Okay, so I've read a few places recently that every 5mph you drive over 60 on the highway you are losing one mile per gallon. So if you normally get 25mpg at 60, then at 75 you'd only get 22mpg. But my question is this, doesn't how much less time it takes to travel the miles make up some of the difference? I mean, say you're travelling 100 miles. At 60mph it would take you one hour and forty minutes, and at 25mpg the trip would use 4 gallons. At 80mph the 100 mile trip would take one hour and twenty minutes and use 4.76 gallons. But doesn't the fact that the car was on and running for 20 minutes less possibly make up for that .76 gallons? Or am I missing something?
-Amber
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04:53 PM
PFF
System Bot
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
Okay, so I've read a few places recently that every 5mph you drive over 60 on the highway you are losing one mile per gallon. So if you normally get 25mpg at 60, then at 75 you'd only get 22mpg. But my question is this, doesn't how much less time it takes to travel the miles make up some of the difference? I mean, say you're travelling 100 miles. At 60mph it would take you one hour and forty minutes, and at 25mpg the trip would use 4 gallons. At 80mph the 100 mile trip would take one hour and twenty minutes and use 4.76 gallons. But doesn't the fact that the car was on and running for 20 minutes less possibly make up for that .76 gallons? Or am I missing something?
-Amber
This is a tough subject, because these magazines and publications are generalizing about cars. Not every car has the exact same drop off point for maxinum fuel efficiency. It's all related to rotations per minute. Depending on gearing, a car can be travelling at 95 mph and getting 20 miles per hour, drop the gearing a full notch (i.e. 3.56 to 2.56), and you could see a rather large improvement in mileage at the same speed. Honestly, in order to find the exact speed for maximizing fuel efficiency, you need to find at exactly which point your engine uses more fuel to turn more rpms, then you need to formulate an equation to include both rpms, final drive ratio, and probably frictional/air-induced force.
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05:40 PM
Formula88 Member
Posts: 53788 From: Raleigh NC Registered: Jan 2001
Don't confuse mpg with gph. GPH = speed (as in, mph) / mpg. In the 60 v 80 mph example you use, and using your 25 and 21 mpg numbers, you get fuel burns of 2.4 gph at 60 mph and 3.8 gph at 80, a 58% increase in fuel burn rate for a 33% increase in raw speed. Being able to shut the car off sooner does not compensate for the increase burn rate, because the burn rate goes up faster than the speed increase.
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06:11 PM
blakeinspace Member
Posts: 5923 From: Fort Worth, Texas Registered: Dec 2001
Which weighs more a pound of feathers or a pound of lead?
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If an ignition module is running in the forest, and no one is around to see it, can it still fail?
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Just supposing... there was a color that was darker than black. And you painted your room this color... you being interested in interior design and all, you could actually put black objects in your room to liven things up a bit.
I believe my truck gets ( or did befoer the cars went south) better mileage going 60 than it gets going 50, its in the gearing. After 60 its all downhill though. Brad
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06:58 PM
BobadooFunk Member
Posts: 5436 From: Pittsburgh PA Registered: Jun 2003
If an ignition module is running in the forest, and no one is around to see it, can it still fail?
AHAHAHAHAHA that right thar is funny.
also i think most cars are best at 65 by the book.... (avg LEGAL highway speed) some may even be 70... but yea gearing as well as how you drive have alot to do with it..
In my experience if you keep the car 80 or under you won't see much effect on mileage. Pick-ups and SUVs have some effect because of more air drag. Back in the 70's during the first gas crunch, I had a huge 69 Chrysler Newport with a big ol 383 engine that got 21 mpg cruising at 75 mph. The government dropped all the speedlimits to 55 and my mpg dropped to 18. The car had been geared to cruise at the higher speed.
I had an 85 Fiero coupe 4 cyl 5 speed that got 33-35 mpg no matter what speed I cruised (60-80 mph) at out on the highway. The lower mpg was in hilly/mountainous terrain and the higher mpg was on flat /level roads. My daughter's 85 sport coupe, set up the same way, has trouble breaking the 30 mpg barrier. It consistently gets 29 mpg, though it hasn't been on any long trips in a few years.
There is no flat rule that works across the board with all cars. I would say that where ever that 1 mpg for every 5 mph comes from that they are taking the worst case and applying it with to big of a brush. It sounds like a government sponsored study that was far to limited in it's scope or something insurance companies are putting out to reduce speed and claims. Your best bet is to try your car in the different speeds and see what happens. Good luck.
Dave
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08:43 PM
SonataInFSharp Member
Posts: 882 From: Minneapolis, MN Registered: Aug 2003
I can't provide a formula, but in my ION I get significantly better mpg at 55 than at 70.
I know this because one summer there was road construction and the speed limit was reduced to 55 mph. Normally the speed limit is 70. This was about an hour and a half strech of road that I travelled often enough to use it to calculate mpg away from stop and go traffic conditions (even during the construction there was no stop and go as it was very rural).
I just remember that I would get something around 34 mpg during the 55mph days and 27-29 mpg during the 70 mph days. For only 15 mph difference, that seems like a HUGE decrease in mpg. (Keep in mind that my ION is only 2.2 liters and personally I think that 5th gear is way too low--I can floor it at 35 mph in 5th and the engine doesn't lug at all.)
As far as calculating a ratio in regards to gas used vs. time saved, I think it is just a matter of priority-- in other words, does the loss of mpg outweight the time you save to drive more quickly. Of course once you break the SPEED LIMIT, the question becomes completely hypothetical.
Alright guys, thanks for the answers. I mainly just wondered if time played into it at all, it seemed to me like the given calculation was missing something because of the time you'd save. Like if you go 0 miles an hour for any amount of time you will burn no gas. Which of course isn't true, so it seemed like maybe since the car would be running for less time maybe that'd make up some of the gas difference. But it doesn't I especially appreciate the creative answers, and we all know an ignition coil can fail anywhere, anytime, for any reason (all the more so in Blake's world).
It's just that you read these figures and hear them on the news and sometimes it's really obvious something is missing making the information either incorrect or completely worthless. On the news the other night they reported that "married couples in which both partners work earn nearly twice as much per year a single person". This was in a report about the benefits of marriage. My question was how two people only earn nearly as much as one.
Anyway.
I personally don't have the patience to drive slower just to improve my gas mileage, so it'll leave it at that.
-Amber
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03:45 AM
AndyLPhoto Member
Posts: 2420 From: Skandia, MI, USA Registered: Nov 2001
Alright guys, thanks for the answers. I mainly just wondered if time played into it at all, it seemed to me like the given calculation was missing something because of the time you'd save. Like if you go 0 miles an hour for any amount of time you will burn no gas. Which of course isn't true, so it seemed like maybe since the car would be running for less time maybe that'd make up some of the gas difference. But it doesn't
-Amber
Well, to be fair, you had a point. Time DOES play into it, just not in comparing direct MPG vs. MPG. By the time you got to the point of asking the question, time had already been considered, because you were comparing (albeit theoretical) mileage rates at differing speeds. So the speed had already been considered when the source you cited noted a drop of 1 MPG per additional 5 MPH.
Look at it this way...if your car burned the same amount of gasoline per minute for any highway speed, then by definition an increased speed would give increased gas mileage, because the engine would be running for less time. However, time is only one consideration. And in this particular example, time has already been taken into account because of the reference to a particular speed. But time was ONE factor in reaching a given MPG at a given MPH.
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07:27 AM
RACE Member
Posts: 4845 From: Des Moines IA Registered: Dec 2002
Gearing does make a huge difference in efficiency but lets look at drag for a minute. Drag increases with the square of the speed.
If you use 25 mph as a base line and double your speed to 50, air resistance will increase 4X.
If you start at 25 mph and then drive 3X faster (75 mph) you will experience 9X as much drag.
You can see that the extra 25 mph will more than double the drag on the car.
Now lets drive 100 mph. Drag will be 16 times higher than it was at 25 mph! You would get there in 1/4 of the time but at a great price in efficiency. All other things being equal and if my math is correct you would burn 15 x as much fuel for the same distance. (Traveling at 100mph fuel burn is 16x what you would burn at 25 mph minus the baseline burn at 25mph = 15x more energy).
Little things become far more critical the farther up the drag curve we go. A flat underbody and sloped rear window would go a long way for our cars aerodynamically. If you could reduce the aerodynamic drag on your car by 5 lbs when traveling at 25 mph it could theoretically reduce the amount of drag by 80 lbs at 100 mph. At 150mph it would be worth 180 lbs of drag.
The reason some vehicles get higher mileage at specific higher speeds is that when you move down out of the sweet spot of the torque band, you lug the engine and it takes a richer mixture (amongst other factors) to keep it running smoothly. Most modern cars have the torque band real low, but if you have a car where the power band starts at 2500-3000rpm like some old muscle cars and you lug it at 1000-1500 rpm like most newer cars do your going to run worse and decrease mileage. The modern car had those extra gears to keep you in the sweet spot and variable cam timing to move that sweet spot around. So if your under 4500rpm, your engine is setup for low end torque, bring it up to 6000rpm and your cams can change lobes, rotate in relation to each other to change overlap, the intake switches tuning to shorter runners, etc... to turn itself into a hi rpm race style engine. With a really small displacement econobox, I bet there are speeds where the car gets lower mpg than at a speed 10-15mph faster. The other variables all still exist... if your driving at a slower speed limit are you also now on more congested roads where your speeding up and slowing down more?
Aerodynamics have improved a bunch, so I don't neccesarily buy random numbers tossed out as "You'll lose X mpg for every 5mph over Y". The numbers given could relate to 1960's pickups. I made a run up almost to the canadian border from Tacoma for a class one morning where driving an automatic 2.3l Escort with a passenger I got right about 35mpg running at about 85 with no traffic about 5am on a saturday morning. Relatively flat drive, hardly any variance in speed, little traffic. My 85 New Yorker with a turbo 2.2 on the other hand would tank the mileage if you drove over 60mph when I first got it. I could just tap 28mpg if I stayed steady state under 60, break 65mph and mileage dropped to about 17mpg. Turned out previous owner had the timing belt off a tooth, and mileage jumped to an average of 28.8mpg mixed driving from 17mpg mixed with it reclocked.
From a physics standpoint, it takes a certain amount of energy to do a certain amount of work. When driving, the energy from gas is to some extent turned into kinetic energy, the energy of a moving mass. When the brakes are used that kinetic energy is turned into heat energy by rubbing the brake pads against the brake rotors. Along the way some of that original kinetic energy is turned into heat energy via air friction, tire friction (tires get hot when rolling, that heat is energy originally from burning gas), and general heat losses through the car's radiator and oil, and tranny fluid if an auto. At higher speeds all the frictions increase, tires, mechanical, air, so the faster you go the more energy is lost as heat per mile travelled.
Air resistance increases much faster than the rate of speed increase, so if you double your speed you quadruple your air friction. At speeds below 40-50 MPH this isn't an issue, but at speeds above 70-80 it becomes a real issue. Lower engine RPMs produce less internal engine friction losses, wider tires produce more rolling resistance, etc, etc. It's all about tradeoffs, designing engine efficiencies to match particular expected driving RPMs and rolling speeds, etc. So, the 1mpg loss thing is really only a rule of thumb, and specific applications can vary widely from that.
