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Old 05-24-08, 09:32 PM   #1
Pheard
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How does weight affect gas mileage?

Is there some kind of scale? xlbs=xmpg???

Is it different depending on certain variables? I'm trying to figure out how much weight extra will affect my mpg over a long drive.
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Old 05-24-08, 09:36 PM   #2
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I don't think there's a simple equation for it. It depends on the car, the amount of power the car has, how much weight you're adding, how well the car can take the extra weight, etc., etc.

Given two otherwise identical vehicles driven the same way, the lighter one will accelerate (and stop) faster and use less fuel in the process. The lighter/heavier the vehicles, the more pronounced this effect is.
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Old 05-24-08, 09:41 PM   #3
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Well yea, of course. I was trying to figure out a very general rule for how much extra weight can be added, before the MPG will be adversely affected. Most of the time mpg pronounced for a car isn't correct, and it never says if this is the mpg with the full weight capacity or not.

I speaking of the chevy cobalt. Says it gets 33 HW mpg. I checked people reports of it's mpg. One person got 33-35mpg on tons of mountainous driving with 4 people and their luggage in the car. Another person reported almost getting 40mpg on some of the legs of his trip with 2 people in the car.
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Old 05-24-08, 09:43 PM   #4
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Adding any weight will negatively affect mpg. Adding more weight will affect mpg more negatively. The factors involved are too great for a simple formula.
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Old 05-24-08, 10:24 PM   #5
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I'm beginning to think, to borrow a cliche, there are lies, there are whoppers, then there are MPG figures. The type of driving someone does is king if they are going to get good MPG or not. If going through a city, having to idle often will kill gas milage, unless its a hybrid which can shut off the gasoline engine and just sit on batteries. If one's commute is primarly highway, MPG will be a lot better.

The biggest factor that kills my MPG is stop and go driving in town.
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Old 05-24-08, 10:31 PM   #6
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It's really not necessary to fill the entire trunk of the rental car with pudding.
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Old 05-24-08, 11:45 PM   #7
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The more fuel efficient the car is, and the lighter it is, the more will extra weight negatively affect gas mileage. A huge SUV with a massive engine (that even at freeway speeds will hardly be more than idling) will not see much of a difference between empty and fully loaded.
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Old 05-25-08, 01:52 AM   #8
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The key thing is to differentiate the effect of weight on stop-and-go driving vs. steady-state cruising.

In steady-state cruising on level roads and with everything else being equal (e.g., a car made of aluminum vs. an indentical car made of steel), weight doesn't have much effect on mileage. Weight slightly increases the rolling resistance of the tires and slightly increases the drag of the bearings in the wheels, but at anything but very low speeds, these are minor contributors to overall vehicle drag - the big factor is aero resistance which is not affected by weight at all.

But in stop-and-go driving, weight has a big effect on mileage because you're constantly accelerating the weight up to speed, and then wasting this energy as heat when you apply the brakes.

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Old 05-25-08, 08:14 AM   #9
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It depends completely on the car and how it's set up. A better way to help the mileage is to do basic maintenance.
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Old 05-25-08, 08:24 AM   #10
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Quote:
Originally Posted by Pheard View Post
How does weight affect gas mileage?
Have you ever been on a bicycle?
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Old 05-25-08, 10:11 AM   #11
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Quote:
Originally Posted by Pheard View Post
Is there some kind of scale? xlbs=xmpg???

Is it different depending on certain variables? I'm trying to figure out how much weight extra will affect my mpg over a long drive.

On the highway, assuming the weight is inside the vehicle, no hills, and you don't accelerate hard/often... not much.

Take away any one of those caveats - it will be quite negatively impacted.
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Old 05-25-08, 12:25 PM   #12
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If the weight is a significant proportion of the total vehicle weight, it will have a large impact.

Even smaller amounts of weight (like carrying around unnecessary stuff in your trunk) will have an effect over lots of miles.
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Old 05-25-08, 12:42 PM   #13
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one way to remove weight is to buy lighter wheels those cheap pepboys 400$ fast and furious rims are 7lbs to 12lbs lighter than factory wheels another is fiberglass or carbon fiber hoods if its applicable to the car you own
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