Lessons in Fuel-Efficient Driving

One of the interesting features of our Prius is that it keeps a running tab on your current gas mileage. You can see both the mileage at any given moment or the average over your trip. Having such easy access to this information while you’re driving subtly teaches you how to drive more efficiently. Here are a few things we’ve learned.

Coasting makes a huge difference on your gas mileage. One thing this data has taught me is the huge value of coasting, particularly through a series of stoplights. Stopping and starting eats a lot of gas – our gas mileage during acceleration goes down to as low as 10 miles per gallon. Coasting, on the other hand, uses virtually no gas at all.

Before adjusting my driving, I had a strong tendency to leave a stoplight, accelerate to the speed limit in town, then often find myself hitting the brake and stopping again as I approached the next stoplight. That meant I was doing a ton of acceleration, then losing most of that speed by braking again just a block later.

Instead of doing that, I’ve found it’s just as quick (and way more energy efficient) to coast as much as possible through long strings of stoplights. I accelerate up to roughly the speed limit, then I coast for a while, particularly if the light ahead of me is red. Almost without fail, I catch up to the car ahead of me just as they’re accelerating away from the stop – and I already have some momentum going forward, which means I don’t have to accelerate nearly as hard to get back up to the speed limit. It doesn’t take any longer and it saves money.

I tested this out driving through the town where I live and the difference was tremendous – doing this added about 25 miles per gallon to my mileage through town.

Driving 75 on the interstate is substantially less fuel efficient than driving 55 on a two-lane highway. One regular trip for us is driving south to the West Des Moines area, about a 35 mile trip or so. We have two routes to get there that are roughly equal in length, but the interstate is a bit faster. On the interstate, of course, we drive around 75 miles per hour to keep up with the traffic. On the other hand, we can take the highway and go around 55 miles per hour. The highway usually takes us about six minutes longer to get to our destination, so before getting our Prius, we’d simply always use the highway.

But here’s the kicker. If we take the interstate, we would get around 38 miles per gallon. If we take the highway, we get about 52 miles per gallon. So, if we take the interstate, we use 0.92 gallons, but on the highway, we use 0.67 gallons. That’s a savings of about $0.48 on the trip, even in our relatively fuel efficient car.

This changes the equation just a little bit. The two lane highway is far more scenic than the interstate as well – there are many more interesting things to see and talk about along the highway route (meaning it’s easier to engage the kids). When you also toss in the fact that it’s cheaper – and it would be a much bigger difference in a less fuel-efficient car or if the price of a gallon of gas were higher than $1.94 – the balance starts to shift towards the slower route. Does the balance actually shift? Not entirely – for us, it still depends on a number of factors (the time of day, the presence of kids, and so on) – but the balance of values has changed.

Wind resistance makes a tremendous difference in your drive. Simply put, driving on a windy day (unless the wind is consistent and at your back) is incredibly inefficient.

On a recent windy day, my family and I embarked on a lengthy road trip where the wind was mostly in our face. This forced us to accelerate quite a bit more to maintain speed – and it pushed the gas mileage down about 35% (29 versus 44). As a test, I drove with the wind on another windy day and found that it improved our mileage by only about 10% (48.5 versus 44).

Thus, unless the wind is very, very consistent and at your back, a windy day will hurt your gas mileage. If you have an optional trip to make and there’s a heavy wind outside, you’re better off delaying the trip. That’s what I’ve already done twice since seeing the impact that a heavy wind can have on gas mileage.

Turn off your cruise control in hilly areas. In virtually every car I’ve used, cruise control has been a great tool on flat roads. It helps me control my slight lead-foot tendencies and seems to do a good job with gas mileage. The data from our Prius backs this up – on flat roads, that is.

If you enter a hilly area, though, cruise control is very inefficient. Instead of maximizing your speed going down hills and using that momentum, cruise control instead tries to keep the car within a few miles per hour of your set speed.

Since it can’t read the road ahead, it doesn’t know what’s coming up. You do. Take advantage of that and turn off the cruise control in hilly areas. I turn it off any time I go downhill or uphill, since it seems to be more efficient to build up speed going down the hill (getting well above your cruise speed) then coasting at the bottom until you get back to your cruise speed, and doing the opposite on hills (allowing yourself to get well below your cruise speed instead of accelerating into a hill).

In the end, our best value from the Prius might be the ability to actually see how our little driving choices affect our gas mileage – and how we can make better choices to vastly improve that mileage. As time goes on, these better choices become ingrained in our driving habits, making the more efficient choices our natural choices – ones that we’ll carry on to other cars. Fuel efficient driving doesn’t cost you time – it just saves you money.

Personally, I’d like to see all cars have a fuel mileage indicator. It’s been an invaluable tool for directly teaching someone how to drive more efficiently – and it’s easy to see the benefit when you go to the gas pump.

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