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I don't want to start a debate, BUT...; New car mileage rant
Topic Started: Dec 29 2011, 02:40 AM (2,360 Views)
metroschultz
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Please just call me; "Schultz"

Woodie
Jan 7 2012, 05:35 AM
horn12007
Jan 7 2012, 03:23 AM
Nerys you do know mjspiess better half is vp of gm....
She's in charge of turning down the gas mileage knob when nerys walks up to the showroom doors.
lmfao
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You say;
she's the V.P. in charge of the 'Conspiracy Against GeoMetroForum Members' ?
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Woodie
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Exactly, I'd hate her if she wasn't so cute.

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Horn


Woodie
Jan 7 2012, 05:35 AM
horn12007
Jan 7 2012, 03:23 AM
Nerys you do know mjspiess better half is vp of gm....
She's in charge of turning down the gas mileage knob when nerys walks up to the showroom doors.
Ahhhahaha good one
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me2


Doesn't adding weight increase the rolling resistance? I know when my fat buddy gets in my XFI to go somewhere with me I can feel a difference in starting the car in motion and in climbing hills AND it doesn't coast downhill as well. I've even told him I want to charge him for the lost mpg's I suffer because he is in the car. Is this scientific? No. But it is what I have observed.

It takes more energy to push a heavy car up a hill and that energy is not regained by flying down the other side. I can push my empty Metro on flat ground. I would bet money that with 3000 lbs added to it I would at least have a more difficult time. Why? Rolling resistance. Now if I had a mag-lev Metro that would not be true. I will start putting refrigerator magnets under it and see if it helps with mpg. I'll get back to ya'll.
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Coche Blanco
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Me2, you're right, but, the difference is small.

A better comparison:
Two cars, identical in every aspect except one has 50% more mass. Start them both at 10mph. (on flat ground) Get out, push them hard enough to maintain 10mph. How hard was it to push the light car? The heavy one?
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bogs
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Duct tape heals all wounds

That would actually be a better comparison if you had something other than a person doing the pushing, preferably something that could measure the amounts of force being applied, under controlled circumstances, etc etc etc.
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Shinrin
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Here's where I think the difference comes in. Ethanol vs non. Ethanol simply doesn't burn as efficiently. Engines keyed towards burning this have to burn differently. Ethanol vs Gas is almost Gas Vs Diesel. Different types of burning fuel get different gas mileage. The only reason(in my opinion) has and ethanol are categorized together is because they're currently mixed and there's no real option against it.

Secondly as has been said, the newer cars are getting city mileage over highway. Most people, this forum excluded, live within a few miles of their job. They drive city mainly and just need city mileage. Improvements that aid city driving hurt highway mileage, which is what we compare here.

Nerys 70mpg was most definitely highway. Add some short tires for better city mileage, change up the transmission gearing, and you have a car with a city driving improvement, and a highway unimprovement.
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me2


I did a quick web search on the subject of weight in relation to fuel economy. Here is a paragraph lifted from the conclusion of a study I found:


Reducing vehicle weight (mass) results in less tractive effort required to accelerate the vehicle and
less rolling resistance from the tires. Drive cycles with more acceleration events (EPA city and
European) show greater fuel economy benefits from weight reduction than highway or steady state
conditions. Also, at higher vehicle speeds the engine is typically at higher throttle (better BSFC)
operating points and provides less opportunity for improvement. Since the tire losses are a greater
percentage of total tractive effort at lower speeds (aerodynamic losses increase by velocity squared)
the potential for fuel economy gain from weight reduction is greater at lower vehicle speeds.


I think this is some of what you said Shinrin.
The fuel economy in the Metro I have is more sensitive to my driving habits than any car or truck I have owned. My experience includes how much weight I am carrying. My 'drive cycles' vary from week to week and my experience will be different from someone making solely long commutes on flat highways. So does weight matter? More so to some than others I guess. I know it makes it harder for me to walk.
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nerys
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Grr

thats because your metro is a extreme performance not unlike an F1 race car just in a different direction (fuel economy instead of speed)

everything just like on the F1 is "right on the edge" almost too light almost too thin metal almost not strong enough parts almost not enough power all "right on the edge" to maximize performance (for us fuel economy)

extra mass will HELP going down kill. if it does not you either have bad wheel bearings or you have under inflated tires. that really is simply physics. more mass means more inertia which means "Drag" will be less effective at slowing you down (this is why you have to brake harder when your heavier)

You can check my eco modder log. empty with just me so car plus my 420 pounds or "full laden" at nearly 3000 pounds total mass including trailer and cargo shows almost "NO" change in fuel economy on the highway.

less than 2mpg for sure usually less than 1mpg.

there are 2 ways more mass can harm your fuel economy.

If you need to overcome inertia.

Inertia is the propensity of a mass to "not change" state. ie to "accelerate" a mass you have to apply force.

accelerate does not mean moving. accelerate means a "change in rate of moving"

so if your holding steady at 10mph your acceleration (on paper) is ZERO.

if you want to go from 10mph to 15mph you now have to apply a delta or "change" in velocity.

THIS is an acceleration. (so is going from 15mph to 10mph btw but that one does not require your engine so we can ignore it)

anytime you have to apply force against inertia you will require more energy and this amount is linked to total mass. so heavier means more energy needed.

the other is a "sub set" of the first (or vice versa) but its easier to think of it as separate since people tend to get wonky when thinking "3D" since they assume cars move 2D but they don't.

Defying Gravity costs ENERGY and it is also linked to MASS so more mass means more energy. Lighter "rocket" needs less propellant than a "heavy" rocket.

so whenever you alter your direction of travel in anyway that is "non perpendicular" to gravity you are now "defying" gravity if its in the "away from earth" direction IE you go "uphill"

gravity is exerting a force on you. this creates a condition of "inertia" since you are now under "CONSTANT" delta velocity changes" or "acceleration" even if you keep the same speed (your VERTICAL speed is now changing it you just don't notice it since your instruments do not measure vertical speed only "rolling speed"

so how about the heavy car and the light car at 10mph.

move them both at 10mph on LEVEL ground (no gravity inertia of relevance)

Now the question is will it be harder to push the heavier car at 10mph than the lighter car at 10mph.

the answer is both YES and NO.

Technically the answer is YES practically the answer is NO.

Here is why.

when you increase mass what changes? well potential energy changes. ie the inertia of the car is higher.

but if you do not engage in an inertial event then there is no change in "energy requirement"

as noted above (thats why I typed all that btw :-) unless you change your velocity go uphill OR some other force "acts" on your car (wind) you have "NO" inertial change.

the extra mass only effect inertia if you try to "alter" the cars current state (IE speed up or slow down)

so why YES and NO?

well we have OTHER sources of drag on a car.

we have the resistance of the wheels on the road and we have the resistance of the "bearings" in the wheels (there are other drags on the car but they are irrelevant for this discussion since they do not "change" during this experiment)

the resistance of your wheel bearings will go up when you add more mass. but unless your bearings are bad or you "overload" them mechanically this difference will be FLAT OUT UNMEASURABLE by you and me. Period.

its so small would likely need to measure your fuel economy to 4 or 5 decimal places to even have a chance of seeing it.

the other is resistance of the tires on the ground.

this also goes up slightly with more mass but this ALSO (with properly inflated tires and proper alignment) is virtually UNMEASURABLE by you and me. you would need 3 or 4 decimal places to measure this difference in real time.

its just so tiny its flat out irrelevant for this discussion.

SO will the heavier car be harder to push? technically? on paper? YES. measurably by you and mean in a real world experiment? Not a change.

IT IS in fact possible for you to test this. get a hanging scale and a "wagon" (red flyer wagon)

find a nice flat smooth surface such as a gym floor (otherwise imperfection in your travel path will alter the results)

now pull the cart and walk a nice steady speed.

now add 50 pounds to the cart (several times its mass) and do it again.

you will note the "pull" force goes up quite a bit to "get it moving" but once you "GET" it moving the force is virtually the same as the empty cart.

Mass on effect energy requirements during a CHANGE in velocity. so if your velocity is not changing (highway driving) for the most part then your FUEL requirements will also NOT CHANGE.

now it does change slightly. you have to accelerate from 0 to get "ON" the highway but you gain a lot of that back when you slow down at the other end if you drive right (not hypermiling just drive smart)

and over 400-500 miles its just not very significant.

now if you drive around the mountains or in the city OK the extra mass is going to murder your fuel economy because now you engaging in LOTS of acceleration changes which means you have to apply the extra energy for that "mass" every time you do that.

This is how I can put 900 pounds or more in the car and 500 pounds on the hitch and still get 50+ mpg in the summer.

because 99% of my driving is "cruising" ie steady state no "accelerations" so the extra mass is simply not very relevant.

this is why "we" can beat EPA estimates as well. we do way more steady state driving than your average "highway driver"

and THIS is why saying newer cars get lower fuel economy only because they are "heavier" is a false argument.

heavier will lower city fuel economy but have a MINIMAL effect on highway fuel economy.

plus we have high mpg examples of cars from 25 years ago JUST AS if not HEAVIER than today's cars.
Edited by nerys, Jan 7 2012, 09:09 PM.
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Shinrin
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Now on paper(or forum) saying once something is moving the weight doesn't make a difference, but I know that is wrong.

At work we have a cart full of batteries. I have a good distance to move it, all in one burst not slowing down or stopping. I can most definitely feel the difference in it if it has one or two batteries, versus the full cart. Getting it started is a lot harder, and it takes a lot more effort to keep it going. I can push it all day empty, but I'll get winded in ten minutes if it's full. And this is assuming flat ground with no variance in speed.
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nerys
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Grr

your comparing an overloaded cart pushed by 1/4HP motor

Versus a 2000 pound automobile moving at 20+ HP

apples and oranges dude :-)

load the cart up with ONE battery.

Now load it with just TWO batteries and now tell me if you can tell a difference.

and the ground has imperfections that are magnified by your WITHOUT BEARINGS small hard solid rubber or plastic wheels.

again apples and oranges.

put some nice 100psi skinnies on that with some quality BEARINGS like you have in your metro.

then load it up with batteries and see the world of difference that makes.

and remember. one of my quid pro quos if you overload the system mechanically.

you can push 1/4HP if your in decent shape.a cart full of batteries "mechanically" overloads "YOU" the engine.

if I put 3000 pounds behind my metro (ie now pushing 5500 pounds) you can bet your butt steady state or not I am going to lose TONS of MPG's because I am not exceeding the mechanical limits of my hardware and power plant.
Edited by nerys, Jan 8 2012, 12:28 AM.
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Shinrin
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Point taken. So we should agree weight does make a difference at steady speed, but only in extreme cases.

Adding 200 pounds to a 2000 pound car doing 55 won't make a noticeable difference.
Adding 2000 points to a 2000 pound car would make quite a difference.

Although unless you're tying a second geo to your roof, I don't think anyone will every worry about that.
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Coche Blanco
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bogs
Jan 7 2012, 05:30 PM
That would actually be a better comparison if you had something other than a person doing the pushing, preferably something that could measure the amounts of force being applied, under controlled circumstances, etc etc etc.
Well...in a perfect world...
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nerys
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Grr

also remember. my BEST tank just over 70mpg over 422 miles was at the cars HEAVIEST. I "EASILY" had 1300-1400 pounds in the car. nearly doubling its mass.

SO much mass I needed to downshift to 2nd gear and was limited to 30-35 mph up some hills !!

but I went 422 miles and I only slowed below 50-55mph 6 times 1 of which does not count

I was below 55 when I left the gas station of course exit station 200 feet enter freeway.

4 hills took me below 50 (around 30-35 for 3 1 was 40-45mph)

and to slow down and get gas on the turnpike after 422 miles.

sure an extreme highway mileage example but fact stands I scored over 70mpg with a total vehicle mass of around 3000 pounds !! (the car masses 1600 empty)
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