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Cadillac & LaSalle Club Forums => General Discussion => Topic started by: "Cadillac Kid" Greg Surfas 15364 on January 30, 2020, 11:03:42 AM

Title: Mileage at 85 MPH and why
Post by: "Cadillac Kid" Greg Surfas 15364 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 
Title: Re: Mileage at 85 MPH and why
Post by: Dave Shepherd 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.
Title: Re: Mileage at 85 MPH and why
Post by: hornetball on January 30, 2020, 11:56:59 AM
Quote from: "Cadillac Kid"  Greg Surfas 15364 on January 30, 2020, 11:03:42 AMI kept in the primaries of the QJ all the way

Having met you, I'm having trouble believing this.   >:D
Title: Re: Mileage at 85 MPH and why
Post by: "Cadillac Kid" Greg Surfas 15364 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
Title: Re: Mileage at 85 MPH and why
Post by: Anderson 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).
Title: Re: Mileage at 85 MPH and why
Post by: The Tassie Devil(le) 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
Title: Re: Mileage at 85 MPH and why
Post by: D.Smith 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.
Title: Re: Mileage at 85 MPH and why
Post by: Tpicks55 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
Title: Re: Mileage at 85 MPH and why
Post by: "Cadillac Kid" Greg Surfas 15364 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
Title: Re: Mileage at 85 MPH and why
Post by: hornetball on January 31, 2020, 12:54:37 PM
Quote from: "Cadillac Kid"  Greg Surfas 15364 on January 31, 2020, 11:48:24 AMShe 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.  ;)
Title: Re: Mileage at 85 MPH and why
Post by: Glen on February 01, 2020, 01:04:40 AM
Quote from: D.Smith 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.   

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.
Title: Re: Mileage at 85 MPH and why
Post by: "Cadillac Kid" Greg Surfas 15364 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
Title: Re: Mileage at 85 MPH and why
Post by: James Landi 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
Title: Re: Mileage at 85 MPH and why
Post by: bctexas on February 01, 2020, 04:08:46 PM
Quote from: Dave Shepherd on January 30, 2020, 11:34:47 AM
Not bad for a brick with 4  wheels  Lol 

Not to mention a spacious rolling living room with two comfy leather sofas!   ;)
Title: Re: Mileage at 85 MPH and why
Post by: Eric DeVirgilis CLC# 8621 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.
Title: Re: Mileage at 85 MPH and why
Post by: "Cadillac Kid" Greg Surfas 15364 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
Title: Re: Mileage at 85 MPH and why
Post by: Eric DeVirgilis CLC# 8621 on February 03, 2020, 10:34:16 AM
Wind resistance is what works against fuel economy as speed is increased.
Title: Re: Mileage at 85 MPH and why
Post by: EAM 17806 on February 03, 2020, 10:44:05 AM
Quote from: Eric DeVirgilis CLC# 8621 on February 03, 2020, 10:34:16 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
Title: Re: Mileage at 85 MPH and why
Post by: "Cadillac Kid" Greg Surfas 15364 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
Title: Mileage at 85 MPH and why
Post by: bcroe 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
Title: Re: Mileage at 85 MPH and why
Post by: Scot Minesinger on February 04, 2020, 11:24:25 PM
Agreed, this is an interesting post. 

Aerodynamics and gearing are important.  My 1978 Olds 88 got about 19 mpg at about 55-60mph (350 V-8, TH 3spd), and my 1985 Chevrolet Caprice got 27 mpg at 75 mph (305 V-8, TH 4spd (overdrive)).  The Chevy was better quality and made it to 300k miles on the original ac, trans, radiator, engine and carb without doing much.  I traded the Olds in to buy the Chevy because at 163k miles the trans was failing.  The Chevy had the 26 gallon tank, so it enjoyed 600 mile range no problem.

And I agree the cost of gas in a Cadillac is a rounding error, and generally of little interest to the owner, except for range of cruising between refueling.
Title: Re: Mileage at 85 MPH and why
Post by: Glen on February 05, 2020, 03:28:52 AM
During the debate about the national 55 MPH speed I waited to see the studies to support the claims that it would save gas.  I assumed magazines like Popular Mechanics, Car and Driver would have test results of various popular cars gas consumption at various speeds.  It did not happen.  Congress passed the law based on testimony of a few people citing wind resistance as the main factor.   No test on cars that I saw. 
I noticed a couple of MPG meters listed in the J. C. Whiney catalog and considered buying one until I found out they were only vacuum gauges with MPG markings.  Granted low vacuum means more air flowing through the carb and more air flow means more fuel consumed.  But it had a major problem, it showed you were getting more than 30 MPG while sitting at a stop light when your gas mileage was actually zero. 
Then I saw an ad by a company called Space Com (IIRC).  It had a product that used a tachometer generator installed in the speedometer cable and a flow sensor in the fuel line to the carb.  I bought it and installed it in my 68 ELDO.  The tachometer generator produced a voltage proportional to the speed of the car.  The fuel sensor used the typical ball in a conical tube, the ball is held in the narrowest part of the cone by a spring.  The fuel flowing through the sensor pushes the ball out of the cone, the more fuel flowing the more the ball is pushed out to a wider part of the cone.   There is a light on one side of the sensor and a photoresistor on the other.  The more the ball is pushed out the higher the resistance of the photoresistor.  The meter then showed the result of the two.  And it showed you getting zero MPG when stopped at a light. 
So what did I find out with this tool?  I was surprised at how much a slight grade affected instantaneous MPG (IMPG).  Going up hill decreased the IMPG more than I would have thought.  And of course, going downhill increased IMPG. 
Another revelation was what I call carb lag.  While sitting at the stop light the gauge showed 0 IMPG, but when I accelerate away from the light the meter shoots up to as high as 30 IMPG or more for a second or two, then plunged to 1 or 2 IMPG.  After a short time, the meter settled to a more reasonable number.  I don’t know the exact cause of this.  It could be it just took a moment for the carb to catch up or it might be that on acceleration the fuel in the float bowl sloshed to the back of the float bowl and supported the floats at a higher level than it should.  As the G forces subsided the fuel evened out and the floats opened the valve to refill the float bowl. 
What this means is that for any reasonably accurate study the readings need to be taken on a level course and after a time of steady throttle. 
So what did I find out?  I did not find the peak IMPG for my ELDO.  I took a number of runs on a stretch of level road on a viaduct.  Each at a higher speed.  Each run at a higher speed showed an increase in IMPG up to 70 MPH.  At that speed the gauge showed 30 MPG.  I could not do the next run at 80 MPH because of the other traffic on the road.  I was going to try at night when the traffic was low, but I was warned that that was a favorite racetrack for the street racers and the police monitored the area.  So my 80 MPH run never happened. 
It did blow the theory that the 55 MPH speed limit saves fuel out of the water. 
Title: Re: Mileage at 85 MPH and why
Post by: The Tassie Devil(le) on February 05, 2020, 04:28:35 AM
Funny thing is that travelling at 55 MPH produces a fuel saving as against 75 MPH, and travelling at 45 MPH saves even more.

The biggest fuel saver is to travel on a flat road, with no changes in speed.

Maximum Speed on the open road in Australia is 65 MPH, and travel 5 MPH over that, and you WILL get booked if you are caught, and the chances of getting caught is very high.

Bruce. >:D
Title: Re: Mileage at 85 MPH and why
Post by: James Landi on February 05, 2020, 08:12:25 AM
Glen,


Such an interesting experiment.  I have owned three large boats that were equipped with "FLo Scan" meters-- these were twin engine cruisers, two over 50 feet, and were propelled with 871 diesels, one "natural" and one  turbo charged, and the last one. a 45 foot boat,, a with Flo Scan had  twin engine gas 454 big blocks .   Unlike cars, boats consume "gallons per hour" and boats never coast-- the engines are always working to move the tonnage against the water, so once one sets the throttles up to"cruising speed," the flow, measured in gallons per hour doesn't change much, and if it does, there's a big problem.  HOWEVER, with the twin gas powered boat, the Flo Scans were "busy" at idle speed and when the throttles got pushed forward--- it was easy to perceive the FUEL bowls filling, and then subsiding, and filling again.   But once up the speed, the fuel flow was "steady" -- indicating that fuel was FLOWING steadily, from the fuel pump, through the carbs and into the intake. Somewhat like a car, however, tide and wind made a difference in the speed over the ground and not the amount of fuel consumed by the engines.    James
Title: Re: Mileage at 85 MPH and why
Post by: Eric DeVirgilis CLC# 8621 on February 05, 2020, 10:24:47 AM
Whenever cars competed in the Mobilgas fuel economy run, where everything was optimized for fuel economy; ie spare tire removed; running with no more than a quarter tank of fuel, flat road surface, etc the cars are always operated at lower speeds ie low 20s -30s mph (just enough for the transmission to be shifted into third) in order achieve absolute maximum fuel economy results.

No matter how good the Cd is, there will always be some level of wind resistance.

Scot... the OD transmission is the reason for the MPG differential between the '85 Caprice and the '78 Delta 88. Has nothing to do with the relative quality between the two cars, which having owned a number of each, I never found much difference in that department. Of course, the better economy of the OD trannies came at some price to performance in the form of balky shifting in/out of OD on highway driving in hilly terrains.
Title: Re: Mileage at 85 MPH and why
Post by: Glen on February 06, 2020, 04:04:48 AM
One thing I wanted to do was put a switch in to cut out the tachometer generator and insert a constant voltage.  The idea was to convert it to a gallons per hour meter.  With that change I could see how much fuel was consumed at idle and how it changed as the speed increased.  Unfortunately, the fuel sensor started leaking so I had to remove the system. 

Today’s cars with OBDII has all that is needed to make an IMPG meter.  A number of years ago I bought the item in the first picture.  It plugs into the OBDII port and you can connect to it via Bluetooth and a app on you cell phone.  I bought it to access the fault codes.  It also can access much more.  It is designed more toward the racing crowd with ¼ mile times, throttle position, 0-60 mph times etc.  I looked through the list of dials I can add and found one that shows MPG, so I added that and a dial that shows speed in MPH.  Second picture.   Should be interesting what I find with my Colorado, S10, and Rendezvous. 
Title: Re: Mileage at 85 MPH and why
Post by: James Landi on February 06, 2020, 08:13:43 AM
Glen,

Looks like that electronic device could provide the user with greater accuracy regarding driving habits and tuning . While I believe a modern car will manage gasoline consumption in spite of driving conditions and a driver's lack of concern for his or her machine, a carburetor car of yesteryear will prove a shade tree mechanic with some interesting opportunities to tune up a severely detuned big block.  For example, I've owned a couple of 1970 SDV Cadillacs and a '72 Eldorado.... By simply by passing the electro-mechanical vacuum cut out that is linked to the speedometer, and connecting the vacuum advance directly to the intake manifold, I had more responsive engines that idled better... not sure how that translated to fuel economy, but with your device, I suspect there would be an interesting answer.  James