Your car will deliver its BEST mileage at around 45-50 MPH. Period. The day you got 27 mpg at 75 mph, you had a 20 mph tailwind…
Air drag increases with the square of the speed, there is just no getting around it. Internal pumping losses in reciprocating piston engines increase with the CUBE of RPM! You can’t get around that either…
Seriously, headwinds and tailwinds make a BIG difference in fuel mileage. Ask any trucker or airline pilot.
First, a 92 Skylark with a 2.3 L engine that doesn’t have a cracked head, that is something all in itself. Most never made it past 40k miles. Serious design flaw in that one.
Next, it sounds like you are comparing a road trip with “city” driving. Stop and go will get you less mileage. Also, as your car ages, it is possible that it will improve in efficiency as carbon builds up in the cylinders and raises compression. Other things could come into play as well. Eventually it will all go down hill though as things just wear out.
Most cars are fairly aerodynamic, at least as compared to a truck. The engine does have a “sweet spot” where maximum efficiency occurs RPM wise. The aerodynamic drag curve is exponential, that is drag increases faster than the corresponding increase in speed. If you look at a graph of the curve, you will see a “knee” in it. That is where it goes from rising slowly to rising faster. The relationship to where that knee occurs and where the sweet spot in the engine occurs determines the speed where maximum efficiency should occur.
In your case, I doubt its 75 mph, but it might be closer to that speed than some might think. If you took the same trip and held 65 all the way, you probably would see a little better gas mileage. Now if you had a truck, you would see the fuel economy drop dramatically above 65 mph due to the increased drag coefficient.
How open is the throttle at 30 mph? 5%, maybe 10% open?
What do you think the engine’s power curve looks like when the throttle is only 5% or 10% open?
Here are a series of power curves of a typical engine at throttle openings smaller than full throttle.
Anyone want to take a shot at why my Lincoln gets 27 MPG on the flatlands here in Oklahoma and also gets 27 MPG in the Colorado Rockies while pulling steep grades and thinner air?
(Verified multiple times by both the on board display and the tank fill/trip meter method.)
Anyone want to take a shot at why my old BMW R100/7 motorcycle would get 40 MPG at 70 MPH and 30-32 MPG at 55 MPH?
(My feeling is that it’s just like my car; at 55 MPH in 5th gear it was an absolute dog and lugging. At 70 it was just starting to breathe well.)
And riding position has nothing to do with it. In other words, I was not leaned over the bars at higher speeds and cutting wind resistance. Matter of fact, I machined a set of handlebar setbacks to allow a more upright riding position as the stock bar position was a pain in the back.
Anyone want to take a shot at why my old BMW R100/7 motorcycle would get 40 MPG at 70 MPH and 30-32 MPG at 55 MPH?
My old R90/6 would get nearly 50 mpg at 55, once I replaced the worn out needle jets and jet needles in the Bing constant velocity carburetors with a set of new ones. These parts slowly wear with use and cause your mixture to go rich and a rich mixture also doesn’t run as smooth as a correct fuel mixture does which helps make the engine feel like it’s “lugging”.
That or you drive 55 like Sammy Hagar does, “one foot on the brake and one foot on the gas”.
You might find that driving 80 gives you better gas mileage then driving 75. Engine torque curve may be better at 80 then 75. May also be better at 80 then at 70…HOWEVER…it then also has to overcome the drastic increase in wind velocity. The better engine performance may be able to overcome a 5mph difference in wind velocity, but I seriously doubt a 10 or 20 mph difference. The BIGGEST FACTOR BY FAR in getting the best gas mileage is wind resistance.
There is another reason most cars get their top efficiency between 45 and 50 MPH: CAFE and the EPA’s test cycle. The older testing for highway fuel economy had an average speed of 48 MPH on the dynomometer. So the manufacturers optimized their cars for this speed to score better on the test.
ok4450 - "A lot depends on the engine, torque, and the power band. Some engines will be at their best in a certain part of that power band. Think of it as a “sweet spot”. It varies by car but sometimes an engine can nearly be lugging at a lower speed. It depends on the car and engine, etc.
I have a Lincoln Mark VIII with the DOHC 4.6 V-8. That car gets better mileage at 70 MPH than it does at 55. (About 1.5-2 MPG better and it’s been verified a number of times.)"
I figure 1-3 mpg better at higher speeds, also consistently verified (helps negate wind factors). Engine torque? Hmm…My car is happy to go 5 mph without me ever stepping on the gas peddle (am I making the right connection here?). In the early 90’s someone told me about what you call the ‘sweet spot’. He said somewhere above 60 a car will get into a rhythm and just keep on churning. My car does coast very well, indeed.
keith - “First, a 92 Skylark with a 2.3 L engine that doesn’t have a cracked head, that is something all in itself. Most never made it past 40k miles. Serious design flaw in that one.”
160k and climbing. The best thing about my car has always been the engine. Just about everything else has broken.
keith - “…as your car ages, it is possible that it will improve in efficiency as carbon builds up in the cylinders and raises compression.”
Makes sense to me. I definitely don’t baby this car. Routine maintenance? What’s that?
NYBo - “There is another reason most cars get their top efficiency between 45 and 50 MPH: CAFE and the EPA’s test cycle. The older testing for highway fuel economy had an average speed of 48 MPH on the dynomometer. So the manufacturers optimized their cars for this speed to score better on the test.”
I was wondering if maybe something might have changed from the manufacturer side over the years, making cars like mine a dying breed. Sad, if true, imo.
Caddyman - “Your car will deliver its BEST mileage at around 45-50 MPH. Period. The day you got 27 mpg at 75 mph, you had a 20 mph tailwind…”
Sorry, no way. I had a 10 mph/16 km/h (WSW) headwind going out and a 7 mph/11 km/h (West) tailwind heading home 3 days later. I also was driving at least 5 mph faster on the way home, as traffic was considerably less. Oh, and for what its worth, I think my car is more aerodynamic than most.
Jeremy_R_Hoyt - “…Is maintaining a constant speed is harder at slower speeds? How is that for a theory? Do you use cruise control? That would eliminate the “speed variation” variable.”
Certainly, my findings aren’t scientific; I don’t drive in a laboratory, and we are talking about just a couple mpg, afterall. I did top off the air pressure in the tires before I left. I do pride myself on driving efficiently, breaking and accelerating as little as possible, and as much as possible, only at opportune times (go with gravity, never against it). I do occasionally use cruise control, but I’ve an unconventional style, so…lets say no. Driving consistently slower is definitely not my style, so I really don’t know what my car will do at a constant of 55 or even 65. I suppose the distinction I’m making is between 70 and 85-90.
GreasyJack - “Being able to go 75+ requires light traffic with no stops and steady speed, which is good for gas mileage…”
There is definitely truth to this. Admittedly, I’ll drive as fast as the next guy in such conditions. Certainly there must be a point at which higher speeds will noticeably hurt my mpg. Yet even going 90+ for sustained intervals has seemed to increase my fuel efficiency.
I do occasionally use cruise control, but I’ve an unconventional style, so…lets say no. Driving consistently slower is definitely not my style, so I really don’t know what my car will do at a constant of 55 or even 65. I suppose the distinction I’m making is between 70 and 85-90.
The speed range doesn’t really matter as long as both extremes are using the same gear. The principle we are debating should apply whether you are talking about a difference between 55 and 65 or a difference of 70 and 90. If you conduct your experiment using the cruise control, even outside a laboratory, I am willing to bet you get better fuel economy at the lower speed than the high speed. My car has no cruise control and I can attest to the fact that it is easier to drive fast at a constant speed than it is to drive slow at a constant speed. Driving at a constant 70 MPH requires finer throttle control than driving at a constant 90 MPH.
We’re not talking about a 4 cyl. race car, where these arguements “could” have some legitimacy. We’re talking about “mall car” with a three speed auto. Any increase in milage as discribed would require a suspension of reality and physics, which I fear we are engaged in. A push rod 4 cyl. Skylark by it’s nature is incapable of reality revision.
We’re not talking about a 4 cyl. race car, where these arguements “could” have some legitimacy. We’re talking about “mall car” with a three speed auto. Any increase in milage as discribed would require a suspension of reality and physics, which I fear we are engaged in. A push rod 4 cyl. Skylark by it’s nature is incapable of reality revision.
Vehicle design is an art. Every car responds differently based on its design and choices by the designer. The high speed problem is that the power it takes to maintain your velocity increases with the cube of the velocity.
(Physics aside: Power is the rate of doing work, work is force times distance. Power consumed is thus force * distance / time. The main force that opposes motion is air drag. Drag is 0.5 mass * drag coefficient * characteristic area * velocity squared. Putting that all together and recognizing that velocity is distance over time, the power consumed to maintain a constant velocity is 0.5 * mass * drag coefficient * characteristic area * velocity * velocity * velocity)
So the high end limit to vehicle travel is due to air resistance.
(Aside: this is the same in commercial aircraft, they fly to optimize time at the point where shock waves almost form (another drag increase) which is approximately 0.85 * speed of sound. If they go faster, they pay a serious power consumption price due to wave drag. By slowing down, they save tons of money and get to the destination only slightly later and save fuel and hence $$$)
Based on the design of the vehicle, you will find an optimum speed for fuel efficiency. This will differ with each vehicle. The manufacturers will probably not give out any tests which are performed in controlled environments. They best way to determine this is to test it. But be aware that you will hit the air resistance wall and your fuel efficiency will rapidly decline. If it does not check your calculation. Also note that you need to consistently get the higher MPG averaged over several tanks for the test to show anything at all.
The Buick Skylark may have been designed to get its best gas mileage at 75 MPH, like other cars.
ok, so a car can be designed to be most efficient at a given speed. how do they decide what that speed should be? from the driver’s standpoint, it should, ideally, be optimized for their normal daily driving speed. is it possible to design a car to have 2 optimum speeds, allowing a driver to select a ‘city’ or ‘highway’ setting? if all cars are different in this way, maybe they should be more open about it and even provide different setups so people can buy for city or highway driving. little difference for each car, sure, but a lot of savings with millions of cars.
well, there is really only one way to know for sure. I may be moving to LA in the near future. If that happens, and I actually drive my car there, I’ll set the cruise to 65 for a whole tank and let everyone know the results!
Make no mistake; MOST cars, your Skylark included do substantially better at <55mph than 75+. CR tested a Camry, at various speeds and a general trend of 5 mpg loss was found for 10 mph increase between 55 and 75 mph. You’re still trying to change the world as we know it. Milage figures for Corvettes also show better fuel efficiency at lower speeds…55 vs 80 mph.
My BMW got these numbers when it was brand new. I bought one of the first R100s manufactured, watched it as it was being uncrated, and picked it up after work when the final assembly was done.
The odds of the jets/needles being worn out is pretty slim IMHO.
And no, I don’t ride with one foot on the brake and the R100 was a different beast than the R90.
I do agree that most cars will get better mileage at a slower speed, and with most cars I’ve owned this was the case. I don’t agree with a blanket assessment that a higher speed always means the fuel economy drops.
When it comes to fuel mileage I’m what could be called “very obsessive” when it comes to fuel mileage and generally pick it to death.
Some of you can say what you want and call my figures error prone or misguided but it’s my car and I’ve done the math too many times.
Around the 4th of Aug. I’m headed to CO for a few days of fly fishing, a few cold ones, and hopefully will become the owner of another guitar.
I’m taking the unobstructed 2 lane highways for about the first 300 miles before hitting the interstate so I’m going to check the mileage by both methods at the slower speeds and on the Interstate. I will post these figures on a separate thread when I return.
(And no, stop signs are not an issue. From the town just north of me to Liberal KS is about 200 miles and there’s only 2 stop signs during the entire length and even those are touch and go in the middle of nowhere. I’ve driven this more than once.)
I’ve done these checks dozens of times and I don’t expect any difference this time.
The Buick Skylark may have been designed to get its best gas mileage at 75 MPH
Please tell me how a car gets LESS wind resistance at 75 then it does at 55!!
A car’s ENGINE may be most efficient at a certain RPM…and in a high gear that may very well be 75…HOWEVER…Even with the engine at peak efficiency I do NOT believe it can overcome a 40% increase in wind resistance at 75 compared to doing 55. Engine efficiency is only ONE factor in determining maximum overall efficiency. And Wind resistance is by far the MOST important factor.
Interesting discussion! As a nurse that makes daily house calls all over several counties, we opted on our new Chevy truck for the Active Fuel Management trick that switches the V8 to a 4 cylinder. The Verdict? Exact same gas mileage as our previous truck sans AFM (same model/engine size). The first issue is the AFM basically is only active when coasting downhill or while traffic drafting at highway speeds. Secondly, the engine RPM’s are higher when it’s active near idle vs. all 8 cylinders at a lower speed under slight power. It’s a 600+ to 1500+ RPM difference that mitigates any mileage savings!
While I agree that some vehicles may be less impacted by the speed difference, my experience is that speed costs me MPG. It is very dramatic in my little 89 Mercury Tracer (1.6L). I can achieve 42 mpg driving at a steady speed of 55. At 70 that drops to 36. I also understand the many variables that effect mileage computations. I track my mileage in an Excel spreadsheet and average the numbers over several fillups. It is less dramatic in my 02 Ford Explorer with a 1-2 drop at 70+ vs 55. Is probably not fair to compare the 70 vs 55 as there are few stretches of road that are interstate quality with a 55 mph speed limit. The roads that you will typically find with a 55 mph speed limit are mostly 2 lane with hills and curves. I can’t bring myself to slow down to 55 in a 70 zone as I feel that is rude and dangerous to other drivers. So a fair test on the same road conditions has never been done by me.
Others have offered several valid explanations as to why a specific engine might be more efficient at higher RPM. It is hard to design intakes and exhausts and fuel delivery systems to achieve optimal efficiency at all RPMs and loads. Another consideration for some cars is that an oversized spark ignition engine operating at low load is a big vacuum pump. As you increase throttle opening, you are spending a larger fraction of your energy doing actual work. That is why diesels are inherently more efficient than gasoline engines.
Those nice smooth curves of speed/resistance correspond to a cylinder in a wind tunnel, and when you look at the raw data, even the curves for a cylinder actually have irregularities in them as flow transitions. Those who have put real world objects in a wind tunnel know that the real curves are not nearly so well behaved.
In conclusion, if we had the time to do scientific tests under controlled conditions at 5 mph increments, most of us would find that the declining fuel economy curve is not a smooth exponential decay curve. A few might even find some bumps in the curve over certain speed ranges. All that being said, for the vast majority, the lowest speed that keeps you in high gear with the torque converter locked up is going to be your optimum speed, and there will be an exponential decay in fuel economy as you go faster.