RPMS and mileage

hybridkiller said:

That's exactly my point - as long as one sees the difference as a percentage then it only comes down to which format is more intuitive for that individual. I get what you're saying about the inverse relationship - one is expressing numerical consumption, the other is expressing numerical efficiency. MPG is much easier for me because I've been working with that unit of measure for decades - it's simply what I'm accustomed to, and when I see it expressed that way I instantly see it as a percentage difference.

You're trying to make it a purely mathematical issue when it's not - it's more akin to differences between languages.

Nationality is relevant because we are conditioned to looking at different units of measurement. I've already stated that I don't think either system is inherently better - your contention, that the system with which YOU have greater familiarity IS better, simply isn't an objective one.

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I tend to agree with you that both formulations have their merit and it boils down to what one is most accustomed too or which one is easier to "get". But this is coming from someone that uses both methods on a regular basis to make evaluations.

DieselBruce said:

Yup! That's where gal/100 miles (or l/100 km) is very useful, it makes it easier to understand which vehicles are ACTUALLY costing you more money and more fuel.

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Easier is a relative term. In the good ole US of A we hear people all the time say they are horrible at math, just don't get it, etc. it is almost a running joke when one is not good at math. While you find GPM easier others find MPG easier.

I am on the fence as to which is "easier" or what should be standard but I don't get how anyone could not understand either method. My wife just cannot grasp MPG but does understand GPM, on the flip side she cannot grasp either hours per gallon or gallons per hour. It just "doesn't make sense" to her.

Math is the subject that children have been allowed to either get or not get in the educational system. Unlike reading where people are embarrassed to not be able to read it is just acceptable to not be able to comprehend math.

psrumors said:

Easier is a relative term. In the good ole US of A we hear people all the time say they are horrible at math, just don't get it, etc. it is almost a running joke when one is not good at math. While you find GPM easier others find MPG easier.

I am on the fence as to which is "easier" or what should be standard but I don't get how anyone could not understand either method. My wife just cannot grasp MPG but does understand GPM, on the flip side she cannot grasp either hours per gallon or gallons per hour. It just "doesn't make sense" to her.

Math is the subject that children have been allowed to either get or not get in the educational system. Unlike reading where people are embarrassed to not be able to read it is just acceptable to not be able to comprehend math.

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I know, I'm from the US. It doesn't change the fact that MPG is a worse measurement. Seriously.

I don't believe it to be "fact"

psrumors said:

I don't believe it to be "fact"

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There is a bunch of information available about this and you can do the math yourself if you really don't believe it but here's a link to a well thought out blog post about this subject:
http://www.datagenetics.com/blog/april12014/index.html

The math never changes (That is the good thing about math 2+2=4 is always the same). How it is then presented as a value can be. People that understand math can understand it represented either way. Those that don't probably won't understand it either way or only the way they are comfortable with. There is no right and wrong here or fact and fiction. There is only what some understand and others don't.

Hm, this thread turned into a liters to gallons discussion.

The worst nightmare I've heard screwing up while converting gallons to liters is a 747 pilot http://en.wikipedia.org/wiki/Gimli_Glider They ran out of fuel 1/2 way into the flight and were lucky to find an abandoned WW2 airfield. How many km is that?? How many more liters should they have gotten? Duh... I dunno...Back to the point...

If the turbo isn't spooled up, you aren't getting economy. If you shift into a gear that is below a point where the turbo is working, your engine is not effficient.

I wish I could find the thread, but several years ago, a woman driving city traffic to and from work, lost 5th gear. Her car continued to be operable, as the lost 5th gear did not disable the transmission. She drove 4th gear as her top gear until it was repaired. Her fuel economy improved. She was forced to work the engine in the proper rpm. She learned the lesson a lot of people need to understand.

In order to get more accurate fuel economy analysis, it does not require completely filling the tank, nor does a tank to tank comparison give you accuracy. A minimum three-tank fill/mileage running average is much more accurate. The larger the sampling, the more accurate the results.

Franko6 said:

Hm, this thread turned into a liters to gallons discussion.

The worst nightmare I've heard screwing up while converting gallons to liters is a 747 pilot http://en.wikipedia.org/wiki/Gimli_Glider They ran out of fuel 1/2 way into the flight and were lucky to find an abandoned WW2 airfield. How many km is that?? How many more liters should they have gotten? Duh... I dunno...Back to the point...

If the turbo isn't spooled up, you aren't getting economy. If you shift into a gear that is below a point where the turbo is working, your engine is not effficient.

I wish I could find the thread, but several years ago, a woman driving city traffic to and from work, lost 5th gear. Her car continued to be operable, as the lost 5th gear did not disable the transmission. She drove 4th gear as her top gear until it was repaired. Her fuel economy improved. She was forced to work the engine in the proper rpm. She learned the lesson a lot of people need to understand.

In order to get more accurate fuel economy analysis, it does not require completely filling the tank, nor does a tank to tank comparison give you accuracy. A minimum three-tank fill/mileage running average is much more accurate. The larger the sampling, the more accurate the results.

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There is no liters to gallons discussion going on. There is a gallon/100 mile (or liter/100 km, doesn't matter which units you use) vs MPG (or KMPL) discussion though. So the discussion is volume per set distance vs distance per set volume. There are no conversions going on, just 2 methods of measuring efficiency/consumption. One is inherently better though.

Also, your argument for RPM vs mileage is a little misleading. Yes lower gears (higher ratios) is better in city driving (due to lots of acceleration). Higher RPMs in NO WAY mean higher fuel efficiency while cruising at steady speed (unless hills are involved). No to mention, spooling the turbo requires more fuel than not spooling the turbo. You need fuel to spool a turbo. The only way that higher rpms are better is during acceleration or any other high load scenario. Lugging an diesel causes it to try to inject lots of fuel to spool the turbo, but since the RPMs are too low, there is not enough exhaust gas flow to spin the turbo fast enough. It takes a lot more fuel to spool a turbo at lower RPMs due to less exhaust gases. Same reason why you need to bring the RPMs at a drag strip to spool your turbo, you need to provide massive amounts of exhaust.

DieselBruce said:

There is a bunch of information available about this and you can do the math yourself if you really don't believe it but here's a link to a well thought out blog post about this subject:
http://www.datagenetics.com/blog/april12014/index.html

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It is a good read but it doesn't make the position of "GPM being better than MPG" fact.

The author asks this question

Ask yourself the question, would you rather know how far you could travel on a gallon of gas, or how much gas you are going to use on your trip?

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As if one cannot get both answers from either measurement.

If a car gets 50 MPG I know I can go 50 miles on a gallon of fuel. Thus if I have a 1000 mile trip I will need 20 gallons of gas (1000/50).

Conversely, if I have a car that uses 2 gallons / 100 miles I know that I need 20 gallons to go 1000 miles (2*10)

Like Lightflyer1 stated "the math never changes"

Lightflyer1 said:

The math never changes (That is the good thing about math 2+2=4 is always the same). How it is then presented as a value can be. People that understand math can understand it represented either way. Those that don't probably won't understand it either way or only the way they are comfortable with. There is no right and wrong here or fact and fiction. There is only what some understand and others don't.

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Very well stated

psrumors said:

It is a good read but it doesn't make the position of "GPM being better than MPG" fact.

The author asks this question



As if one cannot get both answers from either measurement.

If a car gets 50 MPG I know I can go 50 miles on a gallon of fuel. Thus if I have a 1000 mile trip I will need 20 gallons of gas (1000/50).

Conversely, if I have a car that uses 2 gallons / 100 miles I know that I need 20 gallons to go 1000 miles (2*10)

Like Lightflyer1 stated "the math never changes"




Very well stated

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You're not getting it.
This example from that blog demonstrates perfectly how mpg can be misleading.

• An SUV with an efficiency of 16 mpg consumes 62.50 gallons per 1,000 miles.
• A compact car with an efficiency of 35 mpg consumes 28.57 gallons per 1,000 miles.
• Total for both cars is 62.50 + 28.57 = 91.07 gallons per 1,000 miles.
• 91.07 ÷ 2 = 45.54 gallons per 1,000 miles (per car).
• A car with efficiency of 22 mpg has a consumption of 45.45 gallons per 1,000 miles (which is less).

If you just took the 2 mpg numbers and averaged them out you would assume that you would need 2 cars of 25.5 mpg average to break even for fuel consumption. That isn't the case. The actual consumption results (gallons/100 miles) shows that you would actually only need 2 cars of 22mpg to consume less fuel.

Right there shows how terrible mpg is at measuring fuel consumption. It is not linear at all, which is the whole problem. As the mpg numbers get higher, they mean less and less.

DieselBruce said:

You're not getting it.

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Just because I don't agree with you doesn't mean I don't get it.

DieselBruce said:

• An SUV with an efficiency of 16 mpg consumes 62.50 gallons per 1,000 miles.
• A compact car with an efficiency of 35 mpg consumes 28.57 gallons per 1,000 miles.
• Total for both cars is 62.50 + 28.57 = 91.07 gallons per 1,000 miles.
• 91.07 ÷ 2 = 45.54 gallons per 1,000 miles (per car).
• A car with efficiency of 22 mpg has a consumption of 45.45 gallons per 1,000 miles (which is less).

If you just took the 2 mpg numbers and averaged them out you would assume that you would need 2 cars of 25.5 mpg average to break even for fuel consumption. That isn't the case. The actual consumption results (gallons/100 miles) shows that you would actually only need 2 cars of 22mpg to consume less fuel.

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Your assumption is I, or someone else, would use the incorrect formulas for achieving the answer to the question. Why do you believe such a thing?

My contention is if someone doesn't understand MPG and would fall under your assumption they would also have a hard time understanding gallons per mile.

DieselBruce said:

Right there shows how terrible mpg is at measuring fuel consumption. It is not linear at all, which is the whole problem. As the mpg numbers get higher, they mean less and less.

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So where do we set the standard? I've heard gallons per 100 miles. As the author of the blog stated, he used a higher miles number to remove as many decimals to keep down the confusion. That is a shame because decimals play a big part of what is proposed to be standard.

33 mpg = 3.030 gallons / 100 miles
35 mpg = 2.857 gallons / 100 miles
38 mpg = 2.652 gallons / 100 miles
40 mpg = 2.5 gallons / 100 miles
43 mpg = 2.325 gallons / 100 miles
45 mpg = 2.222 gallons / 100 miles
48 mpg = 2.083 gallons / 100 miles

The difference between 3 gallons / 100 miles and 2 gallons / 100 miles is 33 mpg and 48 mpg respectively. Sure, we could then use gallons / 1000 miles and move the decimal 1 place but then one is expecting the buyer to correlate savings based on 1000 miles driven to their every day life which averages to less than 40 miles driven per day.

On EPA's fueleconomy.gov site, Gallons / 100 miles is one of the three fuel consumption units selectable for display, besides MPG and L / 100 km. (click the "Personalize" button)

Each car's entry will also display the combined Gallons / 100 miles rating, regardless of your selected fuel consumption unit, alongside the city and highway consumption in your selected unit (MPG, L / 100 km, or Gal / 100 miles).

When Gal / 100 miles is selected as your preferred unit, the combined Gal / 100 miles ends up displayed twice for each car.

Gal / 100 miles ratings on EPA's fuel economy site is shown with one decimal place.

I prefer to use cubic Angstroms per Furlong.

My car uses about 560,000,000,000,000,000,000,000,000 cubic Angstroms of fuel per 497.097 Furlongs.

DieselBruce said:

You're not getting it.
This example from that blog demonstrates perfectly how mpg can be misleading.

• An SUV with an efficiency of 16 mpg consumes 62.50 gallons per 1,000 miles.
• A compact car with an efficiency of 35 mpg consumes 28.57 gallons per 1,000 miles.
• Total for both cars is 62.50 + 28.57 = 91.07 gallons per 1,000 miles.
• 91.07 ÷ 2 = 45.54 gallons per 1,000 miles (per car).
• A car with efficiency of 22 mpg has a consumption of 45.45 gallons per 1,000 miles (which is less).

If you just took the 2 mpg numbers and averaged them out you would assume that you would need 2 cars of 25.5 mpg average to break even for fuel consumption. That isn't the case. The actual consumption results (gallons/100 miles) shows that you would actually only need 2 cars of 22mpg to consume less fuel.

Right there shows how terrible mpg is at measuring fuel consumption. It is not linear at all, which is the whole problem. As the mpg numbers get higher, they mean less and less.

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Well, MPG is kind of dumb because it is a rate of distance over fuel consumed. It's kind of like measuring speed as hours/100 miles, or time over distance traveled.

Averaging the MPG for two cars is kind of like saying both cars use one gallon of fuel, yet travel different distances. Averaging the gals/1000miles is like saying they both travel 1000 miles yet consume different amounts of fuel.

The problem is that they don't share the same tank of fuel.

So if you hooked up both cars to a shared tank of 2 gallons, the 16 MPG car would use more fuel than the other and your average MPG would end up being 22 MPG.

Likewise, if you made both cars drive for 1000 miles and then took the total fuel consumed and divided by 2000 miles, you'd also get 22 mpg.

The problem arises when you try to average the two numbers. You need to take a weighted average so that distances are the same.

e.g.
2.1875 gallons*16 mpg = 35 miles
1 gallon*35 mpg = 35 miles

70 miles/3.1875 gallons = 22 mpg


So it's not really fair to say MPG is flawed or a terrible way to measure something... you just need to know how to manipulate the data properly to get the results you expect from a rate of fuel consumption (instead of a rate of distance traveled).

That said... MPG is pretty stupid

If I drive in 1st gear for one mile @ 1500rpm that'll be like, 2mpg? 1?

In 6th @ 1500 is like 40mpg?

If we could make considerable torque @ 100rpm and had a ratio of 0.03:1 that'll be like 700 MPG.

Ha.


Yeah, it's goofy. Just pump in fuel and drive it. Keep it simple.


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So, I delve into the alternative reading material "diagenetics"... sounds like Dianetics and equally as interesting.

The equation is fuel/ distance/ time. The power of irridium works better when you are considering time/space warp, but earth-bound Trekkies still have to depend on the internal combustion engine to get around.

Let's just say I'd rather save time then fuel. If I were to save $2 of fuel per hour by going 10mph slower, then I am being inefficient. I'm worth more than that difference per hour. Over the course of a 90 mile drive, if I save $3, it costs me 5x that in lost income stream at my job. The numbers work better if you are employed at minimum wage.

For me, driving faster/ cost of fuel; break-even point is moot, because I'd end up getting jail-time from the excessive speed.

I think I'll just continue to drive at a speed just under that which I would receive a traffic violation. That is my most effective driving option.

Back to my original point... saving fuel...not worth it in more ways than one.

Jason... The K.I.S.S. Principle X2 Put fuel in. Drive.

Geordi said:

Where in Canada can you drive 90-95mph and not have the mounties on your ass?

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Quebec. My commute is always above 75 (120km/h).

Fuel consumption over distance at various speeds (Metric) - Touareg TDI and Hybrid, and Jetta TDI





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Fuel consumption over distance at various speeds (USA) - Touareg TDI and hybrid, and Jetta TDI