Why America's MPG Is A Dumb Unit For Fuel Economy

hello everyone and welcome in this video we're talking about why miles per gallon is a dumb unit so this video has a little bit of something for everyone some of you enjoy math on a whiteboard i'm here for you and i think pretty much the rest of you we can all get behind uh enjoying trashing the units that we use in the united states and today we're going to trash the mile per gallon now i don't really have a problem uh that big of a problem with a mile you know this is a foot a mile is 5 5280 of these that's kind of weird but that's what a mile is all right i get it this is a gallon a unit of volume um there it is you know like a little bit weird but there it is something more weird though happens when you put these together so miles per gallon then something a bit strange happens so to demonstrate this we have a hypothetical scenario so you are the owner of 10 cars in this scenario nine of those cars car one are able to achieve 50 miles per gallon and then one of your cars car number two that is able to achieve five miles per gallon and so i come along to you and i'm like hey i'm an engineer right i know some stuff i can double the fuel economy of either one of your types of cars whether it be car number one of which you have nine of or car number two of which you only have one so that means you would either have nine cars with 100 mpg and one with five or nine cars with 50 mpg and one with 10 mpg and i say you know which one do you want me to do i'll hook you up and you know i think very obviously looking at scenario two that just seems the obvious choice right we're increasing nine cars fuel economy by 50 miles per gallon or we're just increasing one car's fuel economy by five miles per gallon another way of looking at it we're doubling the fuel economy of nine cars or we're just doubling the fuel economy of one car so easy enough we go with scenario two right well let's not rush into it let's at least do a little bit of math to make sure we make the right decision so let's just figure out what the average fuel economy is for each scenario so just as an example looking at scenario one we have nine cars getting 50 miles per gallon plus we have one car getting 5 miles per gallon we have 10 cars total 9 times 50 450 plus 5 455 divided by 10 45.5 miles per gallon so for scenario one that is our average fuel economy 45.5 miles per gallon now for scenario two nine cars at 100 that's 900 plus 5 divided by 10 that gives us 90.5 miles per gallon and then scenario three nine times 50 that's 450 plus 1 times 10 that gives us 460 divided by 10 total cars and that gives us 46 miles per gallon so obviously here if our average fuel economy with scenario number two is 90.5 which was our gut you know hey we should probably go with one where we double nine cars fuel economy that one gives us the best average fuel economy that makes sense that's the scenario we choose right so we're feeling pretty good but it never hurts to check your work so we're going to give the example of say we need to drive our entire fleet 100 miles and we want to know how much fuel are we going to use driving our fleet that 100 miles so if we're driving 100 miles and our car gets 50 miles per gallon well 100 divided by 50 means each one of these cars is going to use two 100 divided by 50 two gallons to travel those hundred miles multiply that by nine because we have nine of those cars and then we have uh one car getting five miles per gallon so 100 divided by five miles per gallon so that's going to use 20 gallons so we've got 2 times 9 18 plus 20 times 1 that gives us 38 gallons used so in scenario one before we get that you know magic applied to our cars that doubles its fuel economy we're getting 38 gallons that we're going to need in order for our entire fleet to travel 100 miles now if we do this same math for scenario number two well we're going to need 29 gallons of fuel so you can see significant savings uh by doubling the fuel economy of those nine vehicles we go down from 38 gallons required to 29 and then if we get to scenario three where we double the fuel economy of this one from five to ten miles per gallon we do that same math how much how many gallons of fuel do we use to travel 100 miles for our fleet 28 which is less than 29 which means this is our best scenario this is the one we should have gone with what okay so let's go back to our fun little demo here miles per gallon so distance per unit volume whereas in much of the rest of the world unlike the us where we say distance per volume they switch that around and say volume per distance so like in europe liters per 100 kilometers so what if we take a look at the average if we swap around our units in the u.s so instead of miles per gallon we do gallons per 100 miles so in this case we have our first car which has a hundred miles per gallon divided by 50 that gives us two miles per gallon we multiply that by the number of cars we have so there's nine of those and then we add to that of course we have our second car which is getting 5 miles per gallon so 100 divided by 5 which equals 20 and there's one of those cars so we have 18 plus 20 and then we divide that by the total number of cars which is 10 so 38 divided by 10 our average being 3.8 gallons per 100 miles and so easily as you can see for these next ones that's going to be 2.9 gallons per 100 miles is going to be 2.8 gallons per 100 miles so initially if i had posed this question and we were giving units of gallons per 100 mile and you did that quick little average to figure out hey which one of these is actually going to give me the best fuel economy would say hey that's the lowest number that's using the least amount of fuel to travel a certain distance on average that is the option i should have gone with but wait a minute you say in the united states we have something called corporate average fuel economy cafe and so this basically states that a company has to meet a minimum average mpg and so you're saying well if average mpg is nonsense if it doesn't even help us out is this all this cafe stuff is that all nonsense uh no they're actually smart enough to know the math behind this and they're actually doing something very similar to gallons per 100 miles rather than mpg and then translating it back so that you can understand it in mpg and so this is called a harmonic mean and so what they're doing is is you're taking the total number of cars so you're taking uh 10 cars here for example with scenario one and then how many cars do we have of scenario one we have nine cars and each car is getting 50 miles per gallon plus we have one car that is getting five miles per gallon so that is our harmonic mean and this gives us an average fuel economy of 26.3 miles per gallon obviously very different than that 45.5 when you do this math for the other ones here so scenario number two you get 34.5 miles per gallon and for scenario number three you get 35.7 miles per gallon so this is the mpg when cafe says you have to meet a minimum this is the one that they're talking about and you can see from this if i were to give you hey your fleet can have an average of 26 34 or 35 what average do you want obviously it's going to be easy to say that one and that is the correct choice whereas in this case 40x 46 mpg is the correct choice even though 90.5 sounds way better and if you actually look at the window sticker of a new car it will actually show you this number so here's a 2017 honda accord hybrid you get the fuel economy and mpg but underneath that as you can see 2.1 gallons per 100 miles versus something that doesn't have a very hot fuel economy here's a 2017 rolls-royce ghost a window sticker 7.1 gallons per 100 miles traveled so this is a unit that the epa uses and displays uh but more commonly it's switched over to mpg so that you know the rest of us can understand what it means even though much of the world is using the other unit gallons per 100 miles or liters per 100 kilometers so who cares what's the point uh i will say that if you have a car that has a 10 gallon tank and you know that it gets 30 miles per gallon well then you know the range of your car you simply multiply the two together you have 300 miles of range nice easy but when you start comparing mpg then things start to get a little weird because two cars gaining one mile per gallon don't save the same amount of fuel both of them gain one mpg you know same number but they don't save the same amount of fuel five to six mile per gallon that saves you 3.33 gallons for every 100 miles traveled whereas if you go from 50 to 51 mile per gallon obviously a much smaller percentage increase well you're only saving 0.04 gallons per 100 miles traveled versus using units of gallons per 100 mile or in other words volume per unit distance two cars saving one gallon per 100 miles they save the same amount of fuel so going from 10 to nine you save one going from two to one you know you still save one gallon per 100 miles traveled another kind of weird thing to think about is if you have a car that goes from 5 miles per gallon to 6 miles per gallon over every 100 miles traveled it's going to save 3.33 gallons if you have a cart 50 miles per gallon well that means to travel 100 miles you need two gallons so if we were to make that thing perfectly insanely efficient uh able to get you know a billion miles per gallon then the maximum potential we can save is two gallons over that 100 miles so no matter how high we got this number we would never save as much as from going from just five to six miles per gallon even if we had a billion miles per gallon it'd be smarter to take this one from five to six so i think that also kind of just indicates like hey the cars that are getting really bad fuel economy we should work on getting up you know once you get started getting to these higher numbers it does make less of an impact overall in conclusion maybe the rest of the world has some idea about what they're talking about when they start using units like liters per 100 kilometers uh you know volume per unit distance and actually tesla here in the u.s gives you watt hours per mile or you could do watt hours per kilometer versus i have seen other electric cars in the us which give you miles per kilowatt hour a lot like you know miles per gallon versus tesla kind of switch that around the nerdy engineering way where you can have these nice clean averages and the numbers make sense so thank you all so much for watching hopefully you enjoyed if you have any questions or comments please feel free to leave them below

Channel: Engineering Explained

Views: 702,307

Keywords: fuel economy, fuel, fuel efficient, mpg, best mpg cars, engineering explained, gasoline, diesel, best mpg, g/100mi, l/100km

Id: oLQmwOX6Xds

Length: 11min 22sec (682 seconds)

Published: Wed Apr 07 2021