Hypermiling – Evaluating common fuel-economy tips on the road

In "Hypermiling - The new menace on the road?" I wrote about drivers who compromise safety and traffic flow in an effort to get the most out of a gallon of fuel. And while some people may employ annoying or even dangerous hypermiling techniques such as driving 45 mph on the highway, turning the ignition key off when coasting, or tailgating a big rig for aero, many people are just looking for a safe and effective way to save fuel.

Some of the responses to my previous blog encouraged me to research and report more on hypermiling. I went to cleanmpg.com  for information from the "mild" to "advanced" ways to hypermile.

Armed with our Prius Touring that we tested for the June issue, I headed out on a weekend camping trip with my son in an attempt to beat the EPA cycle without doing anything unsafe or annoying to my fellow motorists.

After much study and practice, here are a few of the hypermiling tips that I tried, some successfully, some not so much:

1. Maximum tire pressure: Among the most-common hypermiling advice is to fill the tires to the maximum inflation pressure on the sidewall. However, this tip violates my safety criteria: You should inflate the tires to the recommended pressure set by the vehicle manufacturer. High inflation pressures can adversely effect emergency handling by upsetting the balance of the car, minimizing control on bumpy roads, and hurting grip on slick ones. It will also cost you money by prematurely wearing out your tires. So for a trip with my son, this one was out. (Learn more about tire safety and care.)

2. Keeping the speed down: OK, this is an obvious one. In our own fuel-economy testing, just going 10 mph slower can save a lot of gasoline.  Getting in the right lane on a desolate highway and holding the limit is one thing, but my trip was mostly smaller roads. While I tried to go the local speed limits, I quickly found myself being tailgated by annoyed locals wondering what was wrong with me. So I have to admit I did go a bit faster, but only when I was sharing the road with others.

3. Forced Auto Stop (FAS): The Prius Hybrid system allows the engine to turn off when not needed. It does so seamlessly and safely, and it is possible to drive in a way to maximize the engine’s off time. On a conventional car you would have to turn the ignition key off to make this happen (and then quickly back on so the steering wheel doesn’t lock up). But while driving in a car with the engine off may save fuel, the car may not save you if you need to stop quickly, accelerate, or turn to avoid something. Most cars will lose their power steering and brakes very quickly in such a situation, so don’t try this one. Because the Prius does this on its own, I did my best to keep that engine shut–off when I could.

4. Timing the lights: The trick here is to hit the greens as much as you can. Why go full-steam up to a red light, slam on the brakes, and then start off from a stop again when the green comes? Smooth out your driving by slowing before the red to keep some momentum as you go through the light. This takes some practice, but it does work. However, people driving behind you may not understand what you are doing and get frustrated as you slowly coast to the light. Another problem is it is easy to convince yourself that it is OK to coast through a stop sign or go through a light that has just turned red because you are doing your part to save the Earth. Do not do this! Actions like this lead to many deaths each year at intersections.

5. Pulse and glide (P&G): For me, this one was the coolest. For everyone who thought maintaining constant speed was the most fuel-efficient way – all things change when you are in a Hybrid. Under 41 mph the Prius can coast with the engine off without charging or using its batteries if you push down on the accelerator pedal ever so slightly. The P&G technique can only be used on slower roads, preferably with nobody else around you. The trick here is to accelerate briskly to 41 mph, and then coast as long as possible with the engine off and the batteries not being used, and when you get down to, say 34 mph or so "pulse" again back to 41 mph. On my trip, I took a lot of back roads and came back early in the morning. So with the road to myself, I wound up using this technique a lot. While unintuitive to do initially, when you think about the fact that the engine is only running several seconds each minute, the technique begins to make sense quickly!

Bottom line:
I drove 283 miles over the weekend and got (drum roll please)… 68.2 mpg average on the trip computer. Even I was surprised when I looked at the Prius-ometer at the end of the weekend. And 68+ mpg is downright impressive compared to the 42 mpg overall we got in our testing, but then again I didn’t do any of the stop-and-go city driving that we simulate in our tests. At $4.00 a gallon I saved about $10.00. But at what cost? I have to admit that in my concentration on fuel savings sacrifices were made. For instance, I missed a few scenic spots, talked to my son less, and at one point tried to talk him into "holding it" instead of stopping in order to maintain momentum.

When my wife heard about my mileage, she was eager to try out hypermiling. The next day we drove her 2006 Prius to the local supermarket with me at the wheel demonstrating my newfound talents. The trip was all back roads that allowed me to P&G nearly the entire way. The few lights I hit, I was able to time perfectly. As I backed into the space at the market (a hypermiling trick to allow a quick get-a-way, minimizing warm-up time) my wife’s jaw dropped at the display reading 92.7 mpg. On the way home, she tried it out herself. After about five minutes of driving, total fuel savings: None. It took us that long to make it out of the parking lot because she kept making wrong turns as she focused on the fuel economy display. She still tries to moderately hypermile her Prius, but she’s perfectly content getting 50 mpg and still having the ability to hold a conversation with our son.

—Jake Fisher

Also read, "Downsizing for vacation."

Comments

[Leslie]

I note some comments about coasting down hill. That is illegal in my state (Utah) and may be in other states. Utah Code section 41-6a-1709.

[jack]

It is really necessary to save fuel while driving.

[OEM R & H Engineer]

To David:

Some manufacturers will state a split pressure for light loading and another for full vehicle loading, commonly seen on German sedans. For a sports car, the handling characteristics have been tuned by the development team and tire suppliers to fit the targets for the vehicle. But, even for a sports car, considerations like ride comfort and tire wear have to be considered or they will translate into customer complaints. The only exceptions would be a truly high end sports car where the typical customer would accept the trade-offs. Increasing the tire pressure would give a customer a noticeable increase in "sportiness" by reducing some compliance in the vehicle, but there would be a point of diminishing returns due to changes in the tire patch shape.

Regarding Q2, yes, tires are "matched" to the vehicle's performance. Even tires of the same brand and model can be different constructions from car to car, the only true indicator is the DOT code molded on the sidewall (near the 4 digit date of manufacture code). That's not to say that other tires won't perform equally in most respects and better in a few respects, but all of the development work would have been done, and hopefully optimized, on the construction codes available on the vehicle as original equipment.

Regarding the tire pyrometer, I wouldn't recommend it for tread wear investigations. It is often used, for good reason, for racing applications to determine how the tire is working across its width during lateral force generation, but I would suspect temperatures gathered from very low slip, straight line driving would not be repeatable or accurate. But that's branching out of my area of expertise, so take it with a grain of salt...

[Steve]

I've been using the 42/40 tire pressures in my Touring Edition, and I'm very happy with the wear on my tires.

[DAN H]

Toyota recommends 35psi front & 33psi rear for my 2008 Prius Touring with for the factory Bridgestone equipped tires which have a 40 psi MAX rating. i have 3 tire gages and none read the same in the 30-40 psi range (a 3 psi or ~ 10% total variance). IMHO other folks probably have this problem too and since i no longer have access to lab calibration i don't know which one (if any) is correct. i assume that Toyota and Bridgestone take this into consideration along with ride, handling, much empirical data and legal liability when recommending tire pressure. I would not feel comfortable using anything over the MAX rated pressure as shown on the tire unless Bridgestone would give written approval. With the huge legal costs they incurred with the Explorer debacle they are, or should be, gun-shy. Just another opinion.....