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Old 07-13-2007, 04:45 PM   #1
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mpg and weight

I was wondering if there is a theoratical weight to calculate the mpg at a given weight at the same speed if one mpg number and one weight number is given.

Say I know I get 50 mpg at 65 mph with a car that weighs 2200. I look at my car and wonder what the mpg would be at the same speed if I manage to take off 100 lbs of the weight. Is there a formula for this at all?
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Old 07-13-2007, 05:07 PM   #2
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I haven't seen any formulas for any type of mod, since everything is different each tank of gas. It's not like a regular science experiment where you can charge a single variable.

Regardless, I was wondering the same thing. Generally, if everything else stayed the same, I would think you could calculate it like this:

old weight / new weight * mpg at old weight = mpg at new weight.
2200 / 2100 * 50 = 52.38

In any case, you might get +2.38mpg, but in reality it could be less. Can't hurt though!
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Old 07-13-2007, 05:11 PM   #3
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Well, chances are the graph would not be linear. It would probably be doable adding 100 lbs extra and then 200 lbs, then get the mpg's for a controlled trip and then extrapolate (is that English) the graph in the other direction to see where this would get you. Just thought of this.
The reason I started thinking of this is the gastank that I usually fill up to be able to calculate my mpg, but which I usually only drive till half a tank, so I carry 4 to 5 gallons in extra weight with me. Add to that a spare tire that could be thrown out (hey I am an AAA member anyway) and some other things.It made me wonder if it would be worth it...
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Old 07-13-2007, 05:12 PM   #4
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Real hard to guess, you can do an A-B-A test where you remove 100lbs (or add 100 lbs) and get a pretty good feel of the effect in your driving environment.

In P&G in my metro, I haven't seen any correlation, sometimes the extra weight of passengers actually seems to help?
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Old 07-13-2007, 05:15 PM   #5
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Quote:
In P&G in my metro, I haven't seen any correlation, sometimes the extra weight of passengers actually seems to help?
Well that doesn't make much sense theoratically does it? But then again maybe it works that way in Middle Earth
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Old 07-13-2007, 05:24 PM   #6
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Well, there's a lot of variables. But in my tiny metro, I can glide further with the extra weight, so maybe I don't have to relight the engine as many times???

Note, I see you haven't discovered the joys of kill switches and engine off coasting
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Old 07-13-2007, 05:38 PM   #7
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Mass only affects acceleration not constant speed driving.
Force=Mass X Acceleration

It takes more energy to accelerate a heavier car at the same acceleration as a lighter car.
Or using the same energy, (fuel) the rate of acceleration of a heavier car is slower then that of a lighter weight car.

The amount of force (or fuel) it takes to drive a car at a constant speed depends on air resistance, rolling resistance and mechanical losses (friction in transmission, bearing, engine etc..) The weight of the vehicle only slightly affects rolling resistance when traveling at a constant speed. But this factor is insignificantly small.
So the equation you are looking for does not and cannot exist in a way that can be applied to different cars.
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Old 07-13-2007, 06:07 PM   #8
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And the P&G working better with a heavier car does make sense.
Edit: Well, no reason there would be a lessening of FE in P&G with weight.

Though you may be using slightly more gas to accelerate during the pulse, you can coast at least enough longer to cancel out that loss. Extra weight equals extra momentum.

For example, if you apply 10 seconds of acceleration to a light car, you'll get going faster but also slow down sooner once those ten seconds are up.
You apply the same force for that same 10 seconds to a heavy car, you won't go as fast, but you'll take a lot longer to slow down.

Theoretically, in an ideal environment, you do those tests next to each other, the cars will go the exact same distance, but the light car will get there sooner.

Of course, this is all if I remember my physics correctly.
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Old 07-13-2007, 08:03 PM   #9
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skewbe: Yeah... EOC is excellent. I was a bit scared of it at first, but now I actually PREFER the feel of no power steering and non-assist braking! If only we could figure out a way to ACCELERATE with EOC (on an upgrade)! haha..

Bonus of EOC is that you can be moving a ton or 1000lbs... doesn't matter. Each one gets infinite miles to the gallon. Definitely one of the most important techniques for hypermiling!
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Old 07-13-2007, 10:02 PM   #10
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Yeah, I have seen a calculator where you could put n a few different things and it would calc the mpg for you. You would have to look up, estimate, or back into some stuff like Crr and Cd x A.
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