"Break Even" Commute Length

I've watched the fuel economy during long highway trips, say 100miles (160km) or longer, and it often improves all the way to the end. Plus I used to commute 34miles (54km) to work in the Insight-I. On a really good summer day I would be at 103mpg (2.3 l/100km) average for the trip to work when I parked in the garage. But I sometimes continued an equal distance further past work on an errand (roughly another 54km) and found that the average often rose to 110mpg (2.1 l/100km) over the entire 108km. That was about it for me, and I could maintain that 110mpg average on the trip back to work.

A couple of things to keep in mind:
- in order to raise the average, you have to drive at better fuel economy than the average - considerably better to get a noticeable improvement if you have already traveled some distance
- terrain and speeds play a huge role and can vary over your route; if the last section gives better or worse fuel economy that will distort the impression of how long it takes the average fuel economy to stabilize

 

Your question is subjective. What do you consider "reasonable"? A better question would be to ask how long it takes to get to steady state fuel consumption. We would need to specify ambient and driving conditions, but at least that question has an answer.

Tom

 

OP answered OP's question

After that, the end fuel consumption is just a weighted average.

Canard, do you block your radiator or use a block heater in the winter ? You can improve the first 0 - 10 minute consumption rates dramatically by doing so.

 

The mathematical term is "asymptote", and the curve approaches the asymptotically.

Tom

 

Unfortunately it's not that simple. You could generate a family of curves for different speeds, but to generate each curve you'd have to go at constant speed, no wind, no stops/starts, perfectly flat surface, etc. And after all that work the results wouldn't be applicable to the real world with all its variations, not to mention using techniques such as pulse and glide.

The biggest problem is that the fuel economy is so sensitive to how the car is driven, particularly speed. You seem to be imagining that the EPA numbers are a maximum for the car, whereas in fact they are generated by running the car through regimens of speed, load, etc intended to mimic "typical" driving. It's possible to beat the EPA numbers by 30-50% for most cars simply by driving the car more efficiently ("hypermiling").

 

I live where it is dead flat* so my consumption record has no real 'bumps' in it. Around 10 miles in the summer to 15 miles in the winter, I have leveled off to the mileage I will get the rest of the trip

* In the 156 miles from Eden MS to Walls, MS the road gains 60 feet.

 

[People claim they never use calculus once they leave school, every day that curve draws on my MFD and it just screams calculus! The area under that curve divided by my distance is my Average MPG.]

First, I know nothing.

Now here is what I think I know.

The Computers use Engine water temp to decide which of 5 routines to use:
Colder temps try to improve pollution, higher temps try to improve mileage.*
Engine Water temp will not be linear with mileage, but they are almost linear. There should be a fudge factor for outside air temp.

Engine oil, transmission fluid, wheel bearing grease, tire tread, etc. will all have less friction as they warm up. I suspect each has its own curve and that none of them are linear with mileage and all need an outside air temp fudge factor.

Your HV Battery may need to recharge once you get to constant speed, and this can take a variable length of time, but often under 10 minutes.

I run A/C heavily until the interior of my car reaches my desired 'room' temp. This can take 45 minutes, but usually under 30. Higher outside air temp and humidity will fudge this upward unlike the others which go up with lower temps.

As I say, 10 minutes in summer, 15 in winter and I am doing as well as I will that day.

* Or they improve pollution by minimizing the hydrocarbons in, so there will be fewer hydrocarbons out. I sometimes think great gas mileage is merely a side effect of what Toyota was really trying for.

 

that plus installing $60 block heater

 

Question: What have other people found is the "break even point" or minimum single trip distance one has to drive, in order to flatline the fuel consumption at a reasonable level?

Break even point = 50 mpg Fuel Efficiency, ICE coolant > 160 F degrees
(estimated average times/distances)
===============================
Summertime (>68 F degrees, dry smooth roads) = approx. 20 minutes, 7 miles
Wintertime ( 10 F degrees to 40 F degrees, dry smooth roads) = +40 minutes, 12 miles
===============================
Factors that can shorten the time to the most optimum fuel efficiency
===============================
HV battery SoC > 50%,
Car was driven within the last 10 minutes ( coolant is still warm) two trips that occur sequentially can appear to the Prius as one trip. 10 minutes stop over could be considered one long traffic light - or a mini traffic jam ( In the summertime, this 10 minute a stop over rule could be extented as long as 20 minutes if the outside temperatures are over 80 F degrees).

Nice weather: very warm driving temperatures (e.g. > 85 F degrees), no rain-sleet-snow.
engine block heater
grill blocking in temperatures < 60 F degrees
overinflating tires to max sidewall pressure
using a ScanguageII Xgauge FwT to monitor and maintain ICE temperature
hypermiling techniques
Optimum trip route selection = not too hilly, little or no traffic, dry smooth asphalt roads, no stop signs, no pedestrian xcrossings, top speed is 40 mph, two or more lanes in each direction.

hope this helps

Walter Lee
2010 Toyota Prius III, blue ribbon/dark grey, oem floormats
Yokohama Avid S33 (50/48 psi)
Scangauge(FwT,GPH, SoC, RPM)
50% grill blocking as of 09/09/11
100% grill blocking as of 09/16/11
Odeometer 15400, overall 60 mpg
Hypermiler amatuer/student in DC/MD/VA Metro Area
Last tank 695miles/10.18 gallons = 68 mpg

 

i have a 7 mile one way commute, some back roads, some highway. i can start and maintain a tank between 50 and 65 mpg (sorry about the u.s. numbers) depending on the season. if i drive further, it usually improves.

 

If you define the "break even" as 50 mpg (4.7 l/100km), then I can get there about half the time in my 4-mile commute to work, during which the car is powered on for about 13 minutes (including a minute or two of piddling around the parking lot at each end). On the other hand, since my day today involved some mountains and a lot of short errands on the way back, it was hard to even get 50 mpg over 40 miles.

It really depends too much on terrain, speed, wind, climate control, driving style, etc. to make a generalization here. I feel like if you started out right on a highway (after getting out of stage 1, doing say 10 mph for that period), you'd probably be better off, since you'd always be using engine power until the car had warmed up, and could probably get 50-ish mpg with the engine always on... Once it hits 70°C, then you're better off at city speeds, where the engine can turn off.

 

Plug-in Prius will be the right car for short trips (less than 25km).

I hate the short trip in the morning. The engine just could not shut down.