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Old 07-14-2015, 04:41 AM   #1 (permalink)
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Slowing down not really saving you much?

After doing many trips to my grandparents before ecomodding and now more recent trips with ecomodding i struggle to see a big difference in the amount of fuel used for the same trip. a trip usually costs me around 1 bar on the fuel tank level. After this weekends trip with no mods just slowing down my consumption decreased but same amount of fuel seems to be used. So i did some calcs.

Trip highway distance is 120 km or 74.5miles. The speed limit here happens to be 120kph or 74.5 mph. At this speed my cars usual consumption is 8.6 L/100km or 27mpg and the trip takes me 1 hour.

After slowing down to an average speed of around 90kph or 59mph my consumption dropped to 7.5 L/100km or 31mpg. However the same trip now took me 1 hour and 20 min. Which means i drove for 20 min more and had the engine run for 20 min longer

So that begs the question. Yes i slow down to decrease my mpg but the result is an increase in travel time? and at what distance do these balance themselves out?

Does anyone have a link to a L per hour consumption calculator so i can do a proper investigation?


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Old 07-14-2015, 06:42 AM   #2 (permalink)
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MPG is MPG, there is no 'time' component.

At 8.6l/100km your 120km trip used 10.3 litres.

At 7.5l/100km your 120km trip used 9 litres.

Using the bars on your fuel gauge is all but useless in this instance.

Time has no bearing on these calculations, unless you're in a hurry.

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Old 07-14-2015, 07:23 AM   #3 (permalink)
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Fuel economy conversions / calculator - EcoModder.com

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Old 07-14-2015, 05:56 PM   #4 (permalink)
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Originally Posted by oldtamiyaphile View Post
MPG is MPG, there is no 'time' component.
This--don't over think it.

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Old 07-14-2015, 10:39 PM   #5 (permalink)
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It isn't that simple, and we all know it.

Originally Posted by Vman455 View Post
This--don't over think it.
Sometimes, the time lost equates into money lost. Also, length of trip factors into the ecomodding decision.

My wife once worked in marketing as one of those cute convention babes for a large telcom. She was given a stipend to pay for an "appropriate" car and reimbursed by the mile. She blasted all over the American South West. Her Mercedes stayed at home as she burned through 2 used Geo Metros and a Daihatsu Charade. Even at 70-75 mph, the little 3 cylinder cars pulled down double the fuel mileage of the big Mercedes. The fuel savings paid for both cars and left several hundred dollars a month left over for her to spend on "wardrobe". Slowing to 60 mph where the little cars could get another 10 mpg wasn't going to help her bottom line since her mileage reimbursements were set high enough. The extra 45 minutes saved after a blast from San Diego to Phoenix gave her that extra time to jump in the pool before doing her stint at some convention.

When I am pulling a 20,000 lb flatbed out to Texas, I keep the speed at 58 mph even if it is legal to tow at 70 mph in many western states. The difference between 58 and 70 mph is 2 mpg in the Cummins Diesel. The extra 120 miles between fill ups and the reduced cost of fuel is well worth it. But, I am allowing myself several days leeway when I broker the machinery delivery. I can afford to lollygag.

You have to look at your particular situation and see if ecomodding is worth the effort. For many of us, it is a past time. For others, it is time - time that can't be wasted.
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Old 07-21-2015, 07:48 PM   #6 (permalink)
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The time factor

I have to disagree with the comment that time doesn't factor into your MPG when traveling. I think about this one quite a bit and have also wondered at what point am I going too fast, or too slow to offset the differences in my speed compared to others on the road. Where this comes into play is when I am traveling down the interstate on longer trips. I see the same cars and trucks pass me 2 and 3 times before they eventually manage to either break free from my evil MPG conserving ways or they have pulled off for the day or have arrived at their destinations.

I do believe that the old rule of thumb that you hear over the road truck drivers say is... "You have to keep the drivers door shut if your gonna make any money!" The essence of that statement is we are not stopping all the time... Keep on moving until you "need" fuel to minimize the number of stops. I try not to stop unless I "need" gas. The duration of time will depend on which vehicle I am driving and how far I can go on a tank of gas. Now in my Insight I can go farther than my bladder will allow, so I pick a reasonable halfway or some distance that I know I can hold out for a break. Next thing to figure in is how fast do you "need" to go and what is your greatest return on your fuel economy dollar... In other words if you car gets decent economy up to about 62 mph then push it to that point. If you need to go faster then start looking for a "ride". I will lay back and wait for a big rig to pass me going between 65 and 68 mph and then I will suck up in his wake and ride there as long as I can.

If you pay attention to the cars that pass you on the way you will start to see a pattern normally. You will see the same cars and trucks go flying by you at break neck speeds... Somewhere down the road due to need for gas or bladder breaks they stop, then they will go flying past you again. It is a good rule of thumb to basically look at how far could you go if you were driving with no help(no drafting buddy) at the posted speed limit or 5 or 10 mph above the posted limit like most others are driving. Look at how many more times you would have to stop. Figure anywhere from 15-20 minutes lost at each stop. It is a lot like watching people pass you in town only to come pulling up to them at the next light, and the next, and possibly even one more. They are in a hurry, but getting nowhere fast...

So how fast can you go before your going too fast? Only you will know based on how your vehicle performs and the fuel economy it gets at any particular speed and how far you have to go. Can you go too slow? Well if your goal is to roughly get where your going in roughly the same amount of time as if you were driving like everyone else then your going to have to push it along at a faster pace, but still not compromise too much in fuel economy. If you slow down for maximum fuel economy you will just have to accept your maximizing fuel economy and not focusing on the time it takes. I try to blend the two by getting some drafting in there to pick up my speeds and still get amazing fuel economy. So there is more than one way to skin a cat as they say...

You will have to do some math and figure out the number of stops and the average speeds to determine where to try and meet in the middle if that is your goal. It is an amazing feeling though when you manage to pull of both an amazing fuel economy run and still make good time... I made a run from Colorado Springs to Omaha NE and stayed glued to the back of big rigs the whole way... They were moving along pretty quickly and I managed to pull off a 89+ mpg run in my Insight. It was completely awesome... That doesn't happen often for me to have things line up like that and manage to keep a good average speed. It was sweet! So it is time to break out the maps, and do some calculations if you want to figure out the best speeds in return for the best time and economy. Good luck!

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Old 07-21-2015, 11:01 PM   #7 (permalink)
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I get his thinking- what's better, 1 gallon per hour for an hour, or .5 for two. But time doesn't matter, as the fuel is counted per mile. Time doesn't matter in the equation, because we have replaced it with miles, essentially.

It is hard to explain a thought concept through text, but like I said earlier using GPH and time, we have miles to a gallon. I get it, but unless one is counting other aspects of time (possible pay, or leisure), then MPG already factors in the extra drive time.

*thought came back to me! 55 MPG at 55 MPH vs 35 MPG at 85 MPH counts the extended engine run time, as although the mile takes longer to cover, the fuel use is included. It doesn't outweigh the aero drag experienced at the higher speed. I'm sorry, I can't get my focus to put this into words effectively :/

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Old 07-22-2015, 02:39 AM   #8 (permalink)
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So the policeman asks, Why were you driving so fast? And the driver says, I was trying to get home before I ran out of fuel.
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Old 07-22-2015, 09:55 AM   #9 (permalink)
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Originally Posted by Eddie25 View Post
However the same trip now took me 1 hour and 20 min. Which means i drove for 20 min more and had the engine run for 20 min longer
Yes, the engine ran longer, but at lower RPMs, therefore using less gallons per hour.

How much less?

That depends on lots of factors, mostly how much extra fuel was needed to overcome the aero penalty of an extra 30kph.

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Old 07-22-2015, 01:10 PM   #10 (permalink)
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Time isn't a factor if your objective is ultimate efficiency, maximum MPG. If maximum MPG is the focus then you have to set time aside and focus on that objective. If however you live in a world where you are willing to compromise some efficiency to account for time and the maximum rate of return for both time and fuel economy then there is some math involved and understanding your own vehicle. Only you can decide what your objective is.

My position on this is there is a break point where you cannot go any faster or push any harder without going to far and breaking the return on your investment regardless if that is in time or fuel.

For example if I get 50 mpg at 30mph, 45 mpg and at 40 mph, 40 mpg at 50 mph, 30mpg at 60mph, 20mpg and 70 mph you then have to decide do you have time to invest or fuel and if so how much? Your best bet unless you happen to have a lot of time on your hands or a lot of fuel budget is going to be someplace in the middle to try to maximize the most of both time and fuel your investing in the trip.

Of course it isn't that simple and there are lots of other factors like weather, temps, wind speed and direction, hills, elevation, the ability to get a "ride" and draft, vehicle, aero profile, and mods you have made... and last but not least the loose nut between the seat and the controls of the vehicle.

Good instrumentation is the key to all of it, if you don't know what is happening real time then it is really hard to make the proper adjustments when your waiting to fill up the tank to know just how things are going.

So if maximum fuel economy is your goal then I will agree time has to be set aside as a factor. If however your trying to figure out how to split the difference between time and fuel then you need good instrumentation, and the willingness to learn where the sweet spot is for your vehicle and the routes your taking.

The other time factor is how frequently your having to stop. If you have to stop every 200 miles to fill up and it takes 15-20 minutes to get gas and get back on the road then that is an additional 15-20 miles you can be down the road and if you have to repeat that cycle several times you just added a significant amount of time to the trip. If by slowing down you can make fewer stops your saving that time. Now if your going so slow that even that time savings from not having to stop doesn't matter then again you are probably focusing(or should be focusing on maximum MPG and not time). But if your weighing both time and fuel then if you can go longer, not have to stop even one or two additional times the time lost by going slightly slower is made up in making fewer stops. In the end if you play your cards right you will not have added a huge amount of time to your trip, but obtained respectable fuel economy in the process.


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