|
|
|
|
Thread Tools | Display Modes |
03-13-2006, 09:05 PM | #1 |
Scooby Newbie
Member#: 63177
Join Date: Jun 2004
Location: So Cal
Vehicle:2003 WRX Midnight Black Pearl |
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...
* Registered users of the site do not see these ads.
|
03-13-2006, 09:10 PM | #2 |
Scooby Newbie
Member#: 30211
Join Date: Nov 2002
Chapter/Region:
Tri-State
Location: New Jersey
Vehicle:2008 Acura TSX Premium White Pearl |
nm i can't read heh
|
03-13-2006, 10:32 PM | #3 |
Scooby Specialist
Member#: 84006
Join Date: Mar 2005
Chapter/Region:
NWIC
Location: Auburn, WA
Vehicle:2005 Impreza WRX STi WRB with Goldies |
I was turning minute 45 laps out at the track this weekend and averaged 8.8 MPG. Does that help?
|
03-13-2006, 10:33 PM | #4 |
Scooby Specialist
Member#: 38397
Join Date: Jun 2003
Chapter/Region:
MWSOC
Location: Ann Arbor, MI
Vehicle:2002 Lovemachine Silver |
Do your own HW?? Use the search, this has been talked about many many times.
|
03-13-2006, 11:19 PM | #5 |
Scooby Specialist
Member#: 104207
Join Date: Jan 2006
Chapter/Region:
NWIC
Location: Rock Creek (Portland)
Vehicle:2006 Stage 2 STi Blue |
The short answer is this... Stop and go traffic requires much more energy than consistent speed travel.
|
03-13-2006, 11:37 PM | #6 |
Scooby Newbie
Member#: 108772
Join Date: Feb 2006
Chapter/Region:
South East
Location: Atlanta
Vehicle:2006 WRX TR Black |
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.
|
03-14-2006, 12:11 AM | #7 |
Scooby Guru
Member#: 86322
Join Date: May 2005
Chapter/Region:
MWSOC
|
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.
|
03-14-2006, 05:06 AM | #8 |
Scooby Newbie
Member#: 63177
Join Date: Jun 2004
Location: So Cal
Vehicle:2003 WRX Midnight Black Pearl |
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 |
03-14-2006, 05:24 AM | #9 |
Scooby Specialist
Member#: 82378
Join Date: Mar 2005
Chapter/Region:
South East
Vehicle:<--- Hot Donuts Now |
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.
|
03-14-2006, 05:31 AM | #10 | |
Scooby Guru
Member#: 82989
Join Date: Mar 2005
Chapter/Region:
SWIC
Location: Arizona
Vehicle:2002 WRX Hybrid EJ257 |
Quote:
|
|
03-14-2006, 06:47 AM | #11 | |
Scooby Specialist
Member#: 37617
Join Date: May 2003
Vehicle:2007 |
Quote:
|
|
03-14-2006, 10:36 AM | #12 |
Scooby Specialist
Member#: 14141
Join Date: Jan 2002
Location: [email protected] @ 5800 ft on 13T
Vehicle:2002 Impreza WRX |
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 |
03-14-2006, 01:26 PM | #13 | |
Scooby Specialist
Member#: 12344
Join Date: Nov 2001
Chapter/Region:
MAIC
Location: Mountains of Maryland
Vehicle:2002 2JC blue |
Quote:
|
|
03-14-2006, 01:33 PM | #14 |
Scooby Specialist
Member#: 59692
Join Date: Apr 2004
Location: Cincinnati
Vehicle:2005 Impreza WRX STi WRB-Gold |
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.
|
03-14-2006, 01:41 PM | #15 | |
Scooby Specialist
Member#: 94706
Join Date: Aug 2005
Chapter/Region:
TXIC
Location: Dallas, TX
Vehicle:1995 Miata |
Quote:
|
|
03-14-2006, 01:43 PM | #16 | |
Scooby Specialist
Member#: 21999
Join Date: Jul 2002
Chapter/Region:
Tri-State
Location: Central PA
Vehicle:2017 Wrx DGM |
Quote:
just my 2 gallons, Fess |
|
03-14-2006, 02:09 PM | #17 |
Scooby Guru
Member#: 7374
Join Date: Jun 2001
Chapter/Region:
BAIC
Location: Gold Country
Vehicle:2023 MAGA Hat Model3 grey, or is it gray? |
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.
|
03-14-2006, 03:26 PM | #18 | |
Scooby Specialist
Member#: 94706
Join Date: Aug 2005
Chapter/Region:
TXIC
Location: Dallas, TX
Vehicle:1995 Miata |
Quote:
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. |
|
03-14-2006, 03:32 PM | #19 |
Scooby Specialist
Member#: 70647
Join Date: Sep 2004
Chapter/Region:
TXIC
Location: TX / TN
Vehicle:2004 WRX Blue |
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. |
03-14-2006, 03:46 PM | #20 |
Scooby Newbie
Member#: 49238
Join Date: Nov 2003
Chapter/Region:
MAIC
Location: Fairfax, VA
Vehicle:2004 WRX WR Blue |
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.
|
03-14-2006, 04:01 PM | #21 |
Scooby Newbie
Member#: 100603
Join Date: Nov 2005
|
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. |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
Manually Computed MPG vs OnBoard Computer's MPG | f4phantomii | Off-Topic | 13 | 05-24-2007 09:01 AM |
Highway vs. City Mileage | palpullero | Newbies & FAQs | 1 | 09-23-2002 01:02 PM |
MPG vs Modified Exhaust and CEL.... | Philip | Factory 2.0L Turbo Powertrain (EJ Series Factory 2.0L Turbo) | 4 | 08-06-2001 10:15 PM |
highway miles vs. city miles.. opinions?/ | spunky | General Forum Archive | 2 | 12-24-2000 10:26 PM |