class=\"mr_social_sharing\"><a class=\"mr_social_share_email\" href=\"mailto:?subject=Speed to drive for best Fuel Economy&amp;body=https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\" target=\"_blank\"><img src=\"https:\/\/www.myrideisme.com\/Blog\/wp-content\/plugins\/social-sharing-toolkit\/images\/icons_small\/email.png\" class=\"nopin\" alt=\"Email\" title=\"Email\"\/><span class=\"mr_small_icon\">Email<\/span><\/a><\/a><\/span><span class=\"mr_social_sharing\"><a rel=\"nofollow\" class=\"linksalpha_button linksalpha_link\" href=\"\/\/www.linksalpha.com\/social\/mobile\" data-url=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\" data-text=\"Speed to drive for best Fuel Economy\" data-desc=\"I was cruising the web looking for something and I found this discussion. What speed should you drive to get your best fuel economy? interestingly enough, the discussion was from a story from the site Truth about Cars. That site kicks out a shocking amount of car related content. From the story\" data-image=\"\" data-button=\"\">\n\t\t\t\t\t<img src=\"\/\/www.linksalpha.com\/images\/social_share_button.png\" alt=\"Share\" class=\"linksalpha_image\" \/>\n\t\t\t\t\t<\/a><script type=\"text\/javascript\" src=\"\/\/www.linksalpha.com\/scripts\/loader_iconbox.js?v=2.4\"><\/script><\/span><\/div><p>I was cruising the web looking for something and I found this discussion.  <em>What speed should you drive to get your best fuel economy?<\/em><\/p>\n<p>interestingly enough, the discussion was from a story from the site <a title=\"Content rich car blog site\" href=\"http:\/\/www.thetruthaboutcars.com\/\" target=\"_blank\">Truth about Cars<\/a>.  That site kicks out a shocking amount of car related content.  From <a title=\"Speed for best fuel economy\" href=\"http:\/\/www.thetruthaboutcars.com\/ask-the-best-and-brightest-road-load-power-av-bv2-cv3\/\" target=\"_blank\">the story<\/a> folks at truthaboutcars.com were trying to help everyone understand a formula that actually came from <a title=\"The auto part how stuff works\" href=\"http:\/\/auto.howstuffworks.com\/\" target=\"_blank\">Howstuffworks.com<\/a>, another great site to kill several hours&#8230; try taking the Alligator quiz or see if a platypus could poison you.  The How Stuff works guys did alot better job, but didn&#8217;t really give any background.<\/p>\n<p>After the story, several folks chimed in with their &#8220;truth&#8221;.  In reality, they said alot that was dead wrong and then some close, but not right.<\/p>\n<p>The equation they were using to answer the question was:<\/p>\n<p>Road Load Power = aV + bV^2 + cV^3<\/p>\n<p>where a, b and c are constants and V is the symbol for velocity or let&#8217;s just say SPEED.  There was some mention of where this equation came from, but unfortunately, it wasn&#8217;t quite right.  I also want to say what &#8220;Road Load&#8221; means.  Road Load is when you&#8217;re cruising&#8230; not going faster or slower (not accelerating) so your <em>power <\/em>output is balanced to the <em>power <\/em>required to keep your car moving at that speed.<\/p>\n<p>Where does the Power come from and what are a, b and c?<\/p>\n<p>Back up a sec and first consider &#8220;Road Load&#8221;.  Load is a force. Road Load is the force on your car when you&#8217;re not going faster or slower.  It&#8217;s your resistance or &#8220;drag force&#8221;.  How are force and power related?  Hang on a minute&#8230;<\/p>\n<p>It&#8217;s easy to find your Road Load driving force or resistance by what&#8217;s called a <strong>coast down<\/strong> test.  That&#8217;s a test that&#8217;s alot like it sounds.  You drive to about 80mph (no need to go faster, but why is really hard to explain&#8230; suffice to say 80&#8217;s enough)  then put the car in neutral and measure how much time is needed to &#8220;coast down&#8221; to about 10mph.  The time data, measured in pairs, like 80-70 and 70-60 can be used to find your coast down information &#8211; for that one particular car.<\/p>\n<p>After coast down, you&#8217;ll know the driving <span style=\"text-decoration: underline;\">force<\/span> (not power, not yet) you need to keep your car moving.  The equation is a 2nd order polynomial that looks like: a + bV + cV^2.<\/p>\n<ul>\n<li> a= rolling drag, it&#8217;s a constant that comes mostly from the tires deforming and vehicle weight (notice it doesn&#8217;t care how fast you&#8217;re driving)<\/li>\n<li> b= mechanical friction, pumps and other things (depends on speed, usually very small compared to the others)<\/li>\n<li>c= wind drag, depends on frontal area (think about shining a flash light into the front of your car, the shadow it casts is the frontal area.)<\/li>\n<\/ul>\n<p>A term more people might have heard of is Cd.  Cd is related to the &#8220;c&#8221; of this equation by:<\/p>\n<p>Wind resistance or wind drag = c V^2 = 1\/2 x air density x A x Cd x V^2<\/p>\n<p>A is the front area.  The Cd you hear about has more to do with the SHAPE of a car.  A low Cd means a vehicle with the same frontal area (think about a passenger van vs. a bullet train) makes less wind resistance because the vehicle&#8217;s shape is better (more aerodynamic).<\/p>\n<p>Let&#8217;s move on to the question about power.<\/p>\n<p><strong>Power<\/strong> = force x velocity. (in our case Road Load x velocity)<\/p>\n<p>Enjoy the math&#8230; (a + bV + cV^2) x V = aV + bV^2 + cV^3.<\/p>\n<p>You can find the a, b &amp; c from the EPA website for new cars and they&#8217;re the same constants found from coast down testing. (I&#8217;ve heard, I tried to give you a link, but couldn&#8217;t find it)  Coastdown data is used to find the EPA fuel economy.  But, don&#8217;t think it&#8217;s coastdown&#8217;s fault that nobody every gets their EPA sticker advertised fuel economy.  It&#8217;s not!<\/p>\n<p>Are you thoroughly confused yet?  You might as well be cause this still doesn&#8217;t even consider the efficiency of the motor, Brake specific Fuel Consumption or BSFC or the power per fuel used.<\/p>\n<p>To really answer this question, you need more data.  And unfortunately, the data comes from experiment or a bunch more calculations with alot of assumptions. Unless you&#8217;re building the car, you probably won&#8217;t know this stuff.<\/p>\n<p>There&#8217;s a bunch more I could say about this.  Does anyone care?  Or would you rather &#8220;Drop the hammer!!&#8221; and have fun!  Last time I checked, &#8220;Bonnie&#8221;, my <a title=\"Fuel efficient?  Hot rod\" href=\"http:\/\/www.myrideisme.com\/Garage\/pikesan\" target=\"_self\">4 cylinder 27 Roadster<\/a> gets 21mpg.  Works for me!<\/p>","protected":false},"excerpt":{"rendered":"\n\t\t\t\t<div class=\"mr_social_sharing_wrapper\">\n\t\t\t\t<!-- Social Sharing Toolkit v2.2 --><span class=\"mr_social_sharing\"><a href=\"http:\/\/pinterest.com\/pin\/create\/button\/?url=https%3A%2F%2Fwww.myrideisme.com%2FBlog%2Fspeed-to-drive-for-best-fuel-economy%2F&amp;media=&amp;description=Speed+to+drive+for+best+Fuel+Economy\" class=\"pin-it-button\" count-layout=\"none\"><img border=\"0\" src=\"\/\/assets.pinterest.com\/images\/PinExt.png\" title=\"Pin It\" \/><\/a><\/span><span class=\"mr_social_sharing\"><fb:share-button type=\"button_count\" href=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\"><\/fb:share-button><\/span><span class=\"mr_social_sharing\"><a href=\"https:\/\/twitter.com\/share\" class=\"twitter-share-button\" data-count=\"horizontal\" data-url=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\" data-via=\"myrideisme\" data-text=\"Speed to drive for best Fuel Economy\"><\/a><\/span><span class=\"mr_social_sharing\"><div class=\"g-plusone\" data-size=\"medium\" data-annotation=\"none\" data-href=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\"><\/div><\/span><span class=\"mr_social_sharing\"><a class=\"mr_social_share_email\" href=\"mailto:?subject=Speed to drive for best Fuel Economy&amp;body=https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\" target=\"_blank\"><img src=\"https:\/\/www.myrideisme.com\/Blog\/wp-content\/plugins\/social-sharing-toolkit\/images\/icons_small\/email.png\" class=\"nopin\" alt=\"Email\" title=\"Email\"\/><span class=\"mr_small_icon\">Email<\/span><\/a><\/a><\/span><span class=\"mr_social_sharing\"><a rel=\"nofollow\" class=\"linksalpha_button linksalpha_link\" href=\"\/\/www.linksalpha.com\/social\/mobile\" data-url=\"https:\/\/www.myrideisme.com\/Blog\/speed-to-drive-for-best-fuel-economy\/\" data-text=\"Speed to drive for best Fuel Economy\" data-desc=\"I was cruising the web looking for something and I found this discussion. What speed should you drive to get your best fuel economy? interestingly enough, the discussion was from a story from the site Truth about Cars. That site kicks out a shocking amount of car related content. From the story\" data-image=\"\" data-button=\"\">\n\t\t\t\t\t<img src=\"\/\/www.linksalpha.com\/images\/social_share_button.png\" alt=\"Share\" class=\"linksalpha_image\" \/>\n\t\t\t\t\t<\/a><script type=\"text\/javascript\" src=\"\/\/www.linksalpha.com\/scripts\/loader_iconbox.js?v=2.4\"><\/script><\/span><\/div><p>Email I was cruising the web looking for something and I found this discussion. What speed should you drive to get your best fuel economy? interestingly enough, the discussion was from a story from the site Truth about Cars. That site kicks out a shocking amount of car related content. From the story folks at [&hellip;]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ngg_post_thumbnail":0},"categories":[1815],"tags":[2512],"_links":{"self":[{"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/posts\/1503"}],"collection":[{"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/comments?post=1503"}],"version-history":[{"count":4,"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/posts\/1503\/revisions"}],"predecessor-version":[{"id":5423,"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/posts\/1503\/revisions\/5423"}],"wp:attachment":[{"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/media?parent=1503"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/categories?post=1503"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.myrideisme.com\/Blog\/wp-json\/wp\/v2\/tags?post=1503"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}