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80mph mpg

sm625

sm625

Diamond Member
My car gets 45mpg at 35-55 mph. Then it rapidly falls off a cliff to the point where it is only getting 37mpg at 80mph. I am wondering if there is a website or magazine that has an article which goes over this "mileage curve" for all common makes and models. I dont really care which car has the best gas mileage. What I care about is what car gets the best mileage at 80mph.
 
coefficient of drag is likely going to play a much more important role at 80mph than at 35-55mph ...

I don't know if there is a consolidate source for what you ask for, but it would indeed be very interesting to see.

I suspect "sports" cars that tend to get pretty mediocre MPG at lower speeds will actually be pretty competitive at higher speeds due to being more aerodynamic in design.
 
well I can help you a bit with this..
the higher the speed, more air is displaced, and to move through "thicker" air, you will need more energy. The technical term is coefficient of drag. The lower Co drag number the more slippery you are going to be the liquid air.

So what you need is a super efficient body with a big enough engine to efficiently cut through the air without much loss of power.

You could try using blue painters tape to tape up the gaps to make air flow more smooth over the body of the car. However your 80mph drive will eat gas due to the energy required to keep the car moving at 80mph.
 
Drag coefficient in a particular vehicle is a constant. If you have-

Vehicle 'A'
drag coefficient = .35
MPG at a constant 55mph cruising speed = 35

Vehicle 'B'
drag coefficient = .50
MPG at a constant 55mph cruising speed = 25

as rough examples with fairly round numbers, can basic math give a somewhat accurate prediction of what would happen to their mileage as speed increases? Practical experience says 'no;' but my mind wants to say that logically, you should be able to make a rough guess. I know there will be subtle nuances from engine and trans differences. But couldn't you draw a [somewhat] parabolic line for their mileage, which would basically be a function of 'power required to sustain x speed in a vehicle with y drag? (parabola because the required power increases more steeply as you approach top speed)

Also, another random musing I'll throw in 'cause it's always bugged me...vehicle weight doesn't have any effect on speed, right? As in, If two cars are identical in every way, except one is heavier...they still have the same top speed, as dictated by aero and power, right? The heavier one just takes longer to accelerate to said speed. I think this is true, but may be completely wrong.

The related question is, how does this influence MPG? I want to say that it doesn't. At speed, that is. But in this case, I'm almost sure that I'm wrong. My mind wants to think of the heavier vehicle having a greater inertia proportional to it's weight or something...

[edit: I know people hate my random ADD thoughts. But I'm a curious person, dammit]
 
The Stigenator said:
well I can help you a bit with this..
the higher the speed, more air is displaced, and to move through "thicker" air, you will need more energy. The technical term is coefficient of drag. The lower Co drag number the more slippery you are going to be the liquid air.

So what you need is a super efficient body with a big enough engine to efficiently cut through the air without much loss of power.

You could try using blue painters tape to tape up the gaps to make air flow more smooth over the body of the car. However your 80mph drive will eat gas due to the energy required to keep the car moving at 80mph.
Click to expand...

Cd is only part of the equation. frontal area is the other half.

otherwise, get a high-displacement, torquey engine and gear the crap out of it.

see: corvette. the engine turns like 1500rpm @ 80mph. low rpm = low fuel consumption.
 
Fenixgoon said:
Cd is only part of the equation. frontal area is the other half.

otherwise, get a high-displacement, torquey engine and gear the crap out of it.

see: corvette. the engine turns like 1500rpm @ 80mph. low rpm = low fuel consumption.
Click to expand...

This. My c5 got amazing mileage cruising at high speed. Low Cd, Low Frontal area. Low RPM at cruise. Only way it gets better is a FWD version. FWD drivetrains tend to lose less power.
 
Are today's cars still built with the "national" 55mph speed limit in mind?

If so, all the gearing and computer settings are probably setup for that speed to be the sweet spot.
 
Fenixgoon said:
see: corvette. the engine turns like 1500rpm @ 80mph. low rpm = low fuel consumption.
Click to expand...

I'm right around 1500 RPM at 75 with a 0.5 ratio in 6th with a 3.31 rear end. I get decent mileage if I can keep it that slow, but it trails off pretty fast and by 95 is down quite a bit (still under 2000 rpm which hits right over 100 mph). Peak mileage is somewhere between 1000 and 1500 rpm in 6th (50-75 mph).
 
BurnItDwn said:
coefficient of drag is likely going to play a much more important role at 80mph than at 35-55mph ...
Click to expand...

This. To get what you're wanting, OP, you're probably going to need a CD of .30 or less (the Tesla model S is .24, the best of any car in production).
 
Also, keep in mind that the faster you go, the smaller component of your mileage weight becomes. Obviously, weight remains a factor at 80 mph, but it is much less of a factor at 80 than it is at 40. Now, the more aerodynamic a vehicle is, the less true this is.

At *very* high speeds, compared to wind resistance, weight it almost a non factor.
 
phucheneh said:
Also, another random musing I'll throw in 'cause it's always bugged me...vehicle weight doesn't have any effect on speed, right? As in, If two cars are identical in every way, except one is heavier...they still have the same top speed, as dictated by aero and power, right? The heavier one just takes longer to accelerate to said speed. I think this is true, but may be completely wrong.
Click to expand...

That's basically right assuming perfectly level ground. Mass will affect acceleration but not top speed. However, as soon as you have any incline at all, weight will start playing a role in top speed again. (In practice, weight will also have an effect on friction drag as the more weight you put on bearings and tires the greater the friction for a given design, but this would be a minimal effect.)

phucheneh said:
The related question is, how does this influence MPG? I want to say that it doesn't. At speed, that is. But in this case, I'm almost sure that I'm wrong. My mind wants to think of the heavier vehicle having a greater inertia proportional to it's weight or something...
Click to expand...

On level ground, you're right that weight doesn't affect mileage much once you get up to speed and are just maintaining that speed, but the weight will have an effect once you throw hills into the equation, and of course there's acceleration as you mentioned.

ZV
 
Zenmervolt said:
but the weight will have an effect once you throw hills into the equation, and of course there's acceleration as you mentioned.
Click to expand...

Only a minor effect, unless your car is so slippery and heavy, or the slope so extreme, that you will have to downhill-brake, because engine braking is insufficient. At 80mph as cruising speed, this is rather unlikely.
So while yes, you consume more going up, you consume less going down again, which usually more or less equals out the loss, if the engine doesn't get forced into a very inefficient mode due to the elevated torque requirements.

As for
Are today's cars still built with the "national" 55mph speed limit in mind?
Click to expand...

No, they are tuned for EPA testing, to give the best possible result in that test. Everything else is moot.
Especially with small turbocharged engines, you have to be careful with the official numbers, as often times, once you exceed a certain speed, the turbo kicks in and fuel consumption increases rapidly. The goal for a manufacturer is to have the turbo boosting minimally in an EPA cycle, but give you plenty of power if you test drive it.
The only thing that's more skewed is hybrid mpg, where they allow the battery to run out. I guess that has some validity for plug-in hybrids (but the amount of battery charge required should still be listed), but for "conventional" hybrids this is also being done, and is, quite frankly, cheating.
 
This testing article quickly outlines common MPG scenarios: http://www.autoexpress.co.uk/car-news/31861/mpg-mythbusters.

Really, the best mileage will be obtained by lower RPMs, lower speed, lower drag coefficient, and proper maintenance (with varying degrees of efficacy: clean air filter, clean intake, unspent engine and transmission oils, properly inflated tires, proper tire alignment/balance, etc).

There's a few quick studies out there showing how after 55/60MPH drag coefficient takes over and drops any vehicle's mileage dramatically. Here's one example.

Here's a calculator for a little math funs. 😀 It works pretty well if you know your numbers. Don't forget to add weight to your curb weight for everything inside, and use your warm tire pressure numbers (usually 20% higher than when you're measuring cold). The only problem is calculating drive train loss, since that's a very dynamic number.

Here's a limited drag coefficient list.
 
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http://fueleconomy.gov/feg/driveHabits.shtml

speed_vs_mpg_2012_sm.jpg
 
Bleh. My Mazdaspeed 6 seems to spin at a fairly high RPM on the highway. At least it is a lower RPM than my Contour SVT. Man, that thing had EPA ratings of 24/28. Barely any spread.
 
80 MPH is like 4200 RPM in my S2000. 🙁 On the highway, I always wish I had another gear (or three) to shift up into. The gearing is designed so that redline in sixth and drag-limited top speed roughly match up, so it's ideal for racetracks, but not so much for getting to the racetrack. I'm lucky to average 25 MPG with the sticky tires.
 
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