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MPG on 472

Started by gjackson15, September 10, 2011, 09:39:00 PM

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gjackson15

Hi folks. New member here. I have a question for anyone that has a 472 under the hood. I recently purchased a "survivor" 73 Sedan DeVille from the estate of the original owner. Original paint, never wrecked, "tight as a tick" as the old timers would say. Just turned over 78K miles today. Needless to say the car had been sitting a good bit, so a carburetor rebuild was in order. Now that I have it back on the road after about 150 miles, I have found I am getting about 8 miles to the gallon. I knew when I bought the car the mileage wouldn't be great, but really thought I would be getting at least double digits. I still think some carburetor adjusting is in order, as it is a bit hard to start when it is hot, so I am hoping to squeeze a few more MPG out, but wanted to see what other folks were getting. Thanks for any info, glad to be here. Greg Jackson
1973 Sedan DeVille
1985 Eldorado Biarritz
1992 Sedan DeVille (Sold!)

TJ Hopland

8 seems a little low if you are driving it like a Cadillac.   Its hard to find someone that knows how to work on carbs now days.  If it was the original carb its not that hard to clean it and throw a kit in it but if its not original then you may have to find someone that really knows those things to get it working correctly.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

Alan Harris CLC#1513

The 1973 and 1974 cars were known when they were new for atrocious gas mileage.

They had detuned engines which were hobbled by primitive emission control devices. They also had the earlist heaviest versions of the federalized bumpers.

I know that we will hear from people that claim much better mileage, but I do not believe that you could do much more than 8-10 MPG.

Before I gave up, I would go through everything and make sure that all of the emission devices are up to spec and that your points, plugs, wiring, and timing are spot on.

Beyond that, I think you have to live with it.

Mike Josephic CLC #3877

I have a '73 Eldorado (with the 500 CI engine) and have had it since new.

Around town 10 MPG was normal and on long highway trips 14 MPG was the
best I've ever gotten.  Your mileage sounds low to me.

Suggest (if you have not already done so) do a complete tune up (points,
plugs, distributor cap, rotor etc.).  Also, as already suggested, get the carb
rebuilt.  Since you say it was sitting for a long time, some maintenance is
in order.

Mike
1955 Cadillac Eldorado
1973 Cadillac Eldorado
1995 Cadillac Seville
2004 Escalade
1997 GMC Suburban 4X4, 454 engine, 3/4 ton
custom built by Santa Fe in Evansville, IN
2011 Buick Lucerne CX
-------------------------------------
CLCMRC Museum Benefactor #38
Past: VP International Affiliates, Museum Board Director, President / Director Pittsburgh Region

62droptop

does sound a bit low, we had a 76 with the 500 and got mid teens on the highway
tire pressure makes a huge deal on such a heavy car
i run mine at 35psi in my 62 conv
rides a bit firmer but pulls a few more mpg that way
also type of tires,cheap tires most times have higher inherent rolling resistance
a good quality tire may pay for it self in gas mileage
was thinking of tryong the michelin lrr(low rolling resistance) tire that is avail in a 235/75/15 to see how much difference it makes

since i switched from an old firestone 721 which i could run at 40psi(max 44 ontire sidewall) to the delta brand (max 35 psi on tire) which i run at 35-36 i lost about 3-4 hwy mpg

like others said,check tune up,timing ,carb etc
buddy had a 73 coupe that was a pig on as compared to the 76 i,but not sure on his maintenance or tire pressure etc or how well his car was tuned

Glen

The previous owner of my 68 EDLO drove the car about 5 miles to work, then 5 miles home again.  When I got the car in 1973 it was getting about 9 MPG.  I started to drive it to work every day, about 15 miles each way and over a small mountain range.  It was not long before I was getting 12.5 MPG with the A/C running. 

I would get your car out on the road and run it, not real hard but make it work a little.  Run it at 50 â€" 60 MPH for a while, up hill if possible.  That should blow the cobwebs out of it.  Then check the gas mileage. I think it will improve some.  If not then look closer at the tune up etc. 
Glen Houlton CLC #727 
CLCMRC benefactor #104

Chuck Swanson

Checked my 69 Limo this summer for the first time.  Got about 10.5 MPG on the 5555lb beast  :)

I did put in Breakerless SE electronic ingnition.  Did not check before converting. 
CLC Lifetime
AACA Lifetime
Like 65-66 Club: www.facebook.com/6566Cadillac
66 DeVille Convertible-CLC Sr Wreath, (AACA 1st Jr 2021, Senior 2022, 1st GN 2022 Sr GN 2023), Audrain Concours '22 3rd in Class.
66 Sedan DeVille hdtp
66 Calais pillar sedan
66 Series 75 9-pass limo
65 Eldorado (vert w/bucket seats)
65 Fleetwood
07 DTS w/ Performance pkg.
67 Chevy II Nova (AACA Sr GN 2018)
69 Dodge Coronet R/T

1937 LaSalle coupe

Hello Greg. I had a  1980 Seville. Only ran great on high octane gas. Regular made it miss and stall out. I know high test gas is more $$$ but in the long haul your car will run better and get better gas milage. Regards... John Lehman... CLC # 26365...

gjackson15

Awesome. These are all excellent tips. A bit of carburetor adjusting as well as the premium gas is in order. I'll report back with results probably in a few weeks.
1973 Sedan DeVille
1985 Eldorado Biarritz
1992 Sedan DeVille (Sold!)

gjackson15

And a picture (finally)

Greg Jackson
1973 Sedan DeVille
1985 Eldorado Biarritz
1992 Sedan DeVille (Sold!)

"Cadillac Kid" Greg Surfas 15364

Greg,
A couple of the replies touched on the subject, and for 1973 with the first EGR, a new camshaft, re curved ignition timing, and other emission controls introduced to keep ahead of the regulations, mileage was sacrificed to keep the performance up a bit.
I am not suggesting you eliminate any of the controls, but without them and the car "set up" correctly, 10-12 MPG is what you can expect (from a non worn engine) in the city, and 12-14 is what you might see on the highway.
You mentioned a mileage of 76K, and that might indicate a worn out (plastic or actually nylon covering of the cam gear teeth deteriorate with mileage and especially age) timing set.  If so, this can and will be detrimental to both performance and mileage.
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-

EAM 17806

My 1976 Coupe de Ville requires unleaded regular gasoline per the owners manual. I've been using regular since 1977 and it runs like it was brand new. Of course you have to make sure all elements within the engine are in tact such as the timing, carb adjustment, filters, etc. If you check with the big oil companies' web sites you will read them telling us don't use high test gas unless it's required per your shop or owners manuals because you're wasting your funds. I know a lot of you guys don't buy that but go to their web sites and maybe you'll be convinced to put a few more bucks in your pockets rather than the oil companies.   EAM
Ev Marabian

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

"Cadillac Kid" Greg Surfas 15364

Ev,
A 76 is not a 73.  The large chamber (cylinder heads) motors seem to digest a lower octane rating fuel than the placarded 91 Octane, probably due to their state of further "detune" than the 71-73's.  Back when I lived in California, I could notice the difference in performance and mileage between Chevron at 92 octane and ARCO which was 91 octane.  The care was tuned for the 92 and believe it or not, a single octane rating was noticeable.  This was primarily in the fuel mileage achieved.
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-

TJ Hopland

Ethanol is another big variable these days that will effect drive ability and economy of theses cars more so than more modern ones.   If the standard grade in your area has ethanol and the premium does not that can easily explain a perceived or measured difference in performance and or economy.   

In my area we have had the ethanol blends for around 20 years now.   Some areas are just getting it now but I believe almost all areas have it now or will have it soon due to federal regulations.  Some states may only require it in some grades some grades, some counties, or some times of the year.  It also varies state to state weather or not they have to label the pumps as such.  Often times they do proudly advertise the blend because 'its better'.   I notice vapor lock issues and reduced MPG on all my older non efi engines with the blends.   How much seems to depend on the specific engine and conditions.   In our state the Minnesota Street Rod Association got together and went to the state and got them to allow stations to apply for a permit to sell a non blended premium grade in the summer months if they choose to.    There are a few hundred that do this but my luck was usually that when I needed gas I was not near one of these not to mention my car did not require premium so it was extra time and expense to seek it out.

Another thing that can cause problems is the additive packs the different brands use.   In my area we have a regional chain (Holiday) that has always made themselves appear to be 'green'.   They have their special 'blue plannet' fuels that I guess are supposed to be cleaner and all that crap.    In my 05 I maybe see a slight reduction in MPG when I use their gas but not enough to be sure or consistent.   What is consistent is any and all carburaterated engines I own start and run like crap on their regular grade gas.  The cars get noticeably less mpg.   Other brands of regular dont seem to have the issues.   If I am forced to use their fuels I buy their premium which does not seem to have the problems.   Not sure why this is but its happened  over several years from several locations with several engines always with the same result.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

olds34dude

try to find a station withoutn ethanol, yet to meet anyone who doesn't get 10% or more without it- such nonsense to raise food prices and lower fuel economy for the benifit of?- also remember that the regular gas we got  thirty odd years ago had a higher octane rating than the premium we get today- as I remember it. D Varney
D. Varney

R Sotardi #11719

My father in law had a brand new 73 sedan de ville. I used to drive is quite a bit. from Long Island to Dutchess Co.  New York. Sorry Greg running Super Shell or Sunoco 260  the car got 9-10 mpg around town, and @13 mpg on the highway. With ethanol lose @ 1mpg.  Ron

Rob Troxel

My 68 Eldorado's 472 squeezes out 13-15 mpg on the road @65 mph. The car has 66,000 miles on it and the Quadrajet has been rebuilt recently.  I still struggle to get a smooth low rpm idle but it runs very well on the road.  I had the intake manifold gasket replaced and that seemed to help immensely.  I also replaced all the vacuum hoses and the headlight vacuum reservoir.  Collectively that brought the old mpg up from 10-11 mpg on the road I was getting!

TJ Hopland

Rob makes a good point about the condition and tune.   His must have been running lean which does not make good power so he had to 'give it more gas' for any given situation which in the long run used more gas.   I think the last dozen or so older cars I have worked on the vacuum advance was not working.  Many have issues with the carb base gaskets. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason