90 vs 60 and Bugatti vs NissanApril 29, 2008 5:26 PM   Subscribe

Help me solve two MPG (and car) related arguments

(quick side-note, neither of these are serious "OMG UR SO DUMB!" arguments - more like random musing. Though, the first argument has very practical applications)

Argument 1 :
I read this article stating that peak fuel efficiency is at 55-60MPH - and that you lose 5-7% efficiency per 5MPH over that. Using cruise control, obviously, increases that even more. My current car won't let me use CC above 90MPH.

But, my argument (really more of a question) is that if I drove my car from, say, LA to Phoenix (375 Miles, est) at 90MPH vs 60MPH, I'd ultimately help the environment because I'd be on the road for about an hour and 40 mins less time.

Does that make up for decrease in fuel efficiency, or no?

Argument 2 :
I was laughing at the MPG estimates on the Bugatti Veyron - which are 6MPG city and 10MPG highway. Going it's top speed, it would run out of fuel in just under 13 minutes. Of course, one would have traveled roughly 55 miles.

This lead to me saying "that means my piddly 6-speed 2.5L Altima could probably take a Bugatti Veyron on a race to Phoenix from LA" to my friend, who's got a hard-on for that Bugatti.

The best math I had to support it (and I suck at math) is this :

LA to Phoenix = 375 miles.
Bugatti Veyron @ 253 MPH = 1.5 hours
Nissan Altima @ 140 MPG = 2.6 hours.
Altima makes (maybe) 1 stop for gas = add about 10 mins.

Totals :
Bugatti = 2.7 hours
Altima = 2.65 hours

This, obviously, assumes both cars could actually go their top speed the whole way. While the road is mostly straightaways (and let's suppose cops weren't a factor in this theoretical race), I still think one would have a much harder time maintaining 253MPH vs 140MPH.

My friend figures the Bugatti would win, hands-down, regardless.

So - which one would win?
posted by revmitcz to Travel & Transportation (24 answers total)

I'd ultimately help the environment because I'd be on the road for about an hour and 40 mins less time.

How would that help the environment? It's not your car being in motion across the surface of a road that I associate with the environmental impact of driving, it's the consumption of fuel and emission of waste products. If you drive at a less efficient speed, you burn more fuel per mile. That means (a) you have a bigger consumption footprint (more fuel needed to cover those 375 miles) and (b) you generate a greater share of emissions from burnt fuel.

That you were in your car for less time seems immaterial. If you sit in your parked car with the engine off for an hour, are you doing an hour of environmental damage?
posted by cortex at 5:33 PM on April 29, 2008

Response by poster: Okay, correction then : It would be less of an impact on the environment because it's an hour and 40 mins less of fuel consumption. The question is about whether it's less fuel consumption or more, because of the increased driving speed vs driving slower for longer.
posted by revmitcz at 5:36 PM on April 29, 2008

If you drive 375 miles and get n miles per gallon, you have consumed 375/n gallons of gas and generated an equivalent pile of emissions.

So, for example, if you're getting 40 mpg at 60mph, and (using 5% per 5mpg over 60 = 30% efficiency hit, 40 * 0.7 = ) 28mpg at 90mph, your fuel use at those speeds are 375/40 = 9.3 gallons, 375/28 = 13.4 gallons.

That you managed to burn that extra four gallons of fuel and pump the associated crap into the atmosphere in four hours instead of six has nothing to do with anything, anymore than me shooting someone ten times with an uzi is less bullet-y than shooting them twice with a musket.
posted by cortex at 5:42 PM on April 29, 2008

The efficiency you are dealing with is in MPG - miles per gallon. It doesn't matter how long you spend on the road, because the measure of efficiency is how many miles you can go per gallon of fuel consumed. Time doesn't enter into it. Travelling at 90mph you will consume more fuel per mile and thus will consume more fuel overall.

Using cruise control, obviously, increases that even more.

Cruise control doesn't make your car less efficient (in fact, it probably makes your car more efficient if your driving habits aren't very efficient).
posted by ssg at 5:43 PM on April 29, 2008

Okay, correction then : It would be less of an impact on the environment because it's an hour and 40 mins less of fuel consumption.

You are making this more complicated than it needs to be. If you drive 375 miles getting 37.5 m.p.g., you will use 10 gallons. If you drive 375 miles getting 30 m.p.g., you will use 12.5 gallons. Time does not enter into this equation at all.
posted by Dec One at 5:50 PM on April 29, 2008

I'd be on the road for about an hour and 40 mins less time.

So what? During that time, you'd use more gas, because you're driving faster, therefore spewing more pollution, than if you drove more slowly. Environmental impact relates to fuel used, not time engine is on.

Totals :
Bugatti = 2.7 hours
Altima = 2.65 hours

Your rounding is a bit off and creates this result, but in reality:
375/253=1.482 travel time +1.167 gas stops= 2.649 hours for the bugatti
375/140=2.678 travel time +0.167 gas stops=2.846 hours for the nissan
(70 minutes = 1.167 hours, 10 minutes = 0.167 hours)

However, since the Bugatti can travel almost 64 miles at full throttle on a tank of gas, in theory, it would need only 6 refuelings, not 7, which saves 10 minutes, reducing its total travel time to 2.482 hours. (This assumes rather carefully spaced fuel dumps.) Also, the Altima traveling with the pedal to the metal all the way would probably need more than one refueling.
posted by beagle at 5:50 PM on April 29, 2008

In a simplified physics model, driving slowly is definitely more efficient. The problem is that wind drag (thus fuel consumption) increases with the square of speed, whereas distance covered per unit of time increases only linearly (by definition) with speed.

Of course at low speeds (i.e. less than about 50 mph, as quoted by various sources including the one you link to) other real-world factors come into play such as rolling resistance, the optimal power range for internal combusion engines, etc.
posted by randomstriker at 5:52 PM on April 29, 2008

Response by poster: ssg : I mean that it increases the fuel efficiency more, not that it increases that 5-7% loss of efficiency even more.

And I see now that my original premise for the question of fuel efficiency was flawed. I'm not sure why neither or, nor the 10 people I've asked prior to this didn't seem to catch on to that. Most said "good point.. I'm not sure".

>Which car would win? That depends on whether your assumptions about duration of pit stops and actual fuel consumption are correct. Are they?

Well, I had assumed between 15-20 MPG in the Altima. Normally, I make it to Phoenix with between 1/2 and 1/4 tank of gas still left. In this theoretical race, I assume I'd probably need to make at least one stop.

On the flipside, another wrench in the Veyron plan is that apparently the tires would blow out after 15 mins of top-speed racing. Makes me wonder how long before those on the Altima would do the same. If you accounted for changing all 4 tires at every gas stop, I'm sure the time difference between the Altima and the Veyron would be even greater.
posted by revmitcz at 5:55 PM on April 29, 2008

I was laughing at the MPG estimates on the Bugatti Veyron - which are 6MPG city and 10MPG highway.

MPG estimates are standardized according to a very strict model, and completely irrelevant to your question.

City estimates are lower because of adjustments for low speed stop-n-go traffic -- wind drag is negligible and almost all energy is devoted to repeated accelerations. Highway estimates assume a more constant speed, where most energy is devoted to overcoming wind drag.

posted by randomstriker at 6:00 PM on April 29, 2008

I'm not sure why neither or, nor the 10 people I've asked prior to this didn't seem to catch on to that. Most said "good point.. I'm not sure".

Because they didn't take or don't remember their high school physics. It's not that complicated, really. See also: "there ain't no such thing as a free lunch"
posted by randomstriker at 6:02 PM on April 29, 2008

Response by poster: >However, since the Bugatti can travel almost 64 miles at full throttle on a tank of gas, in theory, it would need only 6 refuelings, not 7

Assuming the Wikipedia math of 12:43 between refuelings, and going 253MPH, I came up with 53.5 miles between refuelings - not 64.

253MPH / 1 Hour = 4.2 miles per minute
4.2 * 12.75 Mins = 53.5 Miles per refueling = 7 refuelings.

(again, my math generally sucks - but those are the numbers I was going on)
posted by revmitcz at 6:06 PM on April 29, 2008

Sorry, you're right, 7 pit stops. I mixed up imperial gallons and US gallons.
posted by beagle at 6:12 PM on April 29, 2008

Althought, reallly: the tank holds 26 gallons, it gets 2.05 mpg at full throttle, so it can go 53.3 miles on a tankful. So, 375 divided by 53.3 = 7.036 refuellings. In other words, it really could make it on 6 refuellings, and coast the extra distance (1.92 miles, or 0.32 miles at every stop -- piece of cake if you're doing 253 mph). Either way, Bugatti wins.
posted by beagle at 6:17 PM on April 29, 2008

You have the fuel consumption at top speed of the Veyron, but you don't know the fuel consumption at top speed of the Altima. Without that number, you really can't make the comparison you want to make.
posted by ssg at 6:17 PM on April 29, 2008

But, my argument (really more of a question) is that if I drove my car from, say, LA to Phoenix (375 Miles, est) at 90MPH vs 60MPH, I'd ultimately help the environment because I'd be on the road for about an hour and 40 mins less time.

Does that make up for decrease in fuel efficiency, or no?

No.

The fuel consumption figures are given per mile, not per hour. So time is an irrelevant metric.
posted by pompomtom at 6:43 PM on April 29, 2008

10 minutes for refueling seems a bit much. There are a lot of gas stations right off of most interstates. I guess with lights and traffic, maybe. But then again, with traffic, there's no way you can rip down the highway at 200+MPH. So which is it? Open roads, including pulling into the gas station, or normal traffic?
posted by gauchodaspampas at 7:05 PM on April 29, 2008

The article states that 55 to 60 m.p.h. is the optimum speed for fuel-efficient highway driving. It's not the most efficient speed at which to drive, which is a function (mostly) of your drivetrain and wind resistance.
posted by Kwantsar at 7:05 PM on April 29, 2008

Just as a point of technicality, the whole square of speed drag thing depends greatly on the shape of the object in question.

And MPG depends greatly on at what RPM the engine is turning at and whether it is efficient at producing the required power at that speed. You won't save gas by idling to work.
posted by gjc at 7:46 PM on April 29, 2008

Slightly off topic, but a neat way to visualize gas usage:

You can measure fuel not only by miles per gallon, but by square millimeters.

Imagine all of your gas used in 100 miles was contained into a vessel. That vessel would be about 4 gallons.

Imagine then, if that vessel was stretched out to be 100 miles long.

That string of gas would be threadlike. A cross section of that gives an average usage of fuel. A cross section of the thread of fuel from a 50mpg car would be 1/2 of the size of the 25mpg car.

To think we are so efficiently able to move 3000 pounds of car is totally amazing to me, and we aren't even close to maximum.

25 miles per gallon = 0.0940858333 square millimeters
50 miles per gallon = 0.0470429167 square millimeters

That is smaller than the period at the end of the sentence.
posted by bensherman at 8:48 PM on April 29, 2008 [1 favorite]

Response by poster: >10 minutes for refueling seems a bit much. There are a lot of gas stations right off of most interstates. I guess with lights and traffic, maybe. But then again, with traffic, there's no way you can rip down the highway at 200+MPH.

Well, once you thrown in the change of all 4 tires at every gas stop - it's easily 10+ mins per stop. That's supposing you could actually find someone to do it racecar-style. Else the race is more like 3 hours vs 3 days.

The point of the argument I made to my friend was that my car (or maybe any late model sedan) - which is by no means "super", and really more like a leisurely family sedan, could theoretically outrun a \$1.2Mil car in an interstate race. And, though my car's speedometer says it can hit 160MPH, I doubt it's been through the same rigorous tests as the Veyron, so I'm assuming 140MPH is a fairly safe bet for "top speed".
posted by revmitcz at 9:38 PM on April 29, 2008

On the flipside, another wrench in the Veyron plan is that apparently the tires would blow out after 15 mins of top-speed racing. Makes me wonder how long before those on the Altima would do the same. If you accounted for changing all 4 tires at every gas stop, I'm sure the time difference between the Altima and the Veyron would be even greater.

It is a nonsensical argument and requires ridiculous extremes on only one of the participants to approach making sense in your example. This is not reasonable.

The tyres on the Altima would be entirely unaffected by prolonged speeds at 140mph if they were correctly rated. The loads on a tyre at 250mph+ are massively higher than at 140, especially considering the significant downforce that the Veyron produces.

As you have no figures at all for MPG for the Altima at max speed, your math figures are completely irrelevant. The Bugatti will always win unless you limit top speed and/or number of fuel stops. In a straight race (where neither necessarily has to drive at Vmax) then it will still win.

Incidentally, it shouldn't take more than ten minutes to change four tyres (assuming they were there and ready, being as you can't carry spares in the Veyron). I could change all four in that time just on my own and fill up, easily - without anything more technical than a hydraulic jack and a battery wheel gun from Walmart.
posted by Brockles at 10:22 PM on April 29, 2008

The article states that 55 to 60 m.p.h. is the optimum speed for fuel-efficient highway driving. It's not the most efficient speed at which to drive, which is a function (mostly) of your drivetrain and wind resistance.

Up to a point, yes. But it is a pretty reasonable threshold as aerodynamic forces become sizeable (ie worth considering) above that. There really aren't many vehicles that produce anything other than more drag above that speed, just by progressively lesser amounts.
posted by Brockles at 10:26 PM on April 29, 2008

ssg: "You have the fuel consumption at top speed of the Veyron, but you don't know the fuel consumption at top speed of the Altima. Without that number, you really can't make the comparison you want to make."

Also, the statement about peak fuel efficiency is simply wrong, because that will vary dramatically for different cars (different weights, drag coefficient, gearing and other factor swill influence this).

Also also, you can't drive either the Bugatti or the Nissan at full throttle the whole way. There must be some corners in there somewhere? The Bugatti however, can probably spend more time at full throttle than the Nissan.
posted by dg at 2:57 AM on April 30, 2008

I had this argument over the weekend. I went on a road trip with my daughter and parents, and found myself getting scolded for driving 75 instead of 70.

Our car had one of those instant mileage doohickeys, so we could actually answer this question. Driving about 10% faster seemed to reduce our "miles per gallon" by about 10%. I thought this was worthwhile to shorten an eight-hour drive ("hours per mile") by 10%. My dad didn't want to pay the extra 20% in "gallons per hour."

So to answer your first question: driving faster is good for something, but it's not the environment, and it's not your wallet. The two effects add, rather than cancelling out.
posted by fantabulous timewaster at 7:53 AM on April 30, 2008

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