[BRussell]

Which will save more fuel, switching from a 15 mpg car to an 18 mpg car, or switching from a 50 mpg car to a 100 mpg car?

[Chips G]

50 to 100, assuming that there are no other considerations (city vs. highway driving, type/cost of fuel of each car if different, etc.)

[BRussell]

You sure about that?

[Chips G]

15 to 18 - 20% increase in fuel efficiency
50 to 100 - 50% increase in fuel efficiency

50 > 20, unless I'm missing some catch here (which I feel like I am, based on your post)

[subego]

Originally Posted by Chris Gilpin 

unless I'm missing some catch here (which I feel like I am, based on your post)

Which will save more fuel?

P.S. It should be noted that each car is being driven the same distance.

[mduell]

1800 mile drive for easy math:

15MPG uses 120 gallons
18MPG uses 100 gallons
20 gallons saved

50MPG uses 36 gallons
100MPG uses 18 gallons
18 gallons saved

[imitchellg5]

I went from a 27 mpg car to a 22 mpg car, and it really hasn't been much different. I just have to fill up about a day earlier (I don't really drive a ton, only about 10 miles a day most of the time).

[TheoCryst]

I voted 50 to 100, and then caught the trick in the math. That's actually pretty interesting.

[olePigeon]

Originally Posted by mduell 

1800 mile drive for easy math:

15MPG uses 120 gallons
18MPG uses 100 gallons
20 gallons saved

50MPG uses 36 gallons
100MPG uses 18 gallons
18 gallons saved

I'd still take the 100 MPG car. You're using only 18 gallons instead of 100 gallons of gasoline.

[subego]

Originally Posted by TheoCryst 

I voted 50 to 100, and then caught the trick in the math. That's actually pretty interesting.

It makes more sense if you think of it in terms that one car has to drive 50 whole miles before you've saved a gallon.

[subego]

Originally Posted by olePigeon 

I'd still take the 100 MPG car. You're using only 18 gallons instead of 100 gallons of gasoline.

That isn't what this is trying to illustrate.

It's pointing out that the 100 MPG ain't all it's cracked up to be compared to 50 MPG.

Choosing the 50 MPG means you are using 36 gallons instead of 18.

The effort involved to squeeze that extra 50 MPG out of a car that is already fuel efficient isn't worth the effort, whereas making something which isn't fuel efficient slightly more so is worth it.

This is a/k/a diminishing returns.

[subego]

Originally Posted by TheoCryst 

I voted 50 to 100, and then caught the trick in the math. That's actually pretty interesting.

I only noes cuz I hadz duh gooogl,

[stevesnj]

I will be buying a new 2008 Jetta TDi... 45 to 50 MPG (diesel) about 550 miles tank my Toyota Pickup gets 22 Mpg (Gas) about 280 miles tank and Diesel is only $3.29 gallon here. Thats about equivalent to spending $1.65 gallon. I cant wait!!

[Uncle Skeleton]

Originally Posted by subego 

The effort involved to squeeze that extra 50 MPG out of a car that is already fuel efficient isn't worth the effort, whereas making something which isn't fuel efficient slightly more so is worth it.

Sorry, but according to the poll results you're wrong.

[hyteckit]

Quantity or percentage?

Percentage, I'll go with the 50 to 100 gallon.

[Warren Pease]

Originally Posted by subego 

That isn't what this is trying to illustrate.

It's pointing out that the 100 MPG ain't all it's cracked up to be compared to 50 MPG.

Choosing the 50 MPG means you are using 36 gallons instead of 18.

The effort involved to squeeze that extra 50 MPG out of a car that is already fuel efficient isn't worth the effort, whereas making something which isn't fuel efficient slightly more so is worth it.

Sure, the extra effort. But that effort is negligible because even if you don't take into account the improvement of 100 mpg and just stay at 50 mpg, you are still using less fuel than the 18 mpg or the quote-unquote better 36 mpg fuel efficiency.

It's the question percentage value vs. absolute value. To answer your question, a 50 mpg capable car will save more fuel than any 18 or 36 (or 49) mpg vehicle just by the simple fact that it uses, it needs, by definition, less fuel than the others.

[subego]

Originally Posted by Warren Pease 

But that effort is negligible because even if you don't take into account the improvement of 100 mpg and just stay at 50 mpg, you are still using less fuel than the 18 mpg or the quote-unquote better 36 mpg fuel efficiency.

Q.E.D.

Don't waste your time squeezing out 100 MPG. 50 does fine.

[BRussell]

Originally Posted by subego 

The effort involved to squeeze that extra 50 MPG out of a car that is already fuel efficient isn't worth the effort, whereas making something which isn't fuel efficient slightly more so is worth it.

This is a/k/a diminishing returns.

Yep, you got it.

There's an awful lot of focus on super-fancy tech to make already-efficient cars more efficient, when we'd get a lot more bang for our buck just by using already-available tech to improve the gas guzzlers at the bottom of the scale.

Congress just passed a law to raise the standards, but focusing on the low end rather than the average would do more good.

BTW, I got the idea for this here.

[hyteckit]

More reason to increase the car MPG to 30/35 mpg.

The amount of gas saved from going from 15 to 18 mpg is a big jump. Imagine how our dependency on foreign oil will drop dramatically just by increasing the gas mileage by an average of 5 mpg from the current average.

[richwig83]

That quite clever, i voted for 50-100....

[torsoboy]

Originally Posted by BRussell 

Yep, you got it.

There's an awful lot of focus on super-fancy tech to make already-efficient cars more efficient, when we'd get a lot more bang for our buck just by using already-available tech to improve the gas guzzlers at the bottom of the scale.

Congress just passed a law to raise the standards, but focusing on the low end rather than the average would do more good.

I think I would need to see the % of people using the gas guzzlers to follow this logic. It seems to me like increasing the average is a much better idea and gives more bang for the buck . If the MPG for the middle 80% of the population (leaving 10% on the bottom and 10% on the top) goes up 5 MPG that would be more significant than increasing it 5 MPG for just the bottom 10% it would seem.

[jokell82]

Originally Posted by olePigeon 

I'd still take the 100 MPG car. You're using only 18 gallons instead of 100 gallons of gasoline.

Bingo.

[BRussell]

Originally Posted by torsoboy 

I think I would need to see the % of people using the gas guzzlers to follow this logic. It seems to me like increasing the average is a much better idea and gives more bang for the buck . If the MPG for the middle 80% of the population (leaving 10% on the bottom and 10% on the top) goes up 5 MPG that would be more significant than increasing it 5 MPG for just the bottom 10% it would seem.

That's a good point. The original example is artificial, in the sense that it's unlikely that the vehicles with low and high mpg travel the same number of miles per year.

However, my guess is that low-mpg vehicles in general travel more miles per year than high-mpg vehicles - trucks and buses for sure, but also my guess is that people who get small cars are more likely to travel shorter distances, whereas people who need to drive more get bigger, more "luxury" cars with worse mpg. If that's true, it would make the basic point about diminishing returns even more important.

[nonhuman]

C: Switching from a 15 MPG car to a 100 MPG car.

[wallinbl]

Originally Posted by nonhuman 

C: Switching from a 15 MPG car to a 100 MPG car.

That's what I don't really get, either. Why the specific pair of choices? Is this some way to justify buying a more fuel efficient Hummer?

[BRussell]

Originally Posted by wallinbl 

That's what I don't really get, either. Why the specific pair of choices? Is this some way to justify buying a more fuel efficient Hummer?

Read some of the posts and think about it a bit.

[torsoboy]

Originally Posted by BRussell 

Read some of the posts and think about it a bit.

There is no reason to get a more efficient Hummer. If you're going from a 15MPG Hummer to an 18MPG Hummer the difference is big, but to go from a 15MPG Hummer to a 60 MPG hybrid the difference is MUCH bigger.

Forget the 15-18MPG junk... that's for people wanting to justify buying something that is not needed (unless you're a long-haul truck driver of course). Buy a Hybrid or other high MPG vehicle and save yourself some money (along with helping out the planet a little).

[jokell82]

Originally Posted by torsoboy 

There is no reason to get a more efficient Hummer. If you're going from a 15MPG Hummer to an 18MPG Hummer the difference is big, but to go from a 15MPG Hummer to a 60 MPG hybrid the difference is MUCH bigger.

Forget the 15-18MPG junk... that's for people wanting to justify buying something that is not needed (unless you're a long-haul truck driver of course). Buy a Hybrid or other high MPG vehicle and save yourself some money (along with helping out the planet a little).

If you want to help the planet, don't buy a hybrid. Buy an efficient four cylinder or diesel car. Hybrids wreak havoc on the environment.

If you just want to save gas, then by all means go for a hybrid. But don't think you're saving the world while you do it.

[Warren Pease]

If you want to save the planet - walk. Or bike.

[Teronzhul]

I'm just going to keep riding my motorcycle. 55mpg is enough for me.

[stevesnj]

Originally Posted by Warren Pease 

If you want to save the planet - walk. Or bike.

yeah im going to walk 22 miles, one way, a day. I would spend more money for shoes than gas!

[subego]

Originally Posted by torsoboy 

Forget the 15-18MPG junk... that's for people wanting to justify buying something that is not needed.

You're mistaken in thinking people who buy such things need, seek or care about justification.

You're doubly off base in that the exceptions sure as hell aren't going to do so with a math exercise.

[AngelaBaby]

Interesting find.

What I heard is that the EPA is a big joke. Miles per gallon aren't actually determined, it's just a calculated guess.

[spindler]

Really good observation BRussell. On the other hand though, unfortunately, going from 15mpg to 18mpg probably will motivate a lot less people then going form 15 to 27mpg for the same car. And also going from 25 to 40mpg might not motivate some people that going from 25 to 70mpg for the same car would.

So psychologically it's hard to sell the 15 to 18 increase. That would be a point too fine to spread throughout the population. Also the money spent on the technology of going from 50 to 100 does filter down to the 15mpg car maybe taking it to 18mpg all by itself.

[Laminar]

Originally Posted by spindler 

Really good observation BRussell. On the other hand though, unfortunately, going from 15mpg to 18mpg probably will motivate a lot less people then going form 15 to 27mpg for the same car. And also going from 25 to 40mpg might not motivate some people that going from 25 to 70mpg for the same car would.

So psychologically it's hard to sell the 15 to 18 increase. That would be a point too fine to spread throughout the population. Also the money spent on the technology of going from 50 to 100 does filter down to the 15mpg car maybe taking it to 18mpg all by itself.

Truly, you have a dizzying intellect.

[torsoboy]

Originally Posted by jokell82 

If you want to help the planet, don't buy a hybrid. Buy an efficient four cylinder or diesel car. Hybrids wreak havoc on the environment.

If you just want to save gas, then by all means go for a hybrid. But don't think you're saving the world while you do it.

Agreed. For now at least. In the future that will (hopefully) change.

[BRussell]

Originally Posted by torsoboy 

There is no reason to get a more efficient Hummer. If you're going from a 15MPG Hummer to an 18MPG Hummer the difference is big, but to go from a 15MPG Hummer to a 60 MPG hybrid the difference is MUCH bigger.

Forget the 15-18MPG junk... that's for people wanting to justify buying something that is not needed (unless you're a long-haul truck driver of course). Buy a Hybrid or other high MPG vehicle and save yourself some money (along with helping out the planet a little).

Commercial vehicles like semis and buses get something like 5 mpg today. It's nice to tell them to switch to a Prius, but not very practical, is it?

The point is that we would be better off trying to improve the fuel efficiency of low-MPG vehicles, which we already have the technology to do, than trying to squeeze extra fuel efficiency out of already-efficient vehicles, which is harder to do and requires significant changes in technology.

For example, rather than (or in addition to) raising the average mpg, we could require minimum standards. That would probably have more of an effect. Or we could focus research and tax breaks on technology that would improve the fuel efficiency of those low-efficiency vehicles rather than already-efficient vehicles like small cars.

[iLikebeer]

Originally Posted by stevesnj 

I will be buying a new 2008 Jetta TDi... 45 to 50 MPG (diesel) about 550 miles tank my Toyota Pickup gets 22 Mpg (Gas) about 280 miles tank and Diesel is only $3.29 gallon here. Thats about equivalent to spending $1.65 gallon. I cant wait!!

Make that a 2009 Jetta TDi. It won't be available until October now. Hopefully they don't push it back again. I'm expecting anywhere from 40-50 MPG, it has a bigger engine than the 06.

The 50 to 100 MPG leap will still be huge in the future. When there are tens of millions of cars being driven every day, halving the fuel consumption would make a significant difference in oil dependency and pollution. It just seems like diminishing returns because of the way he set up the example.

[ghporter]

Math trick or not, I think I'd save more gas with the 50-100MPG change. I'd drive more, and thus have more opportunity to rack up miles at a lower per mile cost, so I might even decide to drive places I would otherwise have used another method, such as flying. So in driving, I'd save more gas because I'd drive more. Maybe a lot more.

[Uncle Skeleton]

Originally Posted by BRussell 

Commercial vehicles like semis and buses get something like 5 mpg today. It's nice to tell them to switch to a Prius, but not very practical, is it?

The point is that we would be better off trying to improve the fuel efficiency of low-MPG vehicles, which we already have the technology to do, than trying to squeeze extra fuel efficiency out of already-efficient vehicles, which is harder to do and requires significant changes in technology.

What technology are you talking about for making semis and buses more efficient?

[awaspaas]

The question is silly. Change the first part of the question to 20-->24 and you get a different answer. This is not the way to assess fuel efficiency standards.

[Ghoser777]

Here's the generalization of the trick. If you go from a MPG to b MPG over a distance of x miles, then the savings in gallons of gas are given by:

x / a - x / b = (xb - xa)/(ab) = x(b-a)/(ab)

So (b-a)/(ab) is the key part. As a and b get larger, that term tends towards zero (as the product is going to dominate the difference), and hence the savings in gallons of gas is small.

To spell out the reason why this is important in general, it makes more sense to improve low performing parts of a system than it is to improve high performing parts of a system. In education, that means (unfortunately in some ways) it may make more sense to spend lots of resources preparing low achieving students for a standardized test instead of supporting programs that aid all levels of students (like fine arts).

[JohnM15141]

Originally Posted by mduell 

1800 mile drive for easy math:

15MPG uses 120 gallons
18MPG uses 100 gallons
20 gallons saved

50MPG uses 36 gallons
100MPG uses 18 gallons
18 gallons saved

Let put this in proper perspective your wallet! Lets use $3.50 per gallon:

15MPG uses 120 gallonsx$3.50=$420 spent to go the distance of 1800 miles
18MPG uses 100 gallonsx$3.50=$350
20 gallons saved and $70

50MPG uses 36 gallonsx$3.50=$126
100MPG uses 18 gallonsx$3.50=$63
18 gallons saved and $63

Looks to me like the real winner is the guy spending $63 instead of $350 to go 1800 miles, a difference of $287

I've been driving a Honda Insight since 2000, I made up the the cost difference between it and gas Civic back in 2004, its been a bonus since then!

[Ghoser777]

Well it depends. Let's say you have a car and a truck. The car gets 50 MPG while the truck gets 15 MPG. It will save you more gas (and hence more money) if you can improve the truck to have 18 MPG than having the car improved to 100 MPG. That's not to say that improving the car is a bad thing, but it's not as good as improving the truck. The idea in this whole line of argumentation is that if you wanted to focus your attention on improving something that's inefficient and something that is efficient, you should focus on the inefficient thing.

[peeb]

Right - the point of this is 'go for the low-hanging fruit first'.

[Laminar]

Just hope your cat doesn't live by the same philosophy when you're going commando.

[mrtew]

Originally Posted by Ghoser777 

Well it depends. Let's say you have a car and a truck. The car gets 50 MPG while the truck gets 15 MPG. It will save you more gas (and hence more money) if you can improve the truck to have 18 MPG than having the car improved to 100 MPG.

Plus it's really easy to make a truck get 18mpg and impossible to make a car get 100mpg.

[nonhuman]

Originally Posted by mrtew 

Plus it's really easy to make a truck get 18mpg and impossible to make a car get 100mpg.

Eh? Diesel cars are already approaching 75 mpg, if they haven't hit it already. I don't think it will be too long before we actually hit that mark.

A diesel hybrid, or even a petrol hybrid, using the new nanowire batteries should easily hit that mark as well (once the batteries are being commercially produced, that is).

[Uncle Skeleton]

link?

[nonhuman]

The VW Polo gets about 62 mpg (Volkswagen's Polo Blue Motion: 62 MPG of Diesel-Sipping Fun (TreeHugger)). In that same article there's a link to a diesel concept car that gets nearly 160 mpg.