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Old 04-29-2009, 07:09 AM   #1
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Car weight and MPGs

As I stated in my post about my trip mileage in my odyessy, I averaged 36ish mpg.

normally this vehicle gets about 19mpg. So that is an 89% improvement over the vehicle's average.

When I had my civic(1993DX automatic) I would average about 33mpg and get about 41mpg on a highway trip like that. That is only a 24% improvement.

I'm no engineer by any means, but this leads me to believe that the heavier vehicle got the better mileage increase because it weighs more. The extra weight must have helped me keep my momentum during gliding periods, which kept my speed up when it was time to give it some gas.

Has anyone figured this one out already or posted about it?

Maybe this is part of the reason the GM 3.8L cars do so well.
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Old 04-29-2009, 07:23 AM   #2
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The weight of the vehicle doesn't really matter when it's cruising on the highway. It matters when you're accelerating. So when you're doing a long trip, the larger heavier vehicles will catch up (somewhat) to the mileage of the smaller ones. City mileage goes to the smaller cars, for the reverse reasons.
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Old 04-29-2009, 12:00 PM   #3
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Originally Posted by PaleMelanesian View Post
The weight of the vehicle doesn't really matter when it's cruising on the highway. It matters when you're accelerating.
thats not entirely true, but the difference lost to rolling resistance from the weight is easily lost in other variables. for practical application, i agree.

extra weight afford some ease in navigating hills as you are able to store more energy
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Old 04-29-2009, 04:07 PM   #4
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Quote:
Originally Posted by PaleMelanesian View Post
The weight of the vehicle doesn't really matter when it's cruising on the highway. It matters when you're accelerating. So when you're doing a long trip, the larger heavier vehicles will catch up (somewhat) to the mileage of the smaller ones. City mileage goes to the smaller cars, for the reverse reasons.
This is why trains can be soo efficient (CSX claims like 420mpg per ton).
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Old 04-29-2009, 04:08 PM   #5
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Originally Posted by thisisntjared View Post
thats not entirely true, but the difference lost to rolling resistance from the weight is easily lost in other variables. for practical application, i agree.

extra weight afford some ease in navigating hills as you are able to store more energy
Eh, you don't want weight when you're climbing a 22% grade, or some of the grades in San Francisco, which is like what 30%?
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Old 04-29-2009, 06:17 PM   #6
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Originally Posted by karnovking View Post
As I stated in my post about my trip mileage in my odyessy, I averaged 36ish mpg.

normally this vehicle gets about 19mpg. So that is an 89% improvement over the vehicle's average.

When I had my civic(1993DX automatic) I would average about 33mpg and get about 41mpg on a highway trip like that. That is only a 24% improvement.



Maybe this is part of the reason the GM 3.8L cars do so well.
Not sure about the Honda's but one reason for the 3.8 GM cars doing as well as they do it the engine management system and having a large engine turning relatively low revs for a relatively high cruise speed also helps.

Decent aero numbers help as well but I am talking about local models here so it may be different for the cars you are talking about.

Following Moore's Law you can reasonably expect a decent upgrade in engine management capacity about every model or two at the outside.

This may account for the differences between the two Honda cars.

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Old 04-29-2009, 06:55 PM   #7
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Instead of weight consider this ballistically.

Sectional density is the relationship between the aerodynamics of any projectile and the mass relaitive to the frontal area.

It takes more energy to get the heavier projectile to the same velocity, but the heavier projectile retains that energy for a greater distance.

Where greater sectional density begins to have a negative effect is when you have to navigate grades that are too steep to utilize the energy cost of climbing the grade to coast down the same grade. If you must waste energy downhill to keep your speed low enough to avoid a citation, then you will never get the mileage you would if the grade was not soo steep.

In those situations where grades are more steep the difference in mileage will favor the lighter vehicle, because it will not need braking energy to reduce it's maximum speed downhill, because its sectional density is lower, which makes the aero resistance sufficient to keep the maximum speed to a more acceptable level. DFCO will help on downgrades to eliminate fuel consumption on the downhill leg of the combination.

The gradient of the hill or mountain as well as the engine power available can be used to get better mileage than even a flat terrain, but as the grade percentage increases beyond a couple of percent then mileage will fall off almost without exception.

My normal situation consists of grade changes that are never more than an Interstate overpass, with elevation changes of only 100 feet in 20 miles.

I have found that my car can climb very slight grades while maintaining over 70 MPG at 45 MPH, while almost maxing out the MPG bar graph at 120-150 on the slight downslopes. This is in my Insight, and it may not apply to other vehicles.

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gary
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Old 04-29-2009, 07:35 PM   #8
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Eh, you don't want weight when you're climbing a 22% grade, or some of the grades in San Francisco, which is like what 30%?
thats hilled city driving... you waste the energy when you brake going downhill.
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Old 04-29-2009, 09:15 PM   #9
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I'd say that the heavier vehicle will see a greater improvement in mileage due to steady speed driving than a lighter vehicle, but the lighter vehicle still gets better mpg.
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Old 04-29-2009, 10:11 PM   #10
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I'd say that the heavier vehicle will see a greater improvement in mileage due to steady speed driving than a lighter vehicle, but the lighter vehicle still gets better mpg.
I don't know, I haven't found that to be true. If you want to coast the longest for what ever reason, be sure to be heavy as possible (freight train anyone?). The only way what you said would be true is if a comparable but lighter vehicle wasn't really comparable, as the heavy vehicle for what ever reason has really short gearing (high rpms) in lower gears except cruising while the ligher vehicle has longer gearing (low rpms) all around. Think Civic VX vs Civic Si with the Si's 5th gear ratio being equal to that of the VX. Also the Si is heavier than the VX, so this example works in this regard as well.
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