My Final Word on MPG vs. Speed


#1

Since a couple of posters were a bit derogatory towards me in a discussion a few weeks about fuel economy vs. speed, I volunteered to do a thorough check on my Lincoln on an upcoming road trip out of state.



As I’ve maintained, most cars (including most of mine probably) will get worse fuel mileage at higher speeds. I’ve always been obsessive about keeping tabs on fuel mileage and especially so with the Mark VIII. The mileage on this car has been verified a number of times but due to the ruckus I really tried to keep it exact. So here goes starting from home (and omitting a few hundred miles of dinking around in the CO mountains, etc.) Only the trip up and the trip back was checked; all open highway.



Home to Goodland, KS. (2-lane empty highways, cruise on 63 MPH) 393 miles-15.1 gallons-26.026 actual. Display was 26.1 MPG.



Goodland, KS. to Greeley, CO. (all Interstate, cruise on 73 MPH) 226 miles-8.34 gallons-27.098 actual. Display was 27.1 MPG.



Greeley CO. to Raton, NM. (all interstate, cruise on 73 MPH)269 miles-9.96 gallons-27.008 actual. Display was 27 MPG.



Raton, NM. to Alva, OK. (all 2 lane empty highways, cruise on 63 MPH) 353 miles-13.66 gallons-25.841 actual. Display was 25.9 MPG.



And to head a few questions off.

Why did you take the back roads? It’s what I always do; back roads, slower speeds. That’s me; I despise Interstates and I can assure you there’s more slowdowns on the Interstate than there is on the back roads of western OK and KS.



How do we know you didn’t fudge the gas nozzle a bit when filling up (cutting i off short, cramming, etc.)? You don’t. You have to rely simply on my word. Every fillup was performed the same; nozzle in the same depth, set on the slowest fill speed, and rounded to the next nickle when it clicked off.



Besides, I can’t fudge the dashboard display that matches the actual gallons/trip meter method.



(And since I had mentioned owning a BMW R100/7 motorcycle that got better mileage at 70 than it did at 55 someone stated that my carburetor main jets were probably worn out. Not. I bought this bike new as one of the first off of the assembly line and it had this trait throughout the 6 years I owned it.)



So, rip me up if you want or feel free to comment about how the mileage was figured. I’ll only say that I’m not in the business of Bsing people and may not be around this forum much longer anyway.


#2

“I’ll only say that I’m not in the business of Bsing people and may not be around this forum much longer anyway.”

That would be a darned shame.

Ed B.


#3

Don’t let the naysayers get under your skin.

As I recall, there were several well-reasoned explanations on that thread as to why the inverse correlation between speed and mileage is not a smooth line, and may well have some dips in it for certain cars in certain speed ranges.

Those who insist that the inverse correlation always holds true for all cars in all speed ranges simply do not understand all the variables that are at work here, such as variations in engine thermal efficiency at different throttle settings.


#4

It is certainly possible, depending on the specific car, aerodynamics, gear ratios, engine characteristics etc. Based on my own experience, it is not true for my cars. Keep in mind I tend to buy econoboxes. (with the exception of the RX-7)

Normally I would have listed a number of possible errors, but knowing this was from OK, I know most if not all of them would have been considered and ruled out.

What it does say, is that each of us should remember that we need to know our own cars and their characteristics.


#5

Not trying to rip you, but do want to explain a flaw in your logic. The two-lane roads you mentioned are built to a different standard than the interstate. The allowable grades are steeper, and the curves are tighter, explaining the lower speed limit. Also, many of the 2-lane roads I’ve driven on do have stops and lights to contend with. Not many, but even a couple will easily affect the gas mileage you saw.

A more reliable test is to drive the same route at various speeds. I did this recently, and found that on a common-to-me business trip to SC from Atlanta, I get a 5 mpg advantage using 60 mph instead of my previous usual 75 mph. This was the same exact route on the interstate, with the same brand of gas. The only difference was the speed. Of course, I have a Ford Explorer SUV, with a much bigger air profile than your Mark VIII. Go back to 75 mph, and I lose the 5 mpg I saw at 60.

Your advise has always been an asset to this site. It would be a shame to see you go.


#6

Well at least I’m one who’s on your side. In the previous thread that turned into an an all out brawl, I think maybe it was MY comment about My truck that poked a stick in the hornets’ nest. They acted as if I said it was ALL faster speeds ( I didn’t ), All vehicles ( I didn’t ) and then they spewed a line of math and charts without ever driving my truck. At least you understand that on SOME vehicles , at SOME speeds higher than others you can see better mpg. It’s just a matter of you, yourself, doing the research on your own vehicle to know if it’s applicable for that car. To reiterate; only my 92 Explorer…NOT my 06 hybrid Escape…NOT my 79 chevy pickup gets better mpg on trips to Albuquerque at 82mph than at 75…NOT 90mph…NOT 95…and I positively know it would be much better at 55 ( but I ain’t goin’ no 55 to Abq and back zzzzz ).


#7

I respect your opinions and knowledge tremendously (as a regular reader and occasional poster on this board) but I do agree with BustedKnuckles (no relation) that the true test would be to drive the same road for the same distance at the different speeds (and ensuring no head wind, tail wind advantage) to obtain pure data. You did not drive the same road or even the same type of road for your “control” and you did not factor in wind conditions.

Call me a scientific purist but I am hard to convince without proper data and a tight “control” is always required to validate a properly designed experiment.

When all is said and done I believe it is safe to say that while a handful of vehicles MIGHT get better mileage at higher speeds (depending on gear ratios, computer settings, etc) the vast majority will benefit from the physics of lower speed and wind resistance to acheive the best highway fuel mileage.

That’s my story and I’m stickin’ to it!


#8

Someone’s likely to chime in and say I’m a crackpot who doesn’t understand physics - and maybe their right - but it makes sense that every car will have a sweet spot where speed will overcome inertia and allow the engine to run a little more efficiently. Above that sweet spot and the efficiency begins to drop again. That might be 73mph for some cars, 63mph for some, and 55mph for others. It would be different for all cars as a result of many factors including weight, aerodynamics, and engine performance.


#9

To me, this thread has nothing to do with physics. It has everything to do with how we got to a point where one of this forum’s most valued contributors has been made to feel like his contributions are no longer valued.

All or our contributions to this forum are based on our varying amounts of theory and real world experiences. When real world experiences and theory differ, it means the problem is more complex than we understand - it should never mean we should discount what a respected contributor is trying to share.

I have a ton of theory in my engineering schooling background, and yet it means nothing without carefully listening to everyone’s input and experiences.
OK has spent hundreds of hours of his time helping many on this forum. I am one of those who greatly respects, appreciates, and has learned from his input.

While I am in the camp of believing that lower speeds produces better gas mileage, the issue is complicated with more variables than any of us alone fully comprehends. I will never discount or question the real-world experiences of of a valued contributor like OK. The day I do that is when all of you should stop listening to me.

Joe Mario


#10

Actually, I’m not even slightly mad at anyone here; even the ones who got a bit crude. Sometimes I get a bit irritated but that’s about it.

In regards to the comment about different standards on 2-lane roads, stops, etc. roads are much different in the plains than the ones down south.
Out here most of the roads are arrow straight with sweeping curves if any at all.
Towns are sp;ead out and most of them one goes through with the cruise still set because there are no stop signs or traffic lights.
(Example: I’ve made several trips to the Liberal, KS aircraft museum and there’s one traffic light and 2 stop signs the entire distance; and that’s 200 miles. The light is not a deterrent because it’s easy to catch while green.)

I made more stops and slowdowns on the Interstate than I did the back roads due to traffic, construction zones, etc.

I’ve always watched the mileage closely on the Mark because the dashboard indicator is a neat little toy that lets me keep tabs on how things are running and if there is a potential problem breweing.

My daughter used to live in TX and I’ve made probably close to a dozen trips down there with varying speeds. Almost the entire trip was Interstate because she only lived about a 1/2 mile off the highway. Since I would fill up at the Interstate station which is about 20 miles from me you can see that it was simply a set the cruise and go type of trip.

I’ve checked the mileage repeatedly both coming and going and at different speeds and the Mark actually got better MPG at a higher speed than it did at lower ones.
As I’ve mentioned, I think that most cars, including most of mine probably, will consume more fuel at higher speeds but I don’t think that could be true of every car due to what I feel are the biggest factors; engine type, gear ratio, power curve, etc.

Regarding my BMW motorcycle, that bike lived on the open road and mileage was verified countless times, including 3 2000 mile round trips to Sturgis, SD and back.
At 55 MPH the Beemer was close to being an absolute dog and lugging to some extent.
About 65-70 MPH one could start to feel the engine starting to breathe a lot better and about 80 it was really settling in.
My Lincoln feels the same way due to the 2:73 rear axle and 4.6: at 55 it’s lugging and around 70 it’s starting to settle in.

Probably the best way of thinking about it would be if someone had a manual transission car and simply shifted from 1st to 5th every time. Logic, not mine, would dictate that the mileage would improve dramatically; unless lugging is a factor.
Anyhoo, that’s the basis for my statements. :slight_smile:


#11

I don’t dispute your mileage, but I am not sure if the roads are comparable. I agree with bloody knuckles. The test would have to be done on the same roads under the same conditions. It would be OK to use a round trip; lower speed one way and higher speed the other if the route is flat (start and end at the same altitude). I did the same test in a rental Impala from Orlando to Cape Canaveral and back. I got 32 MPG at 60 MPH and 29 MPG at 70 MPH. There’s a little up and down, but central Florida is essentially flat.


#12

One tank per mpg calculation has the potential for errors considering that your fill levels might be different due to different gas nozzle shutoff sensitivity, fill rates at different nozzles even at minimum fill speed and the levelness of the parking pad at the gas station not to mention headwind differences and different terrain. I have found that different brands of diesel fuel give slightly different mileage figures. Can gasoline be any different? Are road surfaces a factor? I don’t know.

Gas mileage calculations like any scientific endeavor must ideally make all variables constant except for the one being studied which in this case is miles per gallon.

To minimize error in your calculations, you must take the same trip more than once and fill and refill at the same gas station at the same pump at the same parking pad. Even then, you have to hope that the gasoline and the product temperature and therefore measured density are identical.

In the world of engineering, one sample is not enough to cover for the unknown variables.

It seems to be pretty much recognized that faster speeds requires more energy pushing air out of the way but this can be masked by other variables. If you really believe that going faster can require less energy than slower, then get back on a bicycle. Go slow for a while and then go fast for the same distance. What else can I say?

Regarding your comments about leaving, go if you must. You can not fix the world and you can’t fix stupid and it is useless to try to think that you can. Meanwhile you can contribute what you can knowing that some of the good that you want to pass along will occasionally stick.


#13

See…hornets’ nest.


#14

Well I missed seeing that post and the sqirmish that followed but I am ok with that. What I think is perplexing is when someone posts something that they have experienced, then someone of a different opinion reads that, then all get-out breaks loose. I suppose this is because people read and/or hear what they want to read and/or hear which could be attributed to some insecurity or emotional shortcoming in their lifes.
My vehicles never got better MPG at higher speeds but I do not doubt that it can happen. I had a job in which I traveled about 700 miles a week. If I drove like my Taurus was the General Lee I got 28 MPG. But if I drove like a senior citizen then I got 30 MPG. A difference of 2. At a car show a factory rep told my friend amd I that the GM 3800 gets better mileage at a faster rate of speed. Sounds good to me but I don’t own one. My friend did and he verified that.
So with that said, ok4450, I don’t think you need to depart, simply understand that some people will argue even when thay have nothing to stand on.
I have noticed posters on here either

  1. Know what they are talking about from experience.
  2. Have an idea of what they are talking about from limited experience.
  3. Have no clue and are only giving opinions.

All 3 are welcome but lets keep things civil. ok4450 is well versed in automotive related topics and relates these thoughts in clear dialogue and is not mean spirited to anyone.


#15

First, I’m OK with OK4450. I always hope to learn from your experience and look for your posts. The above mileage experiment does have an additional variable, though, which is altitude and the associated air density. When I drive from Austin to see my Pagosa Springs buddy, I always get better mileage at higher elevation because the drag force on a car decreases linearly with air density. This is a substantial effect, about 3 mpg for my Escort. I noticed that the middle portion of your trip (at 73 mph) may have been at a much higher average elevation than the first and last legs and this could account for the difference you observed. I’m in favor of more data. How about visiting Austin?


#16

ummm!

common sense.


#17

Doesn’t the MkVIII lower itself above 65 mph. Seems that I remember that feature in a commercial. It makes it more stable at higher speeds and it also makes it more aerodynamic.


#18

I would like to give my evaluation on the whole speed v/s MPG. I commute 48 miles one way to work every day. My route consists of some back roads and about 95% interstate. But once I get closer to the city, speed varies due to traffic. I had bought a 94 Chevy Cavalier last year to help with fuel mileage. I would set my cruise at 68 MPH and would get an average (city/hwy.) of 31 MPG. I then would set my cruise at 63 MPH and using the same route got 33.5 MPG.

I recently bought a 1998 Crown Victoria. I was leary about buying this car since it has a 4.6L V8. But I wanted a bigger car (since I am a bigger guy, the Crown Vic is more “fat guy friendly”, haha.) I have been surprised that since I set the cruise at 63 on the interstate, I have been getting 27.6 MPG according to the trip computer. It may not be the thing for some people, but for me I am more comfortable in the car, and I am still getting decent mileage. Like you said, the only way I can be believed is to take my word. I guess I could take a digital picture of my digital display on the dash board, but if people don’t want to believe us, then they don’t have to I guess. I take comfort in the fact that I can have a bigger car and still be an economical driver.


#19

I have experienced the same thing in my 2006 BMW 525I. I get better MPG at 90 mph than I do at 70 mph. Most of my friends tell me that that is impossible, however in my research it seams that it is a torque related issue and that at a particular rpm each engine has a sweet spot. The BMW seems to be around 3,000 rpm which is around 90mph in 6th gear. The problem seems to be the cops don’t care about my fuel economy or my need to transverse distance in a shorter given time. The bastards!


#20

Temperature, altitude, humidity, and wind could play a part in the OP’s test. However, they seemed to be consistant, so I choose to believe the outcome.