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Q: Car Effciency ( No Answer,   1 Comment )
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 Subject: Car Effciency Category: Science > Physics Asked by: danielgrimes-ga List Price: \$10.00 Posted: 10 Nov 2006 03:33 PST Expires: 10 Dec 2006 03:33 PST Question ID: 781609
 ```I'm looking for a formula which will help me define how the the fuel efficiency of a car varies with speed``` Request for Question Clarification by omnivorous-ga on 10 Nov 2006 05:40 PST ```Daniel -- A precise formula is unlikely, as there are engine factors and the aerodynamics are also a significant factor. Since cars differ greatly in aerodynamics (wind resistance increases with the squaring of the speed) you're more likely to get an example of fuel consumption vs. speed, rather than a formula. Is that acceptable? Best regards, Omnivoous-GA``` Request for Question Clarification by sublime1-ga on 10 Nov 2006 13:13 PST ```In addition to variations in engine design and aerodynamics, there are numerous other variables which preclude the possibility of a generic formula for all vehicles, such as vehicle weight, differences in tire traction, road conditions, idle speed, etc. However the US government has a website about fuel economy, and they have a graph there that provides a visual estimate of fuel economy vs speed for the average vehicle: "While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph. As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional \$0.20 per gallon for gas." http://www.fueleconomy.gov/feg/driveHabits.shtml You can also see that between 35 mph and 60 mph the fuel economy is relatively stable. Let me know if this satisfies your interests... sublime1-ga``` Request for Question Clarification by sublime1-ga on 10 Nov 2006 13:30 PST ```daniel... On this page from Wikipedia, they provide the formula for the power required to overcome aerodynamic drag, which is only one of the factors involved in fuel economy, but has a major impact: "Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice faster. Since power is the rate of doing work, four times a work in half the time requires eight times the power." http://en.wikipedia.org/wiki/Air_resistance Let me know if this satisfies your question... sublime1-ga``` Clarification of Question by danielgrimes-ga on 11 Nov 2006 08:19 PST ```Thanks - I've seen the graph at fuel economy.gov - this is what I'm trying to replicate using real data. It seems to me that there are three phasesto the fuel economy - less than 20mph the engine is not running at it's optimum, and above 55mph wind resistance kicks in. Much of the variables are available - weight, a published figure for fuel consumption (indicative of engine efficiency) shape of car (SUV, compact etc - indicative of aerodynamics), weather, tyre type. So I'm happy for the figure to be a medium level of accuracy, but I want to try and plot with real data - so need a formula probably of the format: f(x)=speed( f(wind resistance), f(engine efficiency), f(weight), f(tyre resistance)) thanks for so far !```
 ```If this is for serious research or such, you may want to make the distinction between newer/older cars and/or cars of various fuel efficiency in general. The 2 cars i have tested over thousands of miles (checking approximate mpg with every fillup keeping in mind the approximate speeds I was driving) did best at about 75 mph. My 2000 ECHO does 44 mpg at 75 mph (known from several full tanks used in cross country driving at this speed)... It does about 42-43 mpg at 60 mph and also 80 mph. Of course this is a very fuel efficient vehicle which probably has something to do with this, but my other thought is that it wasn't built in the mid 90s. I really think that cars are made to be efficient at higher speeds than they used to be, and the 60 mph rule was from the early to mid 90s and probably for automatic cars (no 5th gear for cruising more efficiently at high speeds).```