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Old 03-13-2006, 09:05 PM   #1
03 Black R3x
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Default Highway MPG vs City MPG

Why do u get better mpg when you're driving on the highway than in the city?

I'm working on a project and need to know hte equation that explains how it works.

And, wat is the efficiency of a regular engine anyways?

thanks...
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Old 03-13-2006, 09:10 PM   #2
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nm i can't read heh
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Old 03-13-2006, 10:32 PM   #3
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I was turning minute 45 laps out at the track this weekend and averaged 8.8 MPG. Does that help?
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Old 03-13-2006, 10:33 PM   #4
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Do your own HW?? Use the search, this has been talked about many many times.
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Old 03-13-2006, 11:19 PM   #5
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The short answer is this... Stop and go traffic requires much more energy than consistent speed travel.
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Old 03-13-2006, 11:37 PM   #6
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In the city, kinetic energy(movement) gets turned into thermal energy(brakes)? On the highway you just keep moving. Correct me if im wrong please.
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Old 03-14-2006, 12:11 AM   #7
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seems like the obvious answer; you break less on the highway and move at a constant speed unlike on the streets where you're constantly accelerating and braking.
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Old 03-14-2006, 05:06 AM   #8
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yea, thanks for the replies..

I understand the reason behind it...or i think i do that u accelerate more in the city and just cruise on the highway..

I had to write up program simulating it to output the two estimated city and hwy mpgs..and was stumped for a bit...but thanks for the responses
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Old 03-14-2006, 05:24 AM   #9
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55 is the magic number. anything higher on the highway will burn more gas. when on the highway you use the accelerator less. city driving you are always on the gas.
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Old 03-14-2006, 05:31 AM   #10
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Quote:
Originally Posted by wrxzboost
seems like the obvious answer; you break less on the highway and move at a constant speed unlike on the streets where you're
constantly accelerating and braking.
Ya. On the highway, how often are you accelerating compared to city driving where you are constantly off and on the gas pedal.
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Old 03-14-2006, 06:47 AM   #11
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Quote:
Originally Posted by wrxblubayou
55 is the magic number. anything higher on the highway will burn more gas. when on the highway you use the accelerator less. city driving you are always on the gas.
My STi gets better fuel economy going 85 mph than it does going 70 mph. If you go 55 on a highway, someone will kill you.
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Old 03-14-2006, 10:36 AM   #12
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In highway driving you have a uniform energy requirement of around 15 - 20 hp to over come aerodynamic drag and rolling resistance.

In the city you have fuel wasted while waiting for stop lights, and fuel used to accelerate up to speed. (energy input to accelerate = change in Kinetic energy hint), you still have the rolling resistance of about 10 hp while moving, but air drag at city driving speeds in not significant.

Figure out how to simulate those factors and you have your answer.

There is no magic number for most effecient speed it varies by driving style and car. Cruising at or near torque peak rpm is usually the most effecient engine speed. Find the gear that lets you do that with very light throttle setting and you usually will get very good economy.

Larry
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Old 03-14-2006, 01:26 PM   #13
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Quote:
Originally Posted by WR^2X
My STi gets better fuel economy going 85 mph than it does going 70 mph. If you go 55 on a highway, someone will kill you.
Well, if you go fast enough you shorten the time the engine is running and consuming fuel. Just stay off the boost, preferably take as many down hills as possible.
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Old 03-14-2006, 01:33 PM   #14
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Actually it has to do with closed and open loop fueling. Typically on the highway people travel at a consistent speed....closed loop (leaner A/F ratio). However, if on the highway, you vary your speed wildly, you can go into open loop, which would bring down your MPG. Since, in the city, you typically travel at varying speeds keeping the car in open loop. The terms highway mpg and city mpg refer to the open and closed loop fueling in the typical sense.
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Old 03-14-2006, 01:41 PM   #15
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Quote:
Originally Posted by wrxblubayou
55 is the magic number. anything higher on the highway will burn more gas. when on the highway you use the accelerator less. city driving you are always on the gas.
Thats just left over government propaganda from when the national speed limit was 55. The actual speed at which a car is most efficient depends on a number of factors, including the car itself (obviously), the terain its driving in, and the atmospheric conditions. Different cars have different gearing, and different hardware to get the power to the wheels, not to mention different tires and aerodynamics which are some of the bigger factors.
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Old 03-14-2006, 01:43 PM   #16
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Quote:
Originally Posted by conker69
Actually it has to do with closed and open loop fueling. Typically on the highway people travel at a consistent speed....closed loop (leaner A/F ratio). However, if on the highway, you vary your speed wildly, you can go into open loop, which would bring down your MPG. Since, in the city, you typically travel at varying speeds keeping the car in open loop. The terms highway mpg and city mpg refer to the open and closed loop fueling in the typical sense.
I am no expert but I do not that think that is the case either. I have read a test (though I don't know how scientific it was) where the mileage achieved on the highway from going WOT to a certain speed and free coasting down to a certain speed was only slightly less efficient than maintaining a constant speed (like 2mpg difference).

just my 2 gallons,

Fess
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Old 03-14-2006, 02:09 PM   #17
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I get *maybe* 1 mpg better mileage on the freeway as opposed to city. But I always have either bikes or snowboards on top of the car. I also keep up with the flow of traffic...which is, um, a little bit faster than 55.
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Old 03-14-2006, 03:26 PM   #18
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Quote:
Originally Posted by fastsubie
I am no expert but I do not that think that is the case either. I have read a test (though I don't know how scientific it was) where the mileage achieved on the highway from going WOT to a certain speed and free coasting down to a certain speed was only slightly less efficient than maintaining a constant speed (like 2mpg difference).

just my 2 gallons,

Fess
the reason this works is because you coast, thus using all of the energy the engine generated for forward motion. if you hit your breaks, you use some of that energy elsewhere. Think about it, when you coast, you get extreemly high gas milage (if you turn off the car completely, you get infinite gas milage while coasting ), so the low milage you got when you went WOT is balanced by the high milage you get while coasting, but in the city, you coast less, instead you use your breaks.

Since gas milage is an average over the period that you drive the car, you can relate that to the average of what you are doing when you drive the car over the same period. That probably doesn't make alot of sense, so ill give an example: if you do as was mentioned and you go WOT for a while and then coast to a stop, thats roughly equivalent to going part throttle over the same distance. if you go WOT and then slam on the brakes and come to a stop, thats roughly equivalent to going part throttle over the same distance, but with the brakes being applied slightly over the distance as well. it doesnt take a genius to realize that driving with your breaks on is going to yield worse milage, so doing more breaking in general will yield the same. its not a perfect matchup, but it gives a good example of whats going on.
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Old 03-14-2006, 03:32 PM   #19
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you all forgot the old physics property that starting from a complete stop requires more force -> more power -> more energy used than accelerating from anywhere else.
u of F, etc etc etc.

so add that in to wind resistance, all else,
if you want a simple answer, i think you still have to include this property, if you want an advanced answer... you got a bt iof work infront of you... as the math gets fun as it gets a few pages long.
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Old 03-14-2006, 03:46 PM   #20
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After driving for almost 5 hours non stop on the interstate at about 75mph my gas light came on, however the odd thing was when the gas pump stopped it only filled up like 11 gallons, so the gas light came on like 3 gallons early, when I did the math I got 34mpg highway on that trip. Because it was about 350 Miles on 11 Gallons, I tried to continue pumping and it stopped so the tank was indeed full, I just find it funny the gas light came on early.
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Old 03-14-2006, 04:01 PM   #21
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brakes? what? That's got nothing to do with how much gas the car will burn.

if you were in 3rd gear WOT going 55mph you'll get MUCH less gas mileage than going 55mph in overdrive gears. WOT and coasting won't yield greater efficiency than cruising speeds. Hence the classic example of the driver who decided to drive greater than 55mph because he was running low on fuel.

THE REASON

The engineers who build cars match up the gears to the engine to produce the maximum amount of revolutions of the wheel for the minimum revolutions of the engine. The more revolutions per minute, the more gas. Going 80mph will require a greater rpm for that gear than 55mph for the same gear.

Generally speaking for flat lands, no wind blah blah blah (hills change the concept), a car in third gear 2300rpm will result in a less amount of revolutions of the wheel than a car in 6th gear at 2300rpm. If you remember a thing or two about the cogs in the tranny, this will make more sense. Small the cog, the greater the revolutions of the wheel per revolutions of the engine.

Jeez can even take this back to geared bicycles. First gear takes the same amount of effort as cruising in the last gear. Except when cruising, every unit of energy you put through the pedals will yield a greater distance than the same amount of energy you put in for the first gear.

Once again, brakes aren't the primary reason behind worse gas mileage on city streets. That's ridiculous. If you put the two identical engines in a lab, with only gas tank and trannys in different gears, this will prove what i just said and negate the brakes-ruining-mpg-figures theory. Having them run at consistant speeds will also negate the fuel looping theory. Varying speeds will result in changing through more inefficient gears which makes mpg even worse. Its the act of accelerating through the first set of gears which end up being the most inefficient.

of course to make any of what i just said significant, we're assuming similar air temp, fuel temp, condition of engine, wind conditions, similar engine/gears/ratios, etc etc.
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