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Discussion Starter · #1 ·
My 2012 Eco came with no spare tire, and the reasoning from GM was that the extra weight (28 pounds for the donut) was detrimental to meeting the 42 mpg EPA target. I added a full size spare to my trunk (39 pounds including a jack from an old Subaru), and I was very curious about the supposed detrimental effect of the weight of the spare. So I checked the mileage (a tank full each) both with the GM "fix-a-flat" system VS. with my spare tire. I actually improved my mileage by nearly two mpg WITH the spare over the same route I drive every day. That may be statistically insignificant, but interesting nonetheless.

Surely GM figured that most folks would carry around some kids, groceries, (wife) etc that would weigh as much or more than a 28 pound donut? And since my experience has been that the added weight, at the very least, had NO detrimental effect on mpgs, the reason for no spare in the Eco MT is even more baffling to me. I mean, 5 gallons of gas weighs as much as the donut!

Anyone else noticed a difference in mpgs with gross vehicle weight variation?
 

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i have been wondering this as well. quick search brought up this >> Gas Mileage Tips - Driving More Efficiently

100lbs is 1-2% more efficient. maybe more in a lighter car.

i carry around 100lbs of tools in the trunk without them i could expect ~.6mpg improvement if i dumped the tools.

spare tire would only be ~.3mpg or so but didn't they drop over 200lbs total. that is likely over 1mpg total.
 

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I have an LS but I none the less have a spare. I ought to take it out and see if my mileage increases. If so, that that begs the question: If you put aftermarket wheels on your car that weigh more, are you decreasing your mileage? Or if I buy the light Eco wheels for my LS will that increase my mileage? Just a thought. Jakkaroo: You have a LS with Eco wheels: Did your mileage go up? Things to ponder. It would be my luck to have a flat when I decide to test the spare hypothesis and take mine out. LOL
 

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Every time you accellerate you use fuel to move the mass of the car. Another way of saying the same thing is every time you use your brakes you pay a fuel penalty. In a steady speed situation the aero drag is more predominant but stop & go driving is more weight dependent.

The last time I had a tire fail on the road was more than 20 years ago.
 

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at .3mpg it will be hard to see a mileage improvement but there is no doubt that there will be an improvement.

on the wheels i would expect the ratio to be higher than just dead weight in the trunk. you not only have to accelerate them in a forward direction you also have to spin them up to speed. that being said it still isn't likely to be obvious in your tank to tank mpg since things like weather and the routes you drive will have more of an effect on mpg.
 

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...here's an equation that approximates the WT vs. MPG trade-off:

(MPG[SUB]2[/SUB]/MPG[SUB]1[/SUB]) = (WT[SUB]1[/SUB]/WT[SUB]2[/SUB])^[SUP]0.72[/SUP]

...where:

MPG[SUB]1[/SUB]= prior fuel economy, mpg
MPG[SUB]2[/SUB]= new fuel economy, mpg
WT[SUB]1[/SUB] = prior weight, lbs
WT[SUB]2[/SUB] = new weight, lbs


addendum: equation is valid for vehicles in the 3,000± weight class (per GM engineer).
 

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...here's an equation that approximates the WT vs. MPG trade-off:

(MPG[SUB]2[/SUB]/MPG[SUB]1[/SUB]) = (WT[SUB]1[/SUB]/WT[SUB]2[/SUB])^[SUP]0.72[/SUP]

...where:

MPG[SUB]1[/SUB]= prior fuel economy, mpg
MPG[SUB]2[/SUB]= new fuel economy, mpg
WT[SUB]1[/SUB] = prior weight, lbs
WT[SUB]2[/SUB] = new weight, lbs
nice! doing the math if i got rid of those 100lbs in tools my last tank would have went from 31.5 up to 32.25. that is substantial.

now doing the math on a spare tire if we go with 40lbs. ((3100/3060)^.72)*31.5 = 31.8
so for me 31.5 to 31.8 or .3mpg

the first thing i do with every car i have is dump the jack and spare. less weight = better mileage, better handling, quicker stops, quicker acceleration. i would think .3mpg is a big deal to the gm engineers and it saves them money too.
 

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Added weight does make a huge difference in your fuel economy. It takes much more energy to get moving and up to speed. Try putting a person or two in your car. Granted, that's a lot of added weight, but you'll get the idea. It's substantial. I can see why GM went to such lengths to reduce the weight of the Eco MT. The Eco AT weighs 3011lbs, and you shave 80lbs going to the manual from the auto, which brings you down to ~2930lbs.
 

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The Eco AT weighs 3011lbs, and you shave 80lbs going to the manual from the auto, which brings you down to ~2930lbs.
...per GM, the manual Eco is 3011 lbs., so the automatic Eco is almost 3100 lbs.
 

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Whenever the spare tire issue for the Eco comes up, people focus on the weight of just the spare. Chevrolet has a different focus where the spare is concerned, because the weight issue is about the whole vehicle. To begin with, Chevrolet has to make their whole fleet of vehicles for a given model year meet the average miles per gallon target for that year. So anything that they can do with one model of car to improve that mileage contributes to the rating for the fleet. And Chevrolet didn't focus on just the spare as the weight issue. They focused on the whole car, and reduced everything they could to get the car 400lbs lighter. The spare is a part of that reduction. And in order to sell the whole fleet this year and meet the mileage target they cannot offer a spare for the Eco. Granted, just adding a spare to the Eco probably won't make noticeable difference in the mileage. But Chevrolet has to look at it differently.
 

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Discussion Starter · #14 ·
Whenever the spare tire issue for the Eco comes up, people focus on the weight of just the spare. Chevrolet has a different focus where the spare is concerned, because the weight issue is about the whole vehicle. To begin with, Chevrolet has to make their whole fleet of vehicles for a given model year meet the average miles per gallon target for that year. So anything that they can do with one model of car to improve that mileage contributes to the rating for the fleet. And Chevrolet didn't focus on just the spare as the weight issue. They focused on the whole car, and reduced everything they could to get the car 400lbs lighter. The spare is a part of that reduction. And in order to sell the whole fleet this year and meet the mileage target they cannot offer a spare for the Eco. Granted, just adding a spare to the Eco probably won't make noticeable difference in the mileage. But Chevrolet has to look at it differently.
I appreciate your analysis of GM's dilema on meeting a fleet average. But IMO I seriously doubt that a donut and jack makes ANY measurable difference in fuel mileage for the Eco. Using that theory, a driver-less car would much get better mileage than one with a driver. :signlol:


Seriously though, it seems there must have been a standard operating weight GM used for determining mileage for the EPA. It would be more meaningful to have a mileage rating based upon gross weight; i.e. an adult driver with at least 1 adult passenger, and a spare and jack (or not). I still say there is no defensible reason for not incluting a spare tire in the Eco.
 

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If it makes a 1MPG difference on the car, and they sell tens of thousands of them, it makes a BIG difference to GM. And using gross weight wouldn't be good for rating because there are too many variances in people and what they carry.
 

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...actually, it's the EPA which sets weight classes, GM just attempts to make their model-year cars "fit" into the lowest class possible so they can achieve a higher CAFE score (what GoldenCruze said above).
 

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I'll take my spare over the 1mpg difference thank you very much.

GM should have removed the back seats and forbid you to have passengers to save weight lol
No spare is the dumbest thing i've ever heard of. With no cruise control being a close second.
 
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Discussion Starter · #19 ·
As I have stated elsewhere on this forum, I think the lack of a spare for the Eco MT has much less to do with achieving a certain fuel mileage goal than it does with creating an incentive for the customer to purchase GM's OnStar service. $240 a year every year for OnStar vs $100 one time for a donut? Me thinks some of you folks work for GM.
:question:
 

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Me thinks some of you folks work for GM.
:question:
...nah, we're all just members of the "...General Motors (in)Voluntary Community (non)Service Action Corps..." (wink,wink)!
 
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