Monday, June 30, 2008

How to Drive for the Best Fuel Economy: Debunking the Myths, Part 1

In today's article about saving that precious clear liquid that almost all of us world-wide are addicted to, you're going to learn a fun trick: Don't drive slow, accelerate slow (but not too slow, I'll explain)

Driving super slow doesn't save gas, in fact, it wastes it. If your car is not in the highest gear possible, you're most likely not getting the best fuel economy possible. I'm sure you've seen those people in economy cars, driving around at some insanely slow speed, thinking about all the fuel they're saving, but in reality their engines are working harder then they need to, spinning faster and sucking down more gas then the fellow who speeds past them.

It all has to do with the gearing of your car's transmission. When you're in first gear, your engine is spinning rather fast, and the faster it spins, the more air it sucks in. The more air your engine sucks in, the more fuel it needs to balance out the mixture (at least on gasoline engines). So, you shift into second (or your car does it for you, if you're one of those unfortunates with an automatic transmission), and now the engine is doing less work to keep the car going the same speed, or the same work to go a higher speed. Each time you go fast enough to shift up a gear (and do so), the engine spins slower, using less gas and wearing out slower.

So, the first reason going too slow wastes gas: If our car is not in its top gear, your engine is doing more work for less distance.

Now, once your car is in its top gear, going faster and faster will lose you fuel economy once you exit the point that your engine is efficient. This point is different for different cars. For example, my 91 RX-7 got it's best fuel economy at an average of 72MPH on a long highway trip. After that point, if I go any faster, the MPG drops. On regular cars, that spot is a bit lower, around the low 60mph range, depending. On vans and SUVs, the fuel economy "happy zone" is a bit lower, (probably around 50 to 60, depending on your car) because wind resistance is such a factor at higher speeds.

So how can you apply this?

First off, the faster you're in the next gear, the better. The longer you're in a low gear, the more gas is being wasted on going less distance. Though, if you shift too soon, the engine will lug (in older cars your engine might even knock and ping!) and you'll end up having to put your foot down more and more just to get up the tiniest of hills with your fuel economy dropping the whole way.

So, since shifting too soon doesn't work, how about flooring it to get up to speed so you can shift faster? That won't work either, as jackrabbit starts are a huge waste of gas. Ideally, you want the engine to be doing as little total work over the entire course of getting up to speed as possible. A good rule of thumb if you don't know how fast to accelerate, is try to keep the engine relatively quiet. If that doesn't sound scientific enough for you, you can buy a decent vacuum gauge for $36 from Summit Racing (dot com!), so that you can tell how hard your engine is working. If you go that route, try and keep that needle above ~9.

Mentioning "Top Gear" so much makes me think of the TV show, so by all means, have a read about what they have to say about fuel economy at Clarkson's top fuel saving tips, the fellows who brought you this video:




If that isn't interesting to you, you can always read about the fellow pulling 59mpg in a plain old Honda Accord.

I hope this article has been helpful to you, feel free to share what tricks have worked for you in the comments.

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