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This started by me reading an article in readers digest, then it continued by the fact that my prof said this in ¡§environmental studies¡¨ class. Apparently if your drive faster, you use more gas, but I disagree, because if u drive faster you arrive quicker, so the engine is running less time therefore the same amount of gas. The only time you are using more gas is when u accelerate faster. Now I tried saying this to my prof, but she will not agree with me, and in an essay question on the final, this came up, and I stated what she taught us, and what I believe, so I am going to loose some points for disagreeing with her, unless I can actually prove to her that I am right, I have been trying to do the math, but I am not sure it is correct or convincing enough, so can someone help me out here.

Do u use more gas if u are driving 100 instead of 80 on the same trip?

Here is the math I cam up with, this is assuming your are steadily accelerating at the same pace, and that the ground is flat (no uphill)

¡§


Assume that the amount of gas used is G, and the speed of the car is S, and the Time of the engine running of T.

The equation for the gas used will be

G = AS + BT

Were A and B are some constants

Say you are taking a 50 km trip. If you drive steadily for 100 km/h, the total time it will take you to reach your destination is 0.5 hours. Therefore

G1 = 100A + 0.50 B

In the second scenario, you are driving 80 km/h, It will take you 0.625 hours to get there. Therefore

G2 = 80A + 0.625B


EQUATE BOTH EQUATIONS, YOUR GET

100A + 0.50 B = 80A + 0.625B

„Ã 20A = 0.125 B
„Ã 160A = B
So if the speed constant the 160 times the time constant then are using the same amount of gas in both scenarios. Therefore for the theory that u use more gas at higher speeds to be true that equation would be at least

G = 161S + B

Which seems very unrealistic.
¡§

 
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man...
too much reading..
not only that..
math formulas...
let me try and read it.

 
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drive at 55 mph.
air resistance is still low at that rate.
plus it give you the best MPG.

good ol' physic's

 
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Too much reading for me too, but higher speeds means higher air resistance and higher rpm's which both eat more gas. So, unless you're driving through a vacuum with different gearing I'm going with better fuel economy by driving slower.

 
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i dunno about alot of cars, but in my dad's hemi there is an instrument (dunno what it is called) that tells you miles per gallon at the speed you are going at...and the higher speed he went the miles per gallon dropped big time...so i think your equations are wrong...but iono...the engineers that made the instrument and vehicle could be wrong...

 
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I think when people say that, they are talking about the amount of gas you waste during hard acceleration up to those faster speeds. Of course if you got to the faster speeds with normal acceleration and then maintained it, you'd get to your destination in less time.

 
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here's your answer....

http://www.physlink.com/Education/AskExperts/ae177.cfm


here's another one ...
http://auto.howstuffworks.com/question477.htm

 
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damn your teacher is dumb...^^ nice find

 
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Your first mistake was you took the wrong class.
Anything that has to do with velocity (speed) needs to be resolved with Physics not Mathematics....

 
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take in to mind the coefficient of friction.

 
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You're an idiot - regardless of what you think, just do whatever your teacher says. You can research and find the right answer, just don't ever let her think her answer is wrong, or else you'll be sorry. There's no room for education in academia

Wind resistance takes its toll, and you neglect to mention that the engine is running at a higher speed when you're travelling faster. I know if I drive 150 miles at 80mph, my mileage is worse than if I do it at 65. There's always exceptions to the rule - if I was driving 60 mph in 4th gear, or travelling 75 mph in 5th, you'd obviously save gas going 75.

 
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Hey Civic_khaos, I'm too lazy to click those links... what's the answer

 
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From the top link:

 
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So its about DRAG on the car

 
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HONESTLY...
WHO CARES!!!!!!
i can care less to save a 1/4 gallon of gas.

 
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Driving at 80mph will eat more gas than driving even at 60mph.

Your teacher is correct--your overly simplified equation fails to take into account the amount of work the engine has to do.

To maintain 100 mph, the engine is running faster than at 80 mph, as is shown in the RPM. Your engine is doing more work to spin the pistons and denote fuel, consuming more gas. But more importantly, to maintain that speed requires a lot more fuel than to maintain say 60 mph, due to our engines not being very efficient in power at speeds of 100 mph (Civics aren't exactly made for high speeds anyway).

Translating that into how you drive, to maintain 100 mph, you'll notice you have to put a lot more gas to the gas pedal to keep the speed. For 60 mph, you'll notice you can just lightly touch the pedal to keep the speed.

Also, the higher the speed the more pronounced the friction will be.

Driving at 60 mph makes a big difference from my experience. I went 400 miles going 60, while I can get about 300 going 80-90.

-Aki

 
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Look I will answer this easy becuase I have driven a route that's around 250 miles two different ways with cruise control on all the way.

One time I was driving the speed limit at 70mph all the way with no stops. When I got to my destination I was half a tank.

Another time I was going steady at 110 with no stops all the way (extremely lucky I guess). Anyways both times the conditions were the same except one factor...speed.

When I drove 110 all the way, I was on empty by the time I got to my destination. So do you burn more gas at higher speeds?? I'd say that's a definate yes. You get to your destination faster, but due to the cost of gas.

 
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I just got back from driving 8000km, of the various tanks of gas I filled up, the ones where I was travelling 100mph vs 75mph went a lot faster, by about a 20-30% variation.

 
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*smack* what myth?

it has little to do with what you said, if any at all.

driving faster and spending less time driving doesn't mean you use less gas.

it can't be a single answer too, because it depends on many condition.

read the links provided. you thought way too much into that little problem without considering the actual factors that affects mileage.

but to answer your question: Do u use more gas if u are driving 100 instead of 80 on the same trip?

YES

if all else is equal (road condition, weather condition, type of car, type of transmission, the gear the car is in, the same tires, the same passenger load, etc. etc.), YES you use more gas.

now if the question is do you use more gas by driving faster? yes, but not all the time. there's a certain point in your car where the best miles per gallon can be achieved, and when you drive slower than that point, or faster than that point.. you'll use up more gas. for most car, this point would be around 50-60 mph.

 
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Actually in todays modern cars it doesnt really matter after you reach a certain speed. In a Civic I would think it would be safe to say after about 65 the wir resistance and time of arrival would equal themselves out. But yeah do the speed limit buddy

 
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I didn't actually read everything said above, but here is what I have to say.
Major factors,

Drag (mechanical parts of the car, probably constant)

Drag (air and tires to road, non linear)

Engine efficiancy, vs load, vs output, vs rpm/gearing. (man don't even wanna think on that one)

Rate of acceloration (basic physics, the more rapidly you accelerate, the more power it takes. not sure but maybe roughly exponential or logarithmic or something, for example if I want to make my 17 sec civic into 16 second one, and then a 15 second one, i may have to add 10hp to drop the first second, but have to add 20 on top of the 10 to drop the second second)

distance traveled

duration at constant speed

range of speed, and range of acceleration

and whatever else I have not thought of.

Now lets travel to the wonderful world of physics (the land of perfect geometry, frictionless surfaces, and 100% efficient energy transfer)
Aw hell lets just go to space.
Now I want to travel in my new honda space cruser from point a to b (if I wasn't just going from a to b I would have got the space cruser si damnit! )
I can do two things,
Use as little gas as possible
Get there as fast as possible

To use as little gas as possible I would just tap the throttle once so that I floated at 1mph or something. only problem is I get to b about 200 years later
To get there as fast as possible I would accelerate full throttle till the half way point, then decelerate at reverse full throttle. 1 day and I'm there, but also used all my gas because I was in i-vee-thechno or something the whole time.

and I'm about to be late for physics ironically
but at some point between these two extremes there is a balance, a rate of acceleration that will yield the best efficiency and best overall speed relative to eachother.
and that is the extreme oversimplification.

 
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If you drive either 80mph or 100mph.. You're doing 2 things..

1) Breaking the LAW... the highest speed limit in America is 75mph on the interstate.
2) Not only are you "using less gas" but if that were the case, you'd be spreading it in a larger area. If everyone hauled *** on the interstate, there would be as much smog there as there would in packed cities such as LA and Downtown Houston.

 
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best efficency is somewhere in the 55-70 range in my civic. I have no fancy answer for it. That has just always worked the best on long trips and in all types of conditions. but yeah there is a happy medium at which you get reasonable speed and good efficency. going faster is not always the answer nor is going slower. here is a dramatic exmple. In my last truck, a silverado, I achevied a city milage of 8.6mpg and a highway milage of around 8.0mpg. one time I was stuck in ventura with no gas and about $10 to get home on. This wouldnt have made it normaly so I decided to buy gas on credit. I drove very carefuly at around 55mph for th ewhole way back and out of curiosity I checked my milage when i got here into simi. 19.4mpg! It was such a shock to get that kind of milage out of that truck but I just figured that 55 was the speed at which it got the best efficency. to give some frame of referance for my other figures I usualy drove that truck between 80 and 85mph on the freeway, those speeds yeilded the earlier number

 
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In my gti.. when i go home from college for the weekend. A trip at 85-90 all the way home and back and i get about 30-50 less miles on a tank of gas than when im going 73-80. 30-50 isnt 2 much of a loss for me though, maybe on someone who is only getting 14mpg is it worth worrying about.

 
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here's the down and dirty.

at a given RPM, the engine is outputting the optimum amount of power for the gas it's consuming, so, unless you know that RPM it's impossible to just say "55 mph or 65 mph" BECAUSE, it depends on what gear you're in. This is a much simpler way of looking at it, especially since you can just ignore drag all together.
Let's take my HX for example, and lets just say the optimum engine rpm is 2500,
in 1st, that means, that driving at 15mph, is the most efficient.
2nd is 25 MPH,
3rd is 37.5 MPH
4th is 50 MPH
5th is 62.5 MPH

these speeds are based on a theoretical optimum engine efficiency rpm of 2500

 
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Exponential functions increase by an order of magnitude in each increment..so going on your add 10hp to drop a second, if it were actually exponential then you would have to add 100HP to drop it another second.

 
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ok heres MY thing.. when i drive normal.. 80ish mph.. i can get about 300 miles on a full tank... i hadda get home really fast a day back.. i was going 120 following mah friends supra.. we were goin for about.. i lost half the tank at 60 miles.. so theres my info to u..

 
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math sucks

 
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I didn't read all that. Plus I'm bad at math. Although that wasn't too difficult. However, it's too early for me. I just think that it's obvious that when you press the gas pedal down harder it pumps more gas through the system, therefore using more gas. The difference between crusing 60 and 50 doesn't matter much, because you aren't pressing the pedal that much harder.

It makes a difference in the city when you are stop and go. If you drive normal you'll use less gas. If you floor it to the next red light you are going to use more gas.

It's more noticeable on older cars. Especially the hopped up V8s. My dads old mustang if you opened it up on the highway you could watch the fuel needle drop pretty quickly.

Oh well, just my .02.