Author Topic: Mileage at 85 MPH and why  (Read 1391 times)

Mileage at 85 MPH and why
« on: January 30, 2020, 11:03:42 AM »
Just got back from a  trip to the coast and looked at the gas receipts.  I filled up before I left and when I got there, and used 12.275 gallons to go 152 miles.  That works out to 12.38 MPG which I think is decent considering I was pushing my barn door (76 Coupe deVille) at a steady 85 MPH. There was virtually no traffic and I kept in the primaries of the QJ all the way, and I've got about 1500 miles on a new motor. 
Me being me I couldn't let it go at simple mileage, so I got out my HP 42S and started figuring.  Short version is that at 85 MPH I was using 41.186 pounds of fuel per hour.  At a nominal 1/2 pound (fuel) per HP/HR that works out to a continuous 82 HP.
I thought it interesting, but then that's me.
Greg Surfas 
« Last Edit: February 03, 2020, 12:25:42 PM by "Cadillac Kid" Greg Surfas 15364 »
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline Dave Shepherd

  • Posts: 2424
Re: Mileage at 85 MPH and why
« Reply #1 on: January 30, 2020, 11:34:47 AM »
Not bad for a brick with 4  wheels  Lol  When I worked for Cadillac back then, drove that car regularly, averaged about 11, 50/50 driving, never over 70 though.  May improve a little after getting some mileage on the engine.

Offline hornetball

  • Posts: 824
  • CLC Number: 32697
  • Name: R. Price
Re: Mileage at 85 MPH and why
« Reply #2 on: January 30, 2020, 11:56:59 AM »
I kept in the primaries of the QJ all the way

Having met you, I'm having trouble believing this.   >:D

Re: Mileage at 85 MPH and why
« Reply #3 on: January 30, 2020, 02:18:47 PM »
Come on now, be nice. It was raining and wet and a steady 85 was fast enough.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline Anderson

  • Posts: 142
  • Name: W.C. Dunn
Re: Mileage at 85 MPH and why
« Reply #4 on: January 30, 2020, 08:00:00 PM »
This is in line with what I observed on a trip to the train station in Richmond and back a few weeks ago in my '70 Eldorado.  The speed wasn't constant and there was a traffic jam at one point, but the mileage is in line with that (I want to say I did about 161 miles on 13.1 gallons).

Offline The Tassie Devil(le)

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  • Name: Bruce Reynolds
Re: Mileage at 85 MPH and why
« Reply #5 on: January 30, 2020, 08:19:02 PM »
It is always surprising just how little Horse Power is used to maintain any speed.   It is what is used to actually get to that speed is what uses the petrol.

I found that out when using the Predator Carby.  When going down a hill, I have to keep a little pedal on as backing right off, as one does with "normal" carburettors, the engine braking is slowing me down.   Normal carbies have sufficient fuel in the idle circuit to maintain speed, and therefore is using more than is necessary.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

Offline D.Smith

  • Posts: 1546
  • CLC Number: 17592
  • Name: Dave Smith
Re: Mileage at 85 MPH and why
« Reply #6 on: January 31, 2020, 10:19:34 AM »
Studies were done in the seventies that showed that you got the best fuel economy at 55 MPH.    Thus that is why the US National speed limited was set at that after the fuel crisis.   

Cadillac also developed the V8-6-4 for the specific reason that the big V8 only needed 40 HP to keep the car rolling on a level surface at 55 MPH.  Thus the cylinder deactivation.   Controversial then, but common place today on many premium cars.

Offline Tpicks55

  • Posts: 168
  • 1975 Eldorado Convert 1994 Deville Concours
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  • Name: Tony Giannantonio
Re: Mileage at 85 MPH and why
« Reply #7 on: January 31, 2020, 10:38:01 AM »
Greg, I loved this.  You must be a engineer or just like numbers.  Keep it up.  btw at 85 arn't you worried about the stray animal crossing your path?  Tony
75 Eldorado Convertible
94 Deville Concurs
2019 Lincoln Continental
2016 Cadillac XTS

Re: Mileage at 85 MPH and why
« Reply #8 on: January 31, 2020, 11:48:24 AM »
Yes I am Tony. The drive between home and Corpus Christi is pretty much flat through some wide open country. About the only animal I have seen on that route besides the cattle off in the distance was a cougar walking along side the grass about 30 feet off the roadway "looking for a little lunch", and he was too smart to mix it up with traffic.
The intent of this post was not looking for mileage, but inversely figuring out the horsepower "consumed" at 85 MPH.
As far as mileage I think my wife is probably a record holder. She got 16.4 MPG driving my 72 Eldo back a while and has never let me forget about it.
The trip back to FLA for the GN a few years back we kept to the speed limit with the same car (76) and cumulative mileage was something on the order of 14.5. Mileage is inversely proportional to the action of your right foot.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline hornetball

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  • Name: R. Price
Re: Mileage at 85 MPH and why
« Reply #9 on: January 31, 2020, 12:54:37 PM »
She got 16.4 MPG driving my 72 Eldo back a while and has never let me forget about it.

Having met her too, I can believe that she had the skill to keep it in the QJ primaries.  ;)

Offline Glen

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  • Name: Glen Houlton
Re: Mileage at 85 MPH and why
« Reply #10 on: February 01, 2020, 01:04:40 AM »
Studies were done in the seventies that showed that you got the best fuel economy at 55 MPH.    Thus that is why the US National speed limited was set at that after the fuel crisis.   

What studies were done?  I remember watching the debate and waited to see the results of the studies.  I saw none.  There were “experts” testifying but no studies were published for the public. 
 I would believe it was true for the small cars like a VW.    But my test saw the opposite for my 68 ELDO.
Glen Houlton CLC #727 
CLCMRC benefactor #104

Re: Mileage at 85 MPH and why
« Reply #11 on: February 01, 2020, 12:25:42 PM »
Realistically there can not be one "sweet spot" for gas mileage across the board for all vehicles. There are too many variables such as weight/engine power, gearing, tire size, engine dynamics themselves. I believe the 55 MPH limit was imposed for political expediency  (looks like we are doing something).
Since this is a Cadillac forum and Cadillac's are definitely not Volkswagen's, lets talk about a 70's era 500 inch motor. Low compression, lots of cubic inches, designed FROM THE OIL PAN UP FOR TORQUE. 
With a 2.73 rear end ratio at 55 MPH a 500 inch Cadillac would be loafing along at about 1750 RPM.  Plenty of torque and HP (395 Lb/Ft, 172 HP) and the efficiencies (volumetric and mechanical) are just about at their peaks.
Something like a small block Chevrolet working at those speeds is really at a disadvantage and the driver is going to have to have his foot in the accelerator to a greater degree.
Our Cadillacs had "road burning" in mind from their very beginnings and are truly not like any others.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline James Landi

  • Posts: 2383
  • 2007 XLR
  • Name: James Landi
Re: Mileage at 85 MPH and why
« Reply #12 on: February 01, 2020, 01:08:16 PM »
My philosophy on owning a Cadillac is that gas purchases are simply a "rounding error"in the total cost of ownership.  Of course, if one owns a fully depreciated Cadillac that's kept in good mechanical condition, the price of gas is, empirically, part of a minor cost compared to depreciation and other costs of ownership of a late model of some car brand other than Cadillac inspirational. "Consumers' Report" used to post the cost per mile of new car ownership  (all in costs, that is)---  I recall on some new cars, the cost per mile was breathtaking, so I started keeping tabs on my daily Cadillac drivers, and figured in a good $1,000 a year for annual repairs and non gas expenses... I came out WAY ahead of the new car ownership blues--- thus I call myself Detroit's worse nightmare...  James

Offline bctexas

  • Posts: 218
  • CLC Number: 31627
  • Name: B Long
Re: Mileage at 85 MPH and why
« Reply #13 on: February 01, 2020, 04:08:46 PM »
Not bad for a brick with 4  wheels  Lol 

Not to mention a spacious rolling living room with two comfy leather sofas!   ;)
1965 CDV
1970 SDV

Offline Eric DeVirgilis CLC# 8621

  • Posts: 7039
  • Name: Eric DeVirgilis
Re: Mileage at 85 MPH and why
« Reply #14 on: February 02, 2020, 02:33:59 PM »
Back in the '70s it was commonly said that the best fuel economy is obtained ~ 35 MPH. May well be similar today.
A Cadillac Motorcar is a Possession for which there is no Acceptable Substitute

Re: Mileage at 85 MPH and why
« Reply #15 on: February 02, 2020, 05:07:11 PM »
I think that today's cars with their multiplicity of tune-able elements ( variable valve timing, variable spark curves, variable fuel deliver, variable cylinder activation, variable and multiple turbo charging), as well as multi multi ( up to 10 or 12) speed or infinitely variable speed transmissions, ALL orchestrated by one or several electronic control modules (ECMs) that coordinate all said devices to obtain the maximum, instantaneous power commensurate with absolute minimum fuel consumption there is no speed at which maximum fuel economy is achieved.
FWIW that is why the fuel consumption verification cycle is a pre programmed series of accelerations and deceleration s to and from various speeds.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline Eric DeVirgilis CLC# 8621

  • Posts: 7039
  • Name: Eric DeVirgilis
Re: Mileage at 85 MPH and why
« Reply #16 on: February 03, 2020, 10:34:16 AM »
Wind resistance is what works against fuel economy as speed is increased.
A Cadillac Motorcar is a Possession for which there is no Acceptable Substitute

Offline EAM 17806

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  • Name: Ev Marabian
Re: Mileage at 85 MPH and why
« Reply #17 on: February 03, 2020, 10:44:05 AM »
Wind resistance is what works against fuel economy as speed is increased.
  Yes, especially with the EL DORADOs with it's two front fenders sticking out to encompass  the wind resistance and hold back the car further.  EAM
Ev Marabian

1976 Cadillac Coupe DeVille, 1989 Chevrolet Caprice Classic, 1990 Pontiac Bonneville and 1996 Buick Skylark

Re: Mileage at 85 MPH and why
« Reply #18 on: February 03, 2020, 12:30:42 PM »
Eric,
That was essentially the thoughts behind my original post.  With a "Cd" of about 0.41 (as compared to a new car's 0.21 or.22) at 85 MPH it takes about 20 HP to overcome aerodynamic drag.
Greg Surfas
Cadillac Kid-Greg Surfas
Director Modified Chapter CLC
CLC #15364
66 Coupe deVille (now gone to the UK)
72 Eldo Cpe  (now cruising the sands in Quatar)
73 Coupe deVille
75 Coupe deElegance
76 Coupe deVille
79 Coupe de ville with "Paris" (pick up) option and 472 motor
514 inch motor now in '73-

Offline bcroe

  • Posts: 3895
Mileage at 85 MPH and why
« Reply #19 on: February 04, 2020, 10:30:29 AM »
The energy consumed by air friction is proportional to the cube of
the velocity.  Just where that becomes a dominating factor varies
hugely with the situation (Sunbeam vs locomotive for example). 
But you get there in less time at higher speed, so the fuel consumed
per mile goes as the square of velocity for air friction. 

The losses for everything else (engine, trans, tires, accessories,
more) were once ignored to achieve nice performance overall.  But
now these have been greatly improved to trim off as much waste
as possible.  I have never seen any curves of efficiency vs speed
for any of the above, the engineers probably have them for recent
decades. 

My own approach has been to try and optimize highway cruising
with a very simple but rugged drive train.  A 2.41:1 axle lets a
big engine turn very slowly, along with a low slip switch pitch trans
with the torque converter in low stall position.  The high stall
position is immediately available to get back more performance. 
This setup does not do well sitting in traffic, but I do not spend
much time doing that.  Bruce Roe

 

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