Slowing down not really saving you much?

[jamesqf]

Originally Posted by Longsnowsm

Time isn't a factor if your objective is ultimate efficiency, maximum MPG.

Yeah, if you just hang on long enough, continental drift will get you there :-)

[Fat Charlie]

Originally Posted by jamesqf

Yeah, if you just hang on long enough, continental drift will get you there :-)

And if you travel using geological movement instead of burning oil, when you get there you have more oil than when you started out!

[Longsnowsm]

You guys are awesome! LOL

Just thought I would work up a quick theoretical example so that someone can maybe attach some of their own numbers to the concepts of both fuel and time management to net a positive result in our investments in time and energy.

1000 mile theoretical trip
- assuming all interstate highway, no construction, no traffic, or reasons to slow down or stop other than fuel, rest, and food. (I know, what a fantasy right???? LOL)

Round numbers estimated driving time per trip at a given speed(again using simple flat numbers for speeds)
75 mph = 13.4 hours driving
65 mph = 15.4 hours driving
60 mph = 16.7 hours driving

Let's make some fantasy assumptions about fuel economy(of course plug your own numbers in for your own vehicle to get an idea)
40 mpg 60 mph
35 mpg 65 mph
25 mpg 75 mph

Fuel capacity:
15 gallon tank with 12 gallons usable before warning

Gallons MPG Tank range
12 x 25 = 300 per tank
12 x 35 = 420 per tank
12 x 40 = 480 per tank

Average/normal people tend to like to stop every couple of hours, stretch, go to the bathroom etc.
Average number of stops during this theoretical trip is 5 times.

Even if we assume people are quick and take just a short 15 minute break per rest break.
For fuel many times it can take a little while to get up to a pump, fill, pay, and get going so let’s say a total time of 20 minutes for fuel stops.

Average/normal people in a hurry will end up stopping more:
They will stop 5 times, 3 for fuel, 2 for rest breaks, and likely stop for food which we will say they go through a drive through fast for another 30 minutes of time. Total non-driving time is 2 hours.
13.4 hours + 2 hours = 15.4 hours

Someone taking their time and trying to stretch more fuel economy might slow down to 60mph, but still budget their time and not stop unless they have to. This means 2 fuel stops and one rest break. You will bring your lunch and drinks with you. Total non driving time will be 55 minutes.
16.7 hours driving + 55 minutes = round up to 17.7 hours

Someone willing to compromise and meet in the middle might pick up the speed to where the fuel economy starts to fall off fast for their vehicle. Let’s say it is 65mph in our theory car at 35mpg.
We still will budget our time, we will make 2 stops for fuel, 1 rest stop, and will bring lunch and drinks with us. So 55 minutes non-driving time.
15.4 hours + 55 minutes non-driving time = round up to 16.4 hours.

So on this trip we could pick up the pace, make one less fuel stop, still get an OK mpg figure, budget our time and only stop when we have to, and even save a few bucks by bringing out own food and drinks with us saving even more time. In the end you would take 1 hour longer than the 75mph trip.

So the choices are up to us. I like to figure out where my sweet spot is for my vehicle and keep it in that zone where I get the best return for the fuel burned, look for rides(drafting partner) so that I can go faster than my car’s natural sweet spot, get even better fuel economy than I can get by myself, and cut the trip time even shorter.

If you have plenty of time then back off, use hypermiling techniques to coast and stretch every gallon. If however you have a time budget as well then look for ways to budget that time more wisely. Pack that lunch and a cooler. Ration what you eat and drink so that you can reduce the number of stops. And look for ways to get free rides, possibly even go faster, and cut the budget across the board.

Of course a shorter trip changes the dynamics of the stops and time costs. Longer trips than our 1000 mile example would likely net even more savings for those of us who time budget as much as we fuel budget. In the end you really can get where your going a reasonable amount of time if you are frugal with your time and your fuel together.

Now I only wish it was this simple, but the math will work itself out if you take the time to think your trips out and just what you can do with some time and fuel management.

Longsnowsm

[Longsnowsm]

To the original poster, the question about a calculator for figuring out the break even is you and your vehicle. Your vehicle will give the the feedback on where it gets the best fuel economy, as you increase speeds you will see that figure drop reflecting the additional fuel burned. At some point as you continue to push your speeds higher you will see your vehicle fuel economy start to suffer in greater percentage/proportion for the speed travelled. At that point back off and see where your fuel economy starts to make the biggest moves upwards. That is generally what I refer to as the sweet spot where your making the best return on the fuel burned as long as your willing to accept that your burning fuel at a faster rate than if you slowed down to the place you see the best fuel economy your vehicle can return.

Then from there decide what you have more of at that point. Time or fuel. Some vehicles when idle or coasting burn little fuel and have a fuel over run where the engine is basically acting as a pump, but not really consuming much fuel. So don't let the run times of the car be the indicator. If you have a Scangauge you can set the liters per hour or gallons per hour as the case may be and note the burn rates when coasting, idling, running steady speed, accelerating etc. It isn't a constant so actual time your running your vehicle really cannot be used as the yardstick for determining if your wasting fuel by taking longer.

Over time if your driving the same routes you start to figure out where you can coast, where to watch for traffic lights, where to accelerate, and where to shut it down to coast or idle down to reduce your fuel rate over the same distance and likely you will improve the time it takes to do it in the process as well. Practice makes perfect. On unfamiliar ground you really have to be scanning ahead, watch your gauges, and know your vehicle and you will figure out where the break even point is in maximizing your time and fuel.

[Fat Charlie]

Driving is mainly about getting somewhere. If you know when you need to be there and how much time you have, that tells you what your minimum average speed is to get where you're going. If it's not your mos efficient speed then you're probably not getting a record tank.

Hypermiling has two major points:
1. You've got to do what you've got to do.
2. Do the best you can while doing #1.

You're not going to make yourself late to get better mpg, so unless you're time is open ended you're going to compromise at the expense of mpg.

[Frank Lee]

One ought to stop periodically anyway to stretch and hopefully prevent blood clots.

[user removed]

Today the wife and parents rode to lunch in my car. AC from the start with ambient temps in the 90s. Just a few miles, maybe 8 altogether. I think I came close to 40 mpg, where my lifetime average is close to 54.18 since purchasing the Mirage.

Best mileage was with 3 board members, we met at Sandston (Byrd Airport) and rode east about 42 miles mostly on old US 60, mostly divided 4 lane with almost no traffic. I followed Dirty Dave, Whatmaycome and Baby Diesel and matched their EOC P&G methods and my mpg gauge was reading 72.6 when we got to the park. Driving up to the meeting point averaging close to the limit which was mostly 55, I averaged 58 mpg (instrumentation).

Most of my driving is in traffic, sometimes heavy and I don't like to affect the flow of traffic. Last night coming home, when I made my LEFT exit from I64 I had a huge ancient Ford pickup about 5 feet from my rear bumper as I made that exit which requires me to be in the left lane. Unfortunately not fast enough for the Ford truck driver and I could only see his grille in my rear view mirror, then he moved right and cut in front of me with only a few feet to spare, probably uninsured.

AT NIGHT!!!!!!!!!!

By the time I got completely off the Interstate the same truck driver was less than 10 feet behind the car that was in front of me before I exited.

regards
mech

[chefdave]

Hi i'll throw another factor into the equestion. For me travelling over roads that resemble a roller coaster for my daily commute 55-60mph i get better mpg than travelling at 50mph or less. Found this out by accident when following a friend back to my house. Think this is due to carrying momentum up the slopes and free wheeling down slopes. I start to accerate when my speed drops to 40mph. Have gained a 20% improvement in mpg by doing this. Now matching gov mpg figures for cruising at 55mph on level roads.

[Frank Lee]

One theory is that 1000-1200 ft/mn piston speed is where optimal efficiency is found; I use it when BSFC data is not available.

I've been driving a Mercury Tracer; 1000-1200 piston speed is 42-52 mph. I figure I can't be too far off from optimum going 47 mph- smack dab in the middle.

I don't think there is a single best speed for fe under all conditions. For example when pushing a headwind fe is probably improved by going closer to 42 than 52; conversely when enjoying a tailwind it could be that fe is better going closer to 52 mph, all due to the compromise between higher engine efficiency at higher loads vs the additional hp requirements of higher speed.

It could even be that the car is most efficient MPG-wise at 37 mph, the speed at which the a/t is in top gear and the torque converter can be held in lock-up. Lacking real-time fe instrumentation, I can't say. But it doesn't matter because I wouldn't go 37 mph over that distance anyway in a car.

Incidentally I got 47 mpg on the last tank, which includes using the a/c some. If I drove at the prevailing speeds the car would get mid to high 30s. Only "mods" on this car are grille and brake vent blocks, passenger mirror delete, engine compartment blanket, and one size taller front tires at 45 to 50 psi; due to the a/t there is almost no coasting or engine off.

I've been doing a 120 mile round trip with some regularity lately. Speed limit is 60 so at the limit it would take two hours and at 40 mph it would take three hours. At 47 mph I am getting 10 miles more per gallon vs 55-60 mph saving .7 gallon each round trip, or about $1.85 these days, but it "costs" me 33 minutes which some would say is compensation of $3.36/hour. But I don't think in terms of my free time being worth x dollars/hour (just had to do this math now) plus I don't have a daily commute and do most errand-running on foot thus I spend very little time in cars so I value the higher fe number more than a lower trip time number.

[slowmover]

There's an upper limit on travel speed. A 75-mph Interstate, unless in a truly rural area (rare) will not allow a travel average that high. There is too much other traffic. It's hard to get above 70-mph, and it's useless for trip planning. Find the engine run time and do the calcs yourself. I have, in running a big truck, where FE is not the concern, per se. It's part of a larger calculation about profitability.

Tire life and the life of other truck/trailer components count in this. The "fastest" truck drivers are not usually the best paid. Just the opposite, in fact. For the casual observer it's hard to generalize as there are so many types of truck driving that relative speed doesn't say much.

But understand that the spread from low to high as to travel time is not so large as some might think. Traffic is traffic. And maintaining a very long distance from the vehicle ahead for braking trumps saving time. Four seconds is almost too close even at 60.

The poor and the stupid are your vibrantly diverse companions on this road. Texting, farting and their tiny minds wandering. Twelve year olds. Just able to hang onto the steering wheel, but not able to make mental calculations worthy of the name.

For someone who claims their time is worth more (gotta get there to pick up the TV remote!) ought to look wider about vehicle wear, and especially at increasing risks.

The envelope from 55-mph up may cut off at 62-mph or lower when major metros, etc, are figured for a long Interstate trip.

About the only thing I miss in a speed-governed truck is to see another pack of bumper riding morons overtaking me when I am also overtaking slower traffic. But now won't be able to pass at my cruise speed. Luckily I am paid such that this doesn't affect my income in main. I'm not likely to get another run before I run out of hours. And at the end of the year it wouldn't be that great an increase against higher truck expense and increased risk. The better companies have figured this closely. So it's quickly a non-concern.

Minimizing risk counts for more than being a dog with his snout stuck out the window. That's the image to retain for being drunk on speed. It's unlikely anyone will maintain an above 65-mph average on a trip of any length. Lane changing, accel and decel events mitigate against it for long term economy. Of which FE is only one piece.