How does weight affect a vehicle's efficiency?

How does weight affect a vehicle's efficiency?

Over the last few decades, the average weight of a vehicle sold in the U.S. climbed steadily after surviving the oil embargoes of the 1970s. Today, however, auto companies are putting a lot of effort into reducing weight – Lotus set up an entire lightweight structures division, BMW is investing millions in the production of carbon fiber and Jaguar loves aluminum – because every ounce you take out of a car improves the vehicle's performance and fuel economy. Options for weight savings that automakers are investigating include installing things like plastic fuel tanks and using carbon fiber instead of steel. As we discovered in a previous Greenlings, carbon fiber is a remarkable, lightweight substance that will begin see wider use as prices (invariably) come down.

Today, one of the main reasons automakers want to reduce weight is because it's a great way to increase MPG numbers. AutoblogGreen reader GenWaylaid sent in a Greenlings question about how, exactly, reducing weight helps efficiency. We investigate his query after the jump.

Let's start with the easy and simple numbers. The EPA says that for every 100 pounds taken out of the vehicle, the fuel economy is increased by 1-2 percent. Based on a gallon of gasoline costing $2.58, this translates to savings of between $0.03-$0.05 a gallon. Of course, 100 lbs. in a small hatchback is going to make a bigger difference than those same 100 lbs. in a Tahoe, so make reasonable assumptions about what going lightweight can offer you.

For a more detailed look at what's possible, consider a report issued by the Aluminum Association, Inc. based on research by Ricardo. The chart below shows that for a small car with a 1.6-liter engine, reducing weight by five percent led to an increase in fuel economy of 2.1 percent on the EPA combined rating. Eliminating ten percent of the weight provides a 4.1 percent mileage boost and a dramatically significant twenty percent weight decrease improved fuel economy by 8.4 percent.

Ways to remove weight in your own car
Since the automakers have already made their decisions regarding how heavy your vehicle is going to be, it's up to each driver to eliminate weight whenever and wherever possible. The place to start is in the trunk and in the back seat. Got some old boxes in there you never use? Put 'em in the garage. Been carrying around a set of golf clubs since last weekend? Put 'em aside for now. Got a dead body in there? Um, that's an entirely different set of issues.

Once the obvious detritus is removed, there are a few other ways to lighten the load. While we have to admit that losing a bit of belly fat can technically make a difference and is probably a healthy choice, we don't want to put too much emphasis on that angle – it's been called out already. People who know what they're doing (and by this, we mean they have a reliable back-up plan, either a cell phone, an Uber app or AAA) sometimes ditch the spare tire and just deal with it when a tire goes flat.

An extreme example of a way to drive around with less weight would be to only fill up the tank half way. Sure, you're trading time for efficiency, but if you live near a gas station and don't drive too often, this could be a reasonable consideration. Gasoline weighs about 6 pounds per gallon and diesel about 7, after all. Filling up to just half of a ten-gallon gasoline tank means you're taking 30 lbs. out of the car. Consider it.

The weight of the future
One of the rarely discussed realities of the U.S. auto industry is that even as fuel economy ratings for most vehicle classes stayed about the same for the past few decades, the vehicles have gotten heavier with all of the added entertainment, comfort, safety and – in some instances – performance features. These numbers were able to diverge because engineers were making the vehicles more efficient in ways that didn't involve saving weight. Now that the industry is focusing on shedding pounds – something that will become even more important once heavy automotive batteries for plug-in vehicles start appearing more often – the gains made with heavy cars can be applied to lighter vehicles. After all, U.S. cars still have a long way to go to reduce weight, and we'll all reap the benefits thanks to reduced fuel usage.

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