Toyota Prius Forum banner

1 - 9 of 9 Posts

·
Registered
Joined
·
1,563 Posts
Discussion Starter #1
Ok, new question...

I have 1 gallon of gas
I have 1 hour to travel

What's the maximum distance I can travel?
What speed(s) do I need to go to achieve that distance?

Assume level ground.
 

·
Registered
Joined
·
6 Posts
Well, to start off with, it will be less than 60 miles. Assume a max readily attainable MPG of 60--that's all the farther you could go on a gallon. But to get that distance in an hour you'd have to go 60 MPH (a mile a minute) and you can't get that speed and that kind of economy.

My guess is that you're talking somewhere around between 45-48 miles. My experience is that I get about that kind of economy at about 55 MPH--so the fuel would run out before the clock did. To get fuel economy in the 60 range I assume you'd have to drive 35 or so--then the clock would run out before the fuel and you would have only gone about 35 miles.

But then again, I could be all wrong. . .
 

·
Registered
Joined
·
18 Posts
Ultimate mpg in this test would also depend on engine temperature before starting the test. Starting cold, the engine will run initially just to come up to operating temperatures.
 

·
Registered
Joined
·
1,215 Posts
Good posts.

But the original question isn't really practical. Fuel economy is best measured "over time." Measring it one gallon and/or one hour at a time isn't practical and may even be confusing (the "is ICE warmed up" question is a good case-in-point).

So I will tackle this one in general terms, avoiding the "one gallon/one hour" question entirely.

I have observed that high speeds (over 55 MPH) cause the biggest fuel economy penalty. More on this later.

Stop-and-go traffic, in my opinion, causes the second most siginificant penalty, even after you account for regenerative braking. Yes, this is contrary to EPA estimates for the Prius, but we all know that the EPA isn't really that "real-world" accurate for most cars anyway.

My logic (based on my observations and lots of time behind the wheel):

My best fuel economy is between 35 and 50 MPH, depending on grade, wind, the weight of the vehicle, passengers, and cargo.

As we already know, when going below 42 MPH, the car will be more willing to alternate between ICE and electricity, or a combination. My car will not usually go to electric-only/ICE-off "stealth" at any speed above 40 MPH, unless my SOC is at 6 or 7 bars (highest blue or lowest green).

Regardless of whether or not the ICE is running, I can drive between 35 and 45 MPH and consistently keep the instant display steadily greater than 50 MPG (after initial accelleration). That's my "feelgood" point; it's a happy medium between being a slowpoke roadhog and a resource-conscious driver.

Sometimes, the instant will float way up there in the 70's, 80's, or 90's, even with the ICE running! The key is to drive at as consistent a speed as possible.

But here's the crux: You pay a HUGE fuel penalty whenever you need to accelerate. Yes, regardless of what's already "in" the battery, it still takes gas to accelerate. I often see my "instant" go down to 20, 16, or 12 MPG while accelerating.

For the length of time it takes me to get back up to the speed limit, that is a penalty, and it mathematically works AGAINST all of that good "at speed" driving where I was getting MPG figures in the 60's to 90's.

So yes, stop-and-go traffic still penalizes me. I have observed my best mileage to be where I can drive no less than a quarter mile or more between slowdowns (for traffic turning off the road in front of me) and stops (for traffic or red lights).

If I can go a half-mile at a time, that's even better (mathematically speaking), because I am accelerating only half as often, for only half the time.

AS I mentioned earlier, high speeds are also gas mileage killers. My average gas mileage when I stay off the freeway is between 49 and 55 MPG.

When on the freeways, I try to keep between 55 and 60 MPH, but often I am forced to speed up just to avoid being killed by tailgating H2's and Escalades.

When I drive on the freeways a lot, and especially when I make long trips, sometimes I will go as fast as 70 to 80 MPH to keep up with traffic. This is a really big "whack" in the gas tank, and it reduces my fuel economy down to something between 43 and 49 mpg (depending on speed).

Here are my latest observations, based on about 11,000 miles of driving with my car, assuming a light load (me and my schoolbooks and other junk), reasonably level terrain and light to moderate wind conditions (0 to 10 mph from varying directions).

At speeds below 50, I can get anywhere from 49 to 55 or more MPG, depending on how often I have to accelerate.

At speeds above 50, I get between 49 and 50 MPG.
At speeds above 60, I definitely get less than 49 MPG.
At speeds above 70, I get significantly less than 48 MPG.
At speeds above 80, the wind-resistance penalty is quite pronounced, and I get less than 43 MPG.

Notice the "curve" gets sharper (more rapidly decreasing fuel economy) as I go faster. I suspect that this is true for any vehicle. On Earth, anyhow....

My overall fuel economy is around 48 MPG, and my car is in the database at http://www.greenhybrid.com .
 

·
Registered
Joined
·
1,563 Posts
Discussion Starter #5
BIF,

I've seen your database record. Pretty thorough entries in there. But I'm curious to hear the answer that the people who've worked out mph vs fuel to four decimal places would come up with, given the limitations stipulated.

For all other factors, let's say hot engine, no wind, level roads, no cops, etc etc etc.... Eliminate all the natural variables and just deal with the Prius itself.

Based on figures presented here, at a constant speed I figure it's roughly around 58 MPH to go 58 Miles in 1 hour. Fuel and Time run out together.

But is a constant speed the best? Maybe doing 20 for 10 minutes, then 60 for the rest of the trip (or vice versa) works better.

Purely a mathimatical puzzle, but if you're ever got an hour left to live, a gallon of gas, and need to get to a hospital x miles away to save yourself... it could come in handy :)
 

·
Registered
Joined
·
1,434 Posts
Need more info/variables.

To be vague, 60mph at 95degrees will take you 60 miles. If you go 61mph you will only have enough fuel for 59.54 miles. Any slower and you'll only get as far as the speed you're traveling.

Different temps, humidity, AC use, tire RR, road RR, barometric pressure, wind, load, summer/winter gas, altitude, etc. all change those numbers slightly.
 

·
Registered
Joined
·
1,215 Posts
(This post attempts to make a point..I think. It's partly in friendly jest, please just bear with me. It will help if you just nod your head and laugh and say "funny, BIF". Maybe then I'll be too tired from school stuff to pursue this any further in the morning...)

Hmmm, yes, there are far too many variables to be able to give anything like a valid answer.

For example, just "when" is it empty? When the gallon is gone? Or when the battery is DOA?

Do we drive it until the SOC is zero, or very near to it? Then do we coast to a stop?

Or do we begin coasting as soon as the ICE dies?

Would it be cheating for me to open my door and push off the ground with my foot? On second thought, the wind resistance of the door would probably cancel out any "help" I might be able to provide ( :) )

And how do we know it's actually out of gas and that the car didn't just go into "stealth?"

Do we try to coast for ultimate distance after our gallon is gone? I ask, because I most certainly would. I have gone a good quarter mile many times under ZERO power. Once or twice, I've even been able to eke out a half-mile or more under zero power, with very little loss of speed.

And how far would our razor-sharp calculations of four-decimal precision take us?

Should we wax the car first? What about that little antenna...I think if I've only got a gallon, I'd probably want to reduce the massive wind resistance that the antenna must be putting up... :p

Who's car shall we use? One with 200 miles, 2000 miles, or 20,000 miles? Break-in makes a difference, right?

Does paint color make a difference? I think maybe my silver is more slippery than "heavier" colors such as red or black.

I say the only way to know is to test it! Somebody run theirs empty, then stop, put exactly one gallon in the tank (measured from a lab beaker), and go for it.



BTW, my guess is in one hour, I can drive about 45 miles on a gallon at speeds ranging from 40 to 50 miles per hour. Coasting would be out, because that would probably take too much time (decelerating, and we only have an hour, y'know...).


..oops, I forgot the decimal points.... that's APPROXIMATELY 45.0000 miles on 1.0000 gallons at speeds ranging from 40.0000 to 50.0000 MPH.

How's that for a high-precision guess? :p

<yawn, is anybody else getting sleepy?>
 

·
Registered
Joined
·
363 Posts
melgish said:
Based on figures presented here, at a constant speed I figure it's roughly around 58 MPH to go 58 Miles in 1 hour. Fuel and Time run out together.
Actually your absolute best 1hr 1gl distance is probably not going to be at a constant speed. Instead you are going to get slightly better distance by very gradually accellerating up to some speed, then shutting down the engine and coasting for a distance, restarting and gradually accellerating back up to speed and repeating.

This is the technique that is used at maximum distance rallys. It is a very unnatural way to drive, but does add a couple percentage points to your distance/quanity figures.

Exactly how you would apply this in a prius is even more complicated. In the prius you can effectively coast under power. (This is what you are doing when running purely on battery power.) so you would need to include that in the equation.

Oh, and pull off that antenna, fold in your mirrors, add solid hubcaps, give the car a good waxing, remove the seats (you, the driver, can have a milk crate) and tape up the keyholes. Can't have any extras when going for maximums.

Might be an interesting experiment. Somebody call discovery channel.
 

·
Registered
Joined
·
1,563 Posts
Discussion Starter #9
SkipHuffman said:
Oh, and pull off that antenna, fold in your mirrors, add solid hubcaps, give the car a good waxing, remove the seats (you, the driver, can have a milk crate) and tape up the keyholes. Can't have any extras when going for maximums
You left off "visit the restroom" :)
 
1 - 9 of 9 Posts
Top