Mileage at 85 MPH and why

Started by "Cadillac Kid" Greg Surfas 15364, January 30, 2020, 11:03:42 AM

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Scot Minesinger

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.
Fairfax Station, VA  22039 (Washington DC Sub)
1970 Cadillac DeVille Convertible
1970 Cadillac Sedan DeVille
1970 four door Convertible w/Cadillac Warranty


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. 
Glen Houlton CLC #727 
CLCMRC benefactor #104

The Tassie Devil(le)

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
'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

James Landi


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

Eric DeVirgilis CLC# 8621

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.
A Cadillac Motorcar is a Possession for which there is no Acceptable Substitute


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. 
Glen Houlton CLC #727 
CLCMRC benefactor #104

James Landi


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