Pondering My Practical Limit on Fuel Economy

Since you'd ask if I didn't, here's the answers to the standard burning questions:

- Have you read This Thread Yet?

Yup.

- What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)

Manual calculations on Fuelly (See link in my signature) MFD shows between 47-50 mpg between fillups

- What fuel economy are you expecting and why?

50 mpg would be nice, but I'm just wondering if I'm getting close to the ceiling for my driving situation. 44-46 mpg is still awesome compared to my 18-20 mpg in my old Equinox!

- What are the approximate outside air temps?

Too warm for winter over here (Lower 40s- Mid 70s)

- How long are your trips?

My daily commute route is 19.8 miles one way. It takes me 24-26 minutes to get there.

How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)

Haven't checked it. I teach Life Science, so no voltimeters in my lab unfortunately

Have you had your alignment checked? Any pulling or abnormal tire wear?

The car was inspected and serviced from tip to tail before I bought it, and the tires had less than 1,000 miles on them when I bought it.

- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?

My driving habits is about 80% highway and 20% city. The highway's speed limit is 65 mph, but when I commute there isn't a lot of traffic. My MFD says the average speed per tank is about 47 mph.

- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)

I live in south Louisiana. We measure terrain relief in inches. There are four overpasses I cross though.

- What are your tire pressures?

44/42

- Is your oil overfilled? (i.e. above the full mark on the dipstick)

Nope, just right.

- Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)

2005 Chevy Equinox with a 3.4L V6; automatic transmission. Over 4 years I got from 18-20 mpg manually calculated no matter how fast or slow I drove, also no matter whether I drove in the city or on the highway. At least it was reliable...

- What region/state are you in? (if you haven't set your location)

See above.

- How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?

When I get on the highway, I'll accelerate briskly to get into the flow of traffic and set the cruise control. Because there's so little traffic and I'm on the open highway, when I get off the highway to the service road I'll coast in order to slow down to get off. The rest of the way to work has me as the only car on the road, so I set it to 55 and do long coasts for the turns.

- What modes are you using, if any? "normal", EV, power, eco?

Eco all the time. If there's a pack of cars when I need to merge, I'll flip to power mode to get a place in the pack, then put it back on Eco mode.

- Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?

I let it warm up by taking it really easy before I get on the highway. It's about a mile from my house to the highway, so I slowly accelerate to the light and the speed limit on the road is just 35 mph. If it's red, I'll sit before turning to get on the highway. If it's green, I'll go straight through and drive to the next overpass to get on, slowly picking up speed as I go.

- Are you driving using D or B mode?

D mode only.

- HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?

Auto on 72. Most of the time the fan is barely blowing.

- Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).

Bridgestone Insignia 195/60R15

With all of that out of the way...

So I've had my Prius for about a month now, and have been thinking about whether I am reaching the practical limit of my mpgs considering my commute or I'm missing something. I'm not expecting 70-80 mpg because it's all high-speed driving (posted speed limits are 65 mph all the way), but hitting the magical 50 mpg would be quite nice. I've done most of the required readings, and did some testing of what I believe to be pulse and glide driving on a trip to New Orleans a couple of weekends ago. I don't think I'm doing right (I accelerate to a set speed [say 65 mph] then glide down to the set floor [say 55 mph] to average your desired speed [say 60 mph]). The YouTube videos seem to all be done with Generation II Priuses, so I don't have the same display to compare to in order to see if I'm doing it right. I do remember seeing a discussion that with Generation III Priuses you can keep the HSD in the first third of the light green area to achieve the same effect, but I'm still losing momentum as quickly by keeping the pedal gently depressed as I would by not holding it at all, so that leads me to believe that the biggest problem with my pulse and glide technique lies between the seat and the steering wheel.

If I am doing it right, then I must still be missing something. I know in several videos I watched there was a lot of driving about in hilly terrain, which would explain the ability to glide for long distances without much trouble and get high mpg readings. Like I said above, where I live, we measure relief in inches, so it's about as flat as you can get. Also unlike most drivers, I don't have to worry about really low temperatures regularly, and winter fuel doesn't exist around here, so I doubt that I'm getting a big drop in mpgs due to the weather, so that doesn't factor into the discussion either.

I'm guessing the biggest obvious impediment to hitting 50 mpg from my POV is that about 80% of my daily commute is driving between 60-65 mph on the highway. From my observations on the instantaneous mpg display and 1-minute graphs, the drop in mpgs from 55 to 65 mpg is almost non-existent, but does show once I get over 65 mph.* I've considered driving below 55 mph, but that would be suicide on the highway, and as a result I would need a new route. The downside to this is that my commute time would leap to over 30 minutes, so whatever mpg gain I got would be offset by running the ICE for almost 20% longer each day, which kind of cancels out the mpg gain at least to me.

So to sum things up, I'm not especially complaining about my fuel economy, but rather looking for some objective advice as to whether there is substantial room for improvement in my car's fuel economy based on my situation, or am I at a place where improving my fuel economy is not all that dramatic, and attempts to do so would be interesting depending on how OCD I feel when I wake up that morning. Oh, and I guess I somehow shoehorned in a question about PNG driving somewhere too.

Thanks!

*I find that interesting, and I guess that's where I'm wondering if my real mpg (I've shown 47-49 mpg on the MFD since I got the car) could hit 50, since the highway test for the EPA has the car driving at 55 mph the entire time. If the fuel economy is looking to be nearly the same from 55-65 mph, why couldn't I get that too?

 

Usually it's 62-63 mph

I'll take a 10-15 minute ride this afternoon and take some photos of the MFD. The differences in the graph are nowhere near as drastic as the one linked below.

 

44/42 front/rear. The service record showed that the tires were changed 1,000 miles before the car was traded in.

I honestly don't think I could find an engine block heater down here. Besides, we get maybe 20 frost days a year, so the warmup time is not as long as elsewhere.

Nope. I put the cruise between 62-64 when I get on the highway, and when I get off, I set it to 50-55. Here's my commute:

S Lewis St/Par Rd 605 to B Edward Boudreaux Middle School - Google Maps

I play podcasts to/from work, so I time my commute when I hit play when I throw the car into gear to when I park in the parking lot. If I start a podcast when I leave I arrive about 25-26 minutes into it.

I've considered changing my commute to this route: S Lewis St/Par Rd 605 to B Edward Boudreaux Middle School - Google Maps since it gets off of the highway almost all of the way (The service road has several stop signs on that stretch, so I figured it would be better to jump up to 65 for a stretch than start/stop several times for no reason other than a stop sign being there. The commute would then push to over 30 minutes one way, so the question becomes whether I want higher mileage or less petrol consumed. I might be off, but I think running the car for 10 more minutes a day would burn more fuel in the long run.

Actually, my city FE isn't too shabby. I regularly get over 50 mpg on trips in town. It's 3-4 miles to get into the town proper, and everywhere I go is 4-5 minutes further away. I take it easy, and oftentimes I'll be running off of the battery when I pick up a head of steam. When I drive to Lafayette and get into some proper traffic, I average well over 60 mpg.

 

Yup. Read them all. But I've got some questions (More on that in a bit)

I've read about warp stealth about a zillion times since I got the car a month ago, and I'm still not understanding it from a practical (e.g. what should my go pedal be doing, what speed, etc.) perspective. Also, almost all links talk about it in the context of a Generation II Prius. Completely different MFDs there, which adds to the confusion.

I'll give it a spin this week for a couple of days and report back.

I went out and put about 60 miles on it driving part of my commuute in a big loop trying to duplicate the seemingly similar consumption readouts for 64 at 55 mph. It went a bit awry as we had some pretty high winds, and when I was driving 64 on the open highway, it was almost straight into 15-20 mph winds. On the way back at 64 mph I got over 50 mpg, but it didn't compare to the over 60 mpg I was getting going 55 on a country road with the tailwind. Driving 55 mph into the wind was getting me somewhere between 50 and 55 mpg. I have the pictures--just need to upload them.

I'll give it a try some time this week.

Humidity is always high here. But seriously, I keep the A/C on auto and it usually blows (if at all) quote low (1 bar) in the recent weather when set to 72. Monday will probably see it blow a little more in the morning (Low is set to be 45), but in the afternoon it shouldn't too much. Summer will be a different story, however.