Getting maximum MPG on freeway

Hello Adam:

I've had my Prius for a couple of months now. I've only begun experiencing "stealth glide" (if that's what it's called) within the last few weeks. Actually, I wasn't even trying for it; it just started happening on its own.

Part of it was the mild weather we started having. But also I think that part of it has to do with the battery becoming better "conditioned". I read somewhere that it takes a while before a new battery can take a full charge.

There seems to be a correlation between the charge state of the traction battery and how readily the car will enter "stealth" or "warp" mode.

Also, it will happen more readily when the load on the car is very light, such as going down slight grades, no significant head wind, etc.

Best wishes,

 

In case you haven't come across Hobbit's webpage on this, there is a graph which shows a relationship between this 'margin for error' which he calls a plateau, and the state of charge (SOC) of your HV battery. The higher the charge the greater your capability to hold these glides.

Alternatively, you might like to try out another technique called Super Highway Mode (SHM) which I think may be a little easier to hold. An essential difference in the glides between these techniques, is that for SHM the ICE is on at a low RPM kept just at the point where it flicks between charging and not charging the battery, whereas in warp stealth the RPM is dropped lower and the battery is assisting with the glide.

You will need to consider getting additional instrumentation to help you with these though (like CanView or ScanGauge II). They give you the confidence to hold a particular pedal position. Sometimes it takes just a few extra seconds to settle down into a perfect glide and see your iMPG soar! I realised after getting SGII that I must have been previously over-correcting my pedal adjustments because of the minimal delays in the updating of arrows on the MFD.

Edit: Oops! I see you've just posted the same link. We must've been typing at the same time!

 

Congratulations to both you and Adam for exploring these techniques so soon after getting your cars. I think its a real tribute to the many masters who contribute to this fantastic website!

 

Good job, your gas mileage shows you understand how to take advantage of how the Prius works and you are trying to maximize the amount of time the ICE off. That's something every Prius owner should be doing.
To answer your question about freeway MPG improvement.....simply slowing down seems to have the greatest positive impact on MPG.
60MPH = 60 MPG+ in a Prius.
Beyond that, Take advantage of hills and intentionally go into a warp stealth on the downhills. Hold it until your speeds begins to bleed off.
I can maintain 60 MPH in a warp stealth on downhills regularly here in GA.
Every hill is an opportunity to exploit this car.
Every hill I crest, the right foot comes off the gas pedal and I lightly reapply to get the warp steath or glide.
Cruise control is remarkably adept at doing all this too.

 

OK, my list>

My Honda has a RPM gage, better seats, better navigation. Everythingelse so far my Prius kicks ###.
So any way to do this without adding a scan gage? I think my wife will say no to that for sure.

 

OK, I read most of the link but how does this work at 65-70 MPH if at all?

 

SHM also is dependent on optimal SOC, around 60% (6 blue bars). It is indeed executed at a low RPM, and I agree that it is much easier with added instrumentation -- though it's still not simple or intuitive. Lacking the instruments, you can try to hit it by gradually easing off the go-pedal until just before the ICE shuts down (watch the Energy monitor), with trial-and-error likely needed before getting a good sense of that threshold.

I find SHM's usefulness rather limited on the typical terrain in my neck of the woods. It seems to work best on level terrain where gradual slowing (into 40-45 MPH territory) doesn't impede traffic or cause a safety issue. I-95 through central VA, for example, doesn't meet those criteria! Instead, I generally use a combination of warp stealth (on hills) and a slightly higher RPM to maintain a steady speed (on the flat). If traffic is light enough and the hills aren't too steep, I'll keep the RPM steady on the hills too, losing speed on the uphill and gaining it going down.

In any situation, city or highway, I seek routes favorable for high-efficiency driving.

Which link?

SHM is rather impractical at those speeds. Warp stealth is quite possible, more practical on hills.

 

Oh, one other thing. If speeds or traffic aren't favorable for SHM, cruise control is probably your best highway option on level terrain. I've compared CC to warp stealth at 60 MPH on level and gently rolling terrain. There is no difference in fuel economy between the two techniques and CC is considerably easier. Be careful to kill CC before hitting significant hills, however. It is very aggressive in maintaining speed and will push RPM through the roof if needed for the ascent.

 

On flat roadways it seems like using CC is just as good or better than trying to not use CC. This could come in handy for those drivers who cannot seem to hold steady pressure on the gas pedal. Instead, they tend to accelerate then slow down over and over again. A few of my friends do this and it drives me insane.

I wonder what the net effect to MPG this type of driving incurs and if this is why some people get terrible mileage despite good driving conditions?

 

Thanks all, it be flat here in Florida so I will use the cruise at 56-70 speeds for long segments.

 

I should have put a disclaimer stating that other methods like SHM or WS P&G would create better results but I am not keen on pulsing on our freeways. It is bad enough driving 65pmh let alone pulsing from 60 to 50mph. I just don't think that kind of driving is safe or curteous on my route (I-80 in Sacramento).

So for high speed freeway driving CC indeed does work well as long as you don't encounter large hills.

 

Hi alevinemi,

Here is a method that requires no add on instumentation to get most of the SHM advantage. It was developed with the use of insturmentation, but as the instrumentation I was using is illegal based on a legal technicality (PC platform is multi-functional), I developed this method instead.

You may know that engines have timing (ignition timing in a gasoline car) due to the speed with which fuel burns. This speed is relatively constant, but the speed with which the cylinder approaches top dead center varies with RPM. As ignition time uses crank angle for triggering, ignition timing has to vary so that the fuel-air mixture develops a peak pressure just after top dead center postion of the piston. Consequently, even without a iginition timing indicator, one can gauge the SHM operation based solely on RPM, or at least most of it. With my instrumentation I found out this was 1280 RPM at 53 mph. And I correlated this to the instantaneous fuel consumption display.

Here is the technique. After getting on the highway stay in the "Energy" screen until you get 6 blue bars on the battery SOC. Now, wait a few more minutes as when the 6 blue bar lights, the battery may be as low as 58% SOC. After those minutes, you can expect the car to be at 60% SOC.

Now switch to the "Consumption" Screen, and find the instanteous fuel economy bar graph on the far right side (should have been on the left side for US cars). Drive along with the car at 55 mpg, and then lift the accellerator slowly, but without letting the IFE go above 75 mpg. The car will slow to 53 mph , at which case you apply an ever so light increase in accellerator til the IFE drops to 70 mpg. In ideal conditions (level, with little to negative head wind) the car will stabilise at 53 mph and 70 mpg. Someimes during the decelleation from 55 to 53, you will feel the jump that Dan speaks about.

This is asensitive condtion, and with even slight head winds, a passing truck may nock the car out of this condition, and it will decellerate. Just reperform the operation above and keep going.

In less than ideal conditions (cold tires, cold road way, cold air, heavy head or quartering winds) the stable speed may be as low as 51 mph. I have not been able to get this condition stable much below 50 mph ever, and in some conditions, the stable point can be as high as 56 mph with even higher fuel economy (hot tires/sunny day with warm roadway on a cold day - like 15 F and a slight tail wind).

Oh, and do this in the RIGHT LANE only.

PS, the tachometer is a better way to do this, as holding the RPM up hill give better mileage than guessing what the proper uphill mileage is for best efficiency. With the Tachometer you drive along decellerating up shallow hills and accellerate down them, and the car stays synced in SHM. With the bar graph fuel economy display, hills are a guessing game.

 

I've had my Prius for a little over 1 year and 21,000 miles. When I first got it I played around with all the techniques you've mentioned. My best tanks (mostly highway) have been ~63 mpg. For what it's worth, here is what I've come to learn...

There are only a few ways to "waste" energy.

1) Braking - to include just letting completely off the accelerator in the Prius (this is why we tell you to use the "glide").
2) Turning (sharply)
3) Air resistance
4) Rolling resistance of the tires
5) Conversion loss from running down the battery and then burning gasoline to recharge it

You already understand how to avoid braking (anticipate stops, use "glide" instead of "coast") so that's good. You minimize rolling resistance of the tires simply by keeping them properly inflated.

Honestly, I think that SHM is stupid because it promotes conversion loss. I only try to drive in "yellow arrows" mode in one situation: if I had to unexpectedly hit the brakes, and now I'm trying to get back up to speed. That way I'm using the energy I just stored via regenerative braking to do it, hopefully (mostly) negating the fact that I had to brake. To me, that seems to be in the spirit of the design. Trying to cruise around in "yellow arrows" all the time does not seem to be in that same spirit.

So really, on the highway, it's all about minimizing air resistance. Since we can't control the speed or direction of the wind, we have only one option: slow down. The physicists here have pointed out that air resistance increases exponentially with speed. There is a big difference between 60 mph and 70 mph. Find a reasonable range such as 55-65 mph that you are comfortable with. Try to just keep your pressure on the accelerator steady, slowing to 55 when you reach the tops of hills, and speeding up to 65 when you hit the bottoms. In good weather this should easily get you 60+ mpg.

Tim

 

on my 2008, i've reached about 27K already. i noticed at 20K+, the battery screen started to get fully charged, meaning (i hope) the battery is settling in. EV modes can be readily felt, gliding is much longer and overall, the battery just feels stronger.

on the freeway, i set it to the consumption mode, (showing the level of mileage on 5 min intervals) and using my PERIPHERALS, i try to keep the yellow bar as high as i can. using the CC can be beneficial, but when going uphill, it really strains the ICE and i prefer to back off the throttle, even if i have to go a bit slower.

bottom line is that i have hi-po potenza's on my touring rims, going 75-80 mph to and from san francisco, i can still get 40-42 on the highway. no, i'm not a hypermiler and yes there were plenty of faster prius' on the road.

 

You misunderstand SHM. Properly executed SHM will draw virtually no power from the battery. You are correct of course that conversion losses from running on the battery are inefficient. Those that first described SHM are among the nation's leading Prius hypermilers and they have a keen understanding of that concept.

 

I mirror your driving exactly, and for the same reasons. That said, you misunderstand SHM. It is an attempt at "optimal gearing" at high(er) speeds. Hobbit explains it best.

I personally don't bother because the speed is too slow, and the gains too minor in my opinion.