Here's my challenge and I am in need for all kind of suggestions and input for driving strategy.

• I live in Texas, where major interstate highway speed limit is usually at 70 MPH.
• The neighborhood I live in, the route I take for everyday commute, usually the speed limit is 45-55 MPH.
• I have my 2007 Prius - Woody, for about 2 months.
• The best MPG I have made it this far is about 45MPG which included few much longer trips for special events. At the moment, current tank which is more close to my everyday usage, 40% tank gone, I am at 44.1 MPG.

As we all have learned here Hybrid 101, Pulse & Glide. Prius achieves better MPG performance with lower driving speed.

With that said, what's the best driving strategy when we get on to I-35 where speed limit is 70MPH ? My husband the other day drove on I-35 at 48 MPH, while he's enjoying the excellent 99.9MPG bar, I was about to have a heart attack and worried the car behind us couldn't stop soon enough or change lane earlier enough before it smash us.

I very much gave him the warning "you either make it to above 55 at least or get off the highway" ( and I will drive ). He's very unhappy about the notice, unwillingly hit the gas panel to speed up, but I can understand the inner struggle. When you hit the gas panel, you get the speed, but your MPG just drops like free fall all the way down to 20 something, while your speed just barely makes it to high 50s or low 60s. I am all OK to have other cars to pass me by or give me the finger, but the safety issue concerned me.

Regardless whether it will be a long while before Woody is fully broken in. What driving strategy should apply to the route I am facing on daily basis ? Completely avoid highway is not an option and I don't think it's even acceptable.

I know that whole MPG testing / number is not realistic, ( whatever...it's 100% accurate for my 98 Accord for the past 9 years and still true today. ) but I just want to know what other hoops I have to jump through before I can achieve and maintain a decent MPG level without causing or even in a car accident due to "driving-like-a-turtle" strategy ? I am not even looking for high 50s MPG, the goal I have now is 46 MPG.

Any input, suggestion and opinions, are most welcome, but please spare me the charts with dots.

Thank you in advanced.

 

Hi,

Let's start with some data:

This chart includes some high-speed driving data from an NHW20 model (your Prius) last summer. As you can see, speeds of 70 mph and higher are pretty rough on both the NHW11 and NHW20 Prius mileage.

Many states have a 65 mph trucker speed limit and some trucking companies have policies of using 65 mph. If you can find one of these trucks, follow them at a comfortable following distance and set the cruise control. You'll get great mileage and still have plenty of time to respond to road debre. Don't follow gravel or construction equipment haulers because they shed bouncing gravel and rocks.

Looking at lower speeds:

There is a boundary speed, 42 mph, between low-speed ICE/EV mode and ICE-on always mode. If you can stay on one side or the other of 42 mph, you'll avoid unnecessary ICE start/stop events. In city driving, try to avoid using the brakes either by route planning or predictive braking. Predictive braking includes a light braking, an early touch, when approaching a red light so you maintain some momentum for a little bit longer. You're giving the light a chance to change so you don't have to come to a complete stop.

One last trick is recognize the engine becomes less efficient at higher rotation speeds. If you hear the engine rev up, try to reduce the accelerator, IF POSSIBLE! One trick when approaching an overpass is to speed the Prius up just a tad and then let the speed decay, not rev up the engine, going up the overpass slope. Then just after the crest on the backside, pick up the speed again. It takes a little patience to 'wait for it' but it pays off in fuel savings.

As a general rule of thumb, try to increase speed on down-grades when gravity helps. When going up a slope, keep the speed constant or better yet, let a little bleed off.

I hope this helps,
Bob Wilson

 

I have a very similar commute in terms of the requirement to access the freeway system (average speeds 75 - 80; no really that's what they are) for approximately 15-20 miles each day. With the same concern for my personal safety, I have to maintain speeds over 65 mph.

What I have found works best is a version of P&G at highway speeds. I no longer use cruise control as it exclusively geared toward maintaining a fixed speed and with with the hilly nature of my commute, I lose 5-6 mpg using cruise control.

Second, I use the anticipatory tactics described by the previous post and anticipate my uphill sections by slowly increasing speed on the downhill or flat sections leading up to the uphill and then allowing the speed to "bleed" down until the crest. My speed will fluctuate, during my commute, by as much as 15 mph so it is important I stay in the right lane thereby reducing the impact on other traffic. I also note that when there is heavier traffic and there are cars in my lane behind me, I will abandon the low end of my mph range and maintain speed out of courtesy to the other drivers.

With more active use of the accelerator and gliding I now regularly achieve mpg in the 52 - 55 range (actual calculations based on fill up gallons vs mileage as opposed to the computer "estimate"). Now, keep in mind this is with a highway/city ratio of 60/40 in my case.

I also found, with a great deal of surprise, that I appear to achieve a better overall mpg if I allow the vehicle to extend into the upper range of the mph spectrum than when I try to stay at or near the posted speed limits (65 - 70). The only explanation I can come up with is that I am able to "glide" for longer periods of time coming off of the higher speeds and I do a more effective job now of "lightly" accelerating back up to the upper speeds. I don't have a computer hooked up to my Prius, so this is simply anectodal evidence and not supported empirically.

The key, however, is to also maximize your mpg when you can use the city streets. This helps to offset some of the inefficiencies of highway driving.

Try it out and see if your results show improvement.

Good luck.

UTpiper

 

I find that all this attention to gaining a few mpg often comes at the expense of one's attention to the road and other vehicles, and by nature of the speed variations, can sometimes even pose a hazard, as you mentioned. Just think what will happen when a million more Prius drivers are "pulse and gliding" to work in the morning. I also think if you are going so slowly as to cause other drivers to flip you off or get upset, it is ultimately a strategy that will backfire by further polarizing the "Greens" vs. the SUV drivers. I hate when I see a Prius holding up traffic like an old VW bus.

Why not just drive the car as you normally would drive a car, perhaps making a few subtle changes? I keep my the freeway speed between 65-70mph, back off slightly on grades, and minimize the A/C use when possible. Sometimes I use electric mode the last few miles home at 38mph on a lightly travelled boulevard. All of this allows me to achieve 45 mpg overall without any stress or anger from other drivers.

I was holding 46.7 mpg, but I recently reduced my tire presssure from 40 to 37 psi to improve the ride and improve crosswind stability, and it dropped the mileage a bit. It is still great mileage, and I enjoy the car a lot more this way.

 

Hi,
Predictive braking depends upon being aware of all traffic. I find I'm more aware, not less of the surrounding traffic.

This is one reason why I advocate steady, slow speeds. I'll take a glide when conditions permit but my testing suggests P&G really needs significant speed changes to be effective. In a Prius, smaller speed changes can eat up the fuel savings with more engine start/stop loads. Starting and stopping a Prius engine takes fuel energy.

This is handled by route planning so you can drive in slower traffic. For example, driving on an access road instead of the divided highway or taking an urban street short-cut instead of a highway. But 100 years of pre-hybrid car designs and subsequent traffic planning has led to what we call "normal" driving practices.
Non-hybrid cars have terrible, low-speed efficiency. Such cars are more efficient at highway speeds without stops. So drivers are conditioned to drive faster even over a longer route to minimize fuel costs. But introduce the hybrid drive and suddenly it is cheaper to drive the direct path at a slower speed than a longer path at highway speeds.

Physics tells us that with regenerative braking, a vehicle should be more efficient at slow speeds. Hybrids give us a chance to get back to driving the shortest path instead of a longer, high speed route. But it takes a hybrid drive system to make this mind-set change.

You are learning the non-normal way of driving that hybrids make possible. For a gas-only driver, this is incomprehensible because the current gas-only cars work so poorly at low speeds.

An active suspension can achieve this same effect with high pressure tires. Today, the 'soft' part comes from air compression and heat loss in the tires. An improved shock and suspension system can 'dial-in' the ride and handling desired without the tire heat loss.

Bob Wilson

 

Great points Bob - thanks. I do find myself taking surface streets across town more frequently, and find it relaxing and pleasant while the commuters are all slogging it out on the 4-lane. And it helps my average mpg, too!

 

Actually I want to thank you. When you posted "normal driving" it reminded me of the abnormal performance of gas-only vehicles.

Vehicle drag follows a fairly simple formula:

drag = (coeff_of_drag * velocity**2) + rolling_drag

This means a speedier vehicle goes up faster than the speed increase by the velocity squared. The Prius and similar hybrids follow this natural law and deliver better City MPG than Highway MPG.

In contrast, Gas-only cars have worse City MPG at low speeds. This is more than just the lack of regenerative braking but also includes operating the engine in inefficient rpm ranges. Internal combustion engines are terribly inefficient at low speeds and though diesels are less bad, they have problems too.

Your comment helped me realize that our hybrids are "normal" because their performance follows the laws of nature. In contrast, ordinary vehicles perform counter to the laws of nature.

Thanks,
Bob Wilson

 

On the highway I get 48 MPG US at 65 MPH - basically level ground, using cruise control and A/C on but it was not hot. Wind will have an effect as well.
I get 45 MPG US at 72 MPH. Same conditions as above.

The original posters targets are achievable. My car is also new, BTW. I'm sure the Texas heat has a lot to do with our differences.

 

First of all, I thank you all for your kind and informative feedback and inputs.

Even though I own a Prius, I don't consider my motive of having it is as noble as most of fellow owners here.

True, MPG performance is important, saving the environment for next generation is important, but, at the end of the day, I want to be able to make it home in one piece without any injury; at the end of the month, knowing I can afford the monthly payment without much struggle. It's the reality no matter what.

People around me consider me a freak ( nut ) because I know how many miles I drive in any given month, I keep track of my cars' MPG for years, I plot the best route to work, the shortest route, or the quickest route from point A to B. Most of the time, I know the distance difference between the shortest route and the quickest route.

To fully utilize a hybrid like Prius, optimize its performance, it will take time and mind. There are plenty of reasons why Operation Research in engineering department is required for at least 3 credits, and 4 credits required in math department. It's a serious business. In my opinion, in order to achieve excellent MPG, you very much require to have good knowledge of physics and math.

While we have Mr. Wilson supplying us with those detail experimental results and chartings, I still believe safety outrank everything.

Driving in low speed is not always safe.

Reading all the feedbacks and inputs so far, I am glad I have proved I am right.
That's enough for me for now.

 

GACK!! You've found me out!!

By 'accident,' I signed up for a graduate level operations research class when I was picking up college classes in the service. It remains one of my favorite courses and great fun. Some of the officers below O6 didn't know what to make of this sargent E5 but I never had any problems with the upper grades.

Bob Wilson