PDA

View Full Version : MPG: Speed vs. Time



bikinpunk
08-09-2006, 11:54 AM
I've had this talk with alot of people over the course of my driving years, and I'm sure some of you have wondered this as well.

If you go slow, everyone knows that you use less gas than going fast. The faster you go the more gas you burn. I've even read that you add the equivalent of 20 cents per gallon for each 5 miles per hour you drive over 60 mph.

That being said, if you're driving faster you get to your destination faster. So, my question is, are you consuming relatively the same amount of gas by going faster thus reaching your destination sooner? Or do you still lose gas by going faster? If you increase your speed you'll get to your destination quicker. So, the question is, is the speed vs. time vs. mpg relatively the same as it would be if you were going slower?

I'm making these numbers up unless I have an equation next to it:
Say you burn 20 mpg going 65. Your trip is 20 miles, so you burn ~ 1 gallon of gas. You make the trip in about 18 minutes, 30 seconds ( (20 miles/65 mph)*60 min/hour).

If you increase to 80 and make the trip in 15 minutes, then are you still consuming the same total amount of gas?

I realize lower rpm's burn less gas, but in terms of speed alone.

luvinthebass
08-09-2006, 11:55 AM
cool a topic that involves math...not even goin try...

TwistedMovistar
08-09-2006, 11:57 AM
you won't have the same 20 mpg cause the engine is using more gas becuase of the excess spinning.... think of it like who will use more gas a person running 2300 rpms at 60 in 4th or a person runnin 1500rpms in 5th going 60 both going 200 miles

bikinpunk
08-09-2006, 12:00 PM
I know that. The question is here, do you still burn the same TOTAL amount of gas. I'm not saying this is true, but this is where the point is always made:

I think speed vs. time would be exponential. Thus, the more speed (rpms whatever) you have the higher the gas expenditure is, but the quicker you get there. You're reaching your destination faster at a faster speed. So, are you burning the same total amount of gas at a faster speed as you would a slower speed but getting there in a longer period of time?

I shouldn't have said anything about the 20mpg until later. I started this post in 2 other forums and everyone says something about the rpms.

danteBirosel
08-09-2006, 12:01 PM
Very good subject. Suscribed. :D

TwistedMovistar
08-09-2006, 12:02 PM
even better for ya.... given a distance of 100ft.... if a car is going thru that set distance of 100 ft with the rpms at 1500 doing 55 it will take him more time to clear it, but he will use less gas than a car doing 70 @2000rpms, the car going 5...............ahhh **** it it has to do with more than i can back up jus ttherory( i know t is right just can't explain it in detail)

TwistedMovistar
08-09-2006, 12:05 PM
i had aready started the post.......u can't just thro out the rpm's thou, becuase it is like an importat variable....more rpm's more gas...also it may have something to deal with the load that is on the engine...what other boards didyouplace this on??

ConcernedMember
08-09-2006, 12:08 PM
This is something for the Myth Busters.

danteBirosel
08-09-2006, 12:08 PM
i had aready started the post.......u can't just thro out the rpm's thou, becuase it is like an importat variable....more rpm's more gas...also it may have something to deal with the load that is on the engine...what other boards didyouplace this on??



So that mean's on sunday when i went long driving back home from a competition i was going up this steep *** hill on the freeway. I was lagging so i gassed it and hit around 4rpm constant. That means i was wasting alot of gas huh?


dante

mikeyboc
08-09-2006, 12:09 PM
I think it would ultimately it depends on the distance driven. In theory this may hold true under ideal or certain circumstances, which you may neve actually encounter in real life.

For Example: Driving X distance @ Y speed burns Z Gas

You can change the variables and still burn the same amount of gas, but the distance driven on any given trip is, for the most part a constant. So under certain circumstances you may be right, but I am willing to bet that most of the time, driving a little slower is going to benefit gas consumption the best.

bikinpunk
08-09-2006, 12:10 PM
i had aready started the post.......u can't just thro out the rpm's thou, becuase it is like an importat variable....more rpm's more gas...also it may have something to deal with the load that is on the engine...what other boards didyouplace this on??

Well, the reason I said for now to forget rpm's is because that's not of concern for this post. I think everyone knows the more rpms' you use the more gas you consume. Which is the reason I come off cruise control on hills and accelerate myself.

I'm really wanting to get this down to a constant speed vs. time vs. mpg issue.

8thcivic.com/forum
sounddomain.com/forum

GrayMatter4tw
08-09-2006, 12:12 PM
i make a 500 mile trip a bunch of times every year and half is 80mph and half is 65.

250 miles @ 80mph
3.12hours 16mpg 16.6gallons

250 miles @ 65mph
3.85hours 21mpg 11.9gallons

it makes no difference that you are there faster.
period.

bikinpunk
08-09-2006, 12:13 PM
I think it would ultimately it depends on the distance driven. In theory this may hold true under ideal or certain circumstances, which you may neve actually encounter in real life.

For Example: Driving X distance @ Y speed burns Z Gas

You can change the variables and still burn the same amount of gas, but the distance driven on any given trip is, for the most part a constant. So under certain circumstances you may be right, but I am willing to bet that most of the time, driving a little slower is going to benefit gas consumption the best.

I agree with you. I'm not trying to convince anyone of this. It's just something that I've wondered for a long time. And anytime I bring it up noone ever has a true explanation of why it wouldn't be correct. I'm in aerospace engineering...maybe I should talk to one of my professors about doing this as an experiment. I was thinking this might be able to be done in a garage (where they do engine tests and etc) but I'd also like to have drag factored in this as well, since it's the most prominent issue in mpg at high speeds.

ConcernedMember
08-09-2006, 12:13 PM
You could test this yourself. ;)

bikinpunk
08-09-2006, 12:14 PM
i make a 500 mile trip a bunch of times every year and half is 80mph and half is 65.

250 miles @ 80mph
3.125 hours 16mpg 16.625gallons

250 miles @ 65mph
3.85 hours 21mpg 11.9gallons

it makes no difference that you are there faster.
period.

Nice. Real Numbers.
Was this constant speed (no engine revs, speed increases/reductions, cruise control, etc)?

danteBirosel
08-09-2006, 12:15 PM
i make a 500 mile trip a bunch of times every year and half is 80mph and half is 65.

250 miles @ 80mph
3.12hours 16mpg 16.6gallons

250 miles @ 65mph
3.85hours 21mpg 11.9gallons

it makes no difference that you are there faster.
period.


What car is this?


dante

GrayMatter4tw
08-09-2006, 12:17 PM
cruise control the whole way
65mph part has maybe 4 or 5 slowdowns due to going through towns but no stops.
the 80mph part is a straight shot though on I-75

edit: livinlife its a 1996 ford explorer sport

bikinpunk
08-09-2006, 12:17 PM
Thanks for posting that.

mikeyboc
08-09-2006, 12:17 PM
Nice. Real Numbers.
Was this constant speed (no engine revs, speed increases/reductions, cruise control, etc)?

You would have to have speed increases and engine revs in any driving situation. The conditions and terrain for any given trip are going to vary, which just contributes to the numerous variable that would have to be considered in the equation. I know you said that you don;t want to take RPM's, so on and so forth, into consideration, but they have to be in order to come to a valid conclusion. By leaving out data, you can manipulate the results to say pretty much whatever you wanted.

bikinpunk
08-09-2006, 12:47 PM
In a real world situation, yes, you have to consider all sorts of elements that you can't control. I totally agree, but for sake of this argument. If you broke it down into simply constant speed with no variations in speed and took 2 speeds I'd like to know the results.

I suppose if you had a good open stretch of highway and started with a full tank and went for a few miles, maybe 10-20 to make it worth it. I dunno, I guess there's no point in arguing it because conditions will hardly ever be ideal. It's just something I've wondered. considering drag is relative to the velocity^2, the force on your car is increased much more at faster speeds. I'm to the point where I'm tired of talking about this, haha. At least some of you understood what I was saying.

ToyTundra
08-09-2006, 12:49 PM
the question isn't speed and getting there faster it's the increased drag caused by going faster. Drag is cV^2 where c is a constant relating to the frontal area and overall aerodynamics. twice the speed = 4 times the drag. I've read 55 is the most efficient, but that depends on the vehicle:

My truck total trip 240mi each way.

@80mph (empty bed)18.3mpg
@70mph (bed loaded) 19.6 mpg

it's not 100% accurate because of the load though

Showrides
08-09-2006, 12:52 PM
This study has been done before.

There is a speed in every car that gets you the most mpg.

The time it takes really isn't even a factor. It all comes down to how far you traveled and how much gas you used. Throwing time into the equation just complicates things. All you really want to know is how much gas did you use to get from point A to point B.

And test it out at different speeds, and see which ones consumes the least amount of gas. Then you will find the best way to save money;)

Eugenics
08-09-2006, 01:26 PM
generally you will lose economy, because when you're driving agressively you're in a lower gear which means you're engine is spinning more revolutions in relation to how far its going then it would be in say the overdrive gear. HOWEVER if you accelerate quickly to get your car into the overdrive gear earlier (for instance speed upto 65 as quickly as you can on the onramp then go into overdrive) you're saving yourself gas by going to the highergear early instead of accelerating in lower gears at high revolutions. make sense? im hungover, so i'll revise this later. but the just of it is still there. feel free to debate it with me as this is just my theroy based upon logic but it could be flawed.

Eugenics
08-09-2006, 01:27 PM
p.s. time dosen't matter because you can figure it as a function of speed and distance

Eugenics
08-09-2006, 01:28 PM
Well, searching for my posts on this topic would probably help ...

It's really quite simple. The 2 main things that affect fuel consumption are the air:fuel ratio and the engine speed.

The lower the air:fuel ratio, the more fuel you consume.
The higher the engine speed, the more fuel you consume.

When you go faster, you must increase the throttle input to overcome the increased drag, which lowers the air:fuel ratio, which increases fuel consumption. In addition to this, you also increase the engine speed, which increases fuel consumption.

It doesn't take a genius to see that increasing speed is going to increase fuel consumption at a much higher rate, thus reducing your MPG ...

For maximum MPG, you would want to use the least amount of throttle possible in the highest gear.

ding

ballstothewall
08-09-2006, 01:36 PM
drag is relative to the velocity^2.



This is the kicker, no you can't go faster and consume the same amount of fuel, if you don't understand, read what I quoted.

phyphoestilic
08-09-2006, 01:53 PM
time is money, so the faster u get there the more money u save, thus u put that extra saved money into gas, in conclusion.

Just like all the delivery services out there, u pay more money to get there faster. So u can have that extra 5 minutes to - 10 hours depending on where ur going, that u would've been stuck in trafic u can already be there and doing w/e u gotta do.

Time is definatly a factor in this, so stop trying to rule it out.


Dante - I heard u should stay under 3k rpm to save the most gas, but Iono.

ndnbolla
08-09-2006, 04:23 PM
The best speed is to go ~60 so to speak.

What is the reasoning for lower gas mileage when driving under this speed?

Railrocker
08-09-2006, 04:30 PM
generally you will lose economy, because when you're driving agressively you're in a lower gear which means you're engine is spinning more revolutions in relation to how far its going then it would be in say the overdrive gear. HOWEVER if you accelerate quickly to get your car into the overdrive gear earlier (for instance speed upto 65 as quickly as you can on the onramp then go into overdrive) you're saving yourself gas by going to the highergear early instead of accelerating in lower gears at high revolutions. make sense? im hungover, so i'll revise this later. but the just of it is still there. feel free to debate it with me as this is just my theroy based upon logic but it could be flawed.

True, the railroad has a big push for fuel economy and download every train to see how your running it in regards to fuel use. They have been doing this for about five years and recommend getting up to speed as fast as you can cause this equals more time in "higher more economical gears" Getting the mass up to speed is where you burn your gas not maintaining speed. Of course this is only relative to areas where the trip is long and has little stopping.

IrTehL33T@DBeez
08-09-2006, 04:56 PM
theres no need to get complicated, its just a matter of if the engine in the vehicle of choice uses constant ratio of fuel throughout rpm range. if you were to look at the fuel mapping, determine rpm points at which each speed will match with in top gear, along with the same load input, you can determine how much fuel is being consumed and then use that quantity to go take into consideration with the time difference.

some fuel maps are rather rich in the lower rpms which could actually mean that in a high geared 5th or 6th at 65, you're revving so low that you would actually lean out a little bit by accelerating to 80 and getting the revs up to a leaner a/f ratio, and possibly require less of a load to maintain speed because of powerband... in which case the fuel consumption may actually decrease.

its all going to depend on the specific vehicle's tuning data and its gear ratios

although, this entire theory doesn deplete the consideration of drag coefficient, it would have to be considered an idealistic theoretical calculation

RandyJ
08-09-2006, 10:13 PM
its all going to depend on the specific vehicle's tuning data and its gear ratios

although, this entire theory doesn deplete the consideration of drag coefficient, it would have to be considered an idealistic theoretical calculation


I was beginning to think no one would ever consider gearing and powerband. Wow it took a while.

`pr0digy
08-11-2006, 04:10 AM
Wow, lots of idiots here. First of all, wind resistance squares as speed doubles, iirc. More gas. Acceleration enrichment if your speeding up quicker. More gas. All kinds of other factors, I garuntee you use more gas going faster though. Go research the german autobahn.

bikinpunk
08-11-2006, 08:05 PM
Wow, lots of idiots here. First of all, wind resistance squares as speed doubles, iirc. More gas. Acceleration enrichment if your speeding up quicker. More gas. All kinds of other factors, I garuntee you use more gas going faster though. Go research the german autobahn.

Are you saying you'd burn more gas going 2 miles an hour than at 0 mph? Based on your theory that's how it seems.

My whole premise for this post was based on the main fact that sitting idle in the car you get 0 mpg. However, if you go 2 mph you drastically improve mpg. I guess what I'm wondering is when this ratio tapers off and at what point higher speed is better than the lower speed. I know there also is a point where a higher speed is worse, but I wonder what types of speed it would take. Are we talking 20's & 30's or 50's & 60's (mph)?

dbhittin
08-12-2006, 02:21 AM
actually, u use more gas at lower rpms cause the engin lags and u give it more gas to go, as to where im cruzing at 4gran in 5th gear going 90, im hardly on the gas pedal at all.. i also notice when im driving fast on long trips i use less gas than if i were to drive the speed limit

RandyJ
08-12-2006, 09:04 AM
I already said it earlier in this thread and like 2 years ago, n00b ...


Sorry, your posts are ignored due to your inability to ever post something wrong/fictional/false/stupid/ignorant. It's no fun to have the thread shut down in the first page ;)

And honestly, I just forgot what you had wrote after I read all the other replies:p:

bumpin_blazer
08-13-2006, 12:30 AM
good thead.

Abneriel
08-17-2006, 12:35 AM
cars are generally geared to get the best mileage @ 55 mph