Need help getting better MPG

Thanks, for the comprehensive post. I am still confused though because I wonder why promote the 1/2 iMPG to MPH rule (I know I must be missing something) when about half away down through your post you mention iMPG being as much as 1.5 times MPH and conclude with a sentence saying that iMPG can be well above MPH and the car still accelerating efficiently?

 

Thanks! My SGII also shows IGN 14 (best efficiency) when accelerating at about 1200 RPM similar to that in superhighway mode (SHM). And this brings a related question: If the engine is more efficient at higher RPM, then why is it that the engine is at around 1200 RMP in the SHM (not at higher RPM)?

 

I think for the Prius pilot who doesn't have the benefit of added instrumentation, keeping the iMPG anywhere above half the vehicle speed will work for a variety of situations. It could be relatively aggressive (iMPG = MPH/2) or gentle (iMPG = MPH+). Exactly where will depend upon his/her (and following traffic's) tolerance for leisurely acceleration. The most important thing seems to be keeping ICE below ~2400 RPM, which the iMPG > MPH/2 rule accomplishes over a wide range of speeds.

These days I choose to keep it gentle when traffic allows. I should emphasize that I always put safety first. From there I try to balance fuel economy and courtesy. During my morning commute, where traffic is quite light at 6:00 a.m., I drive very gently. In the afternoon, much less so.

Here is a detailed discussion on the iMPG/MPH rule of thumb.

While theoretically the ICE supposedly is inefficient at such low RPM ranges, experience demonstrates good fuel economy results. As Hobbit suggests, as long as the ICE is under load it's operating efficiently. Maybe there is something about the car that makes the system (not just the ICE) most efficient when the ICE operates within those lower ranges; I simply don't know. I'll let the technical experts speculate on the what and the why and, meanwhile, I'll let results speak for themselves.

 

It's the system, including the laws of physics and the driver as part of "the system" :_> Energy conversions such as charging and discharging a battery always unavoidably waste some energy. Accelerating on the battery alone (one energy conversion) is generally less efficient than if the engine's output is used directly, because that battery energy ultimately came from the engine (a second energy conversion).

The numerical rule of thumb is probably better than my "briskly" advice, because the rule is at least objective.

 

Yes hopefully someday someone will come up with a model that simulates the entire system.

 

That much I know. I was thinking more of the interaction of all the drive system components.

 

I tend to target about 1/2+ indicated mpg vs. mph up to about 40-45 mph. I classify this as brisk because it is usually faster than the other traffic initially. I've never been one to let the grass grow beneath my vehicle when accelerating. It will still seem slow compared to a leadfoot in the other lane. Ironically, many folks who end up driving faster start out much slower, so it is not until we begin to reach moderate speeds that they overtake and fly by.

Keep in mind that it is hilly here, and most of the time I'm going up a grade when accelerating. Between 0-20 mph the Prius efficiency is hurting with the ICE on, so I don't like to dally in that range. Couple that with charging or AC/heat and higher than 1/2 iMPG figures are difficult at low speed, so I get out of that inefficient range briskly. I don't have a scangauge so I can't see RPM's. I try to listen to the engine to determine if it sounds like it is smoothly making power, working hard, or lugging. Lugging is a problem with ICE on and low load.

I lack the mpg numbers these other guys do, so consider my observations more of a "just drive it" approach.

 

DELETED stuff about RPMs and ICE. Other folks had better explanations, above.

Here are some non-technical rules of thumb I follow in traffic in the DC area, coming off of a light:

1. I watch the person behind me. If they're falling behind me, I have the liberty to ease off and accelerate slower myself.

2. In general, I'll fill or nearly fill the ECO portion of the HSI bar. If I am able to accelerate at 3/4 of a bar or less (though always strongly enough for the ICE to kick in), I might. If it's uphill or if traffic is really moving, I'll go up to half of the PWR bar (maybe even fill it), but be ready to back off as I feel my momentum. I never just push the pedal to X% and keep it there until I reach speed Y.

3. I don't try to keep up with the corresponding person in the lane next to me, but I generally don't let the second car behind them pass me. (Depends on #1, above.)

Really, it all boils down to watching the traffic ahead of and behind you, and the terrain ahead as well. For example, my commute is 22 miles round-trip and consists of 10 blocks of city driving at the beginning and end, plus mostly interstate in the middle. The interstate is either open lanes and 60-65 MPH or stop-n-go traffic, with most of the backup on the return leg. (Home is about 250 feet lower than work.)

I set a personal record today of 54.4 MPG on this commute, which I think it pretty good for those conditions. In the backups on the way home (most of the trip) I actually raised my MPG by burning a couple of bars of battery, then getting the feel of the backup surges and pulse-n-gliding to that. (Which recharged the bars, and raised my MPG by 3-4 MPG.)

 

Good thread. Does this(iMPG) mean that the slower you are going the more acceleration that is required and as you increase speed you decrease acceleration to keep your MPG gauge half of your MPH. So as I start out through 20MPH, I would accelerate hard enough to about 10 iMPG and decreasing as the speed increases. Now if this makes sense then it seems at 60, I would be at 30 MPG and that would probably still be increasing at a significant rate.

So does this imply getting there somewhat quicker and leveling off is more ideal? Whew!

--Mickey

 

This is a GenII thread so there is no HSI bar.

 

Yes, I should emphasize that everything I've said refers to the Gen II. Though I would think the same concepts (e.g., efficiency of gentle acceleration) apply to the Gen III, I simply don't know; I don't have one, I haven't driven one, and I haven't followed the GIII threads closely enough. Nor do I have any sense of whether the relationships between various metrics (iMPG, MPH, RPM, etc.) are comparable.

 

One thing I don't think anyone has clarified yet is the importance of "pedal sensitivity." I've found that I have to pay attention to just how much pressure I place on the gas pedal in order to engage the most efficient mode possible. It's taken me a while to be able to glide. But now that I seem to have figured out just the right touch it's gotten easier. I constantly am feathering the throttle while keeping an eye on the MFD to see just what's working best MPG wise. I'm getting between 48 - 52 MPG, on my 72 mile, mostly congested freeway commute. I should note that I almost never go faster than 63MPH and do so by following slower traffic and or staying in the right hand lane. I am sensitive to the flow of traffic and will not impede flow for the sake of my MPGs.

 

I have a touring so it's wider tires have an MPG penalty, but even with that, if I lead foot and do hard stops, which is rare, the worst mileage I can get is 44 MPG.

I got over 70 MPG over an 8 mile trip to work this morning being careful. Combination of stop and go freeway and expressways