To Boost Gas Mileage, Automakers Explore Lighter Cars

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Analysis

In determining the effect of vehicle weight on vehicle gas mileage, it is important to first understand the type of relationship that exists between weight and energy consumption.

There are a number of inefficiencies and sources of energy consumption in a vehicle. Two terms, however stand out as being the most significant from a weight perspective: the energy required to accelerate the car, and the energy required to overcome road friction. Since of these factors scale linearly with vehicle weight, it can be said that energy consumption as a whole scales roughly linearly with vehicle weight.

It is important to note that fuel efficiency η is actually a measure of distance per energy. Therefore, fuel efficiency in kilometers per liter (1 MPG = 0.425 kg/l) should scale inversely to vehicle mass. Therefore, for this analysis, weight data and fuel efficiency were related as seen in Eq. (1), where η is EPA estimated fuel economy, M is vehicle curb mass and a, b, and c are fitting parameters.

η = a M-b + c (1)

With (1), it is possible to determine how much of an effect vehicle weight has on fuel efficiency.

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5. Ski or roof racks

If you drive an SUV and keep the ski rack on the roof all year, it will impact your gas mileage. Ski racks weigh your car down and put a strain on your tires, which will cause your car to use more fuel. Some ski racks may not be removable, and you may just have to live with it. But, if it is removable and you don’t use it very often, consider taking it off and storing it to increase fuel efficiency.

Understanding the Charts

Cost/Gallon: When driving 50 mph, price per gallon is assumed to be the same as the pump price. At higher speeds, the estimated price per gallon is increased based on the additional fuel you use by driving faster. The price of fuel doesn’t actually go up, but this is a useful way to put the cost of driving faster in context.

Cost/Gallon = Fuel Price at Pump × Est. MPG at Higher Speed÷ Est. MPG at 50 MPH

Cost/100 Miles: This is useful for estimating fuel costs for long trips.

Cost per 100 Miles = Fuel Price ÷ Estimated MPG × 100

Save Money: This graph shows how much money you can save by slowing down. Cost per gallon and cost per 100 miles show the difference in cost at your typical highway speed and the cost at the reduced speed.

Savings per extra time incurred is calculated as the money saved by slowing down divided by the extra time incurred:

Savings = (Fuel Price * (1 ÷ MPGT)-(1 ÷ MPGR)) ÷ ((1 ÷ SpeedT)-(1 ÷ SpeedR))

where

  • MPGT = fuel economy at typical highway speed
  • MPGR = fuel economy at reduced highway speed
  • SpeedT = typical highway speed
  • SpeedR = reduced highway speed

3. Excess weight

It’s always a good idea to keep a spare tire in the trunk. Any other excess weight, particularly heavy items, can weigh your car down and decrease gas mileage. Make sure to clean any unnecessary items out of your trunk and backseat to maximize your car’s fuel economy. This can increase your fuel efficiency by as much as two percent.

What factors affect your MPG?

Another reason that towing affects your gas mileage: drag. Drag is the effect of wind pulling on the vehicle as well as what it’s towing. Drag forces the engine to work even harder. If your load is bulky, if you have a roof rack, or if you’re towing a trailer, you will encounter even more drag as your load offers more surface area for the wind to pull on.

According to a Consumer Reports test, a simple roof rack decreases fuel economy by 5 percent. At highway speeds, more than 50 percent of engine power is used to conquer drag. 

I still didnt answer the original question.Why Mass?

Back to my daughter's question. Why? My first guess was that with a larger mass, you have to use more energy to get the vehicle up to speed. If two cars are both traveling at 70 mph, the more massive car will have more kinetic energy — remember:

But it can't just be the kinetic energy. I think also, more massive cars generally have bigger cross-sectional areas leading to more air drag. More massive cars have bigger engines that waste more gas with more moving parts to lose energy to friction. Bigger isn't always better, I guess.

One final note. I just realized that the data I used for fuel efficiency includes some sport cars. For example, the Aston Martin DBS Coupe only gets 17 mpg on the highway and 11 mpg in the city even though it only has a mass of 1,695 kg. Again, the problem is the big engine.

Top image: David Guo's Master/Flickr

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