Gas Mileage 429 Engine?

[richardbergquist]

What mileage are y’all getting on your 429s depending on where and how you drive? I’ve. Been getting 8-8,8 MPG (60 % City) myself in 67 Calais.

[Julien Abrahams]

I really have never paid attention to it, but i think 8 mpg is on the high side. Or yoy are really stepping on it. I thinkn12 mpg is obtainable.

[Gary McKinney]

1966 Eldorado, original drivetrain, usually 14-15 MPG on the freeway.  I don't know what it would be in the city as I've only ever checked mileage on long trips. 

[bcroe]

If interested in mileage, I'd start by making sure the odometer is
right on, most of mine required trans gear recalibration.  Check the
rear axle ratio.  A TH400 trans allows a stepping down from a 3s
axle ratio to a more economical 2s.  That might appear to slow the
car down a lot, but an electronic control on your switch pitch trans
would get most of that back.  Changing the axle also requires changing
the speedo gears. 

All this assumes the engine is in A1 condition, new timing chain set, etc.
Bruce Roe

["Cadillac Kid" Greg Surfas 15364]

Richard,
You have a Quadrajet on that motor and they were intended to give decent mileage when driven sedately and good performance when you want power.  A "good" number for combination city/highway driving would be just about 10 MPG IF EVERYTHING IS OPERATING CORRECTLY. Slightly out of tune or with a bad vacuum advance mileage will drop like a rock to 7-8.
Greg Surfas

[Maynard Krebs]

I got about 11.5 or 12 in my '66 hardtop Sedan DeVille.   

A caveat, however:  it had the taller rear axle ratio [lower numerically] because it didn't have A/C:  a 2.94 versus the 3.21 in A/C cars.

[richardbergquist]

Yes it is true that I have not been driving all to careful, also something still isn’t right as the car hesitates to accept a wot, it will only accept a wot if I gradually come to wot. Maybe getting that fixed will improve the mileage.

I asked on the Swedish Cadillac Forum the same question and many there seem to believe that they’re getting 18-24 MPG in their 429. One guy said 18 MPG when pulling his camper. Most said they were getting between 12-16 though. I think Swedes like to thInk they’re getting better mileage then they’re actually getting as gas is about $7 a gallon.

Greg I could absolutely live with 10 MPG. I mostly drive around Austin. Im getting about 13 MPG in my Suburban in Austin.

The 2.94 should be a little more economical I would guess. Do you have AC in it now?

Julien, how many km does your Kadett have?

[The Tassie Devil(le)]

When one has to pay the European/English or Australian prices for Petrol, one is really careful about putting the pedal to the metal, sorry, carpet.

Bruce.

[Julien Abrahams]

Yes, I would notmind living in the Dtates with regard to fuel prices. Here inthe Netherlands it is aleo equivalent to about ú dollars per gallon.

[savemy67]

Hello Richard,

Your post included a screenshot of your phone application showing 8.8 MPG as your best mileage to date.  I don't know if the screenshot shows your first use of that application, but if 8.8 MPG is the best to date, you can do a few things to improve your car's mileage.

Your post also indicated your carburetor "bogs' when pushing the accelerator to WOT.  Initially, I would suggest you consider checking that the carburetor is adjusted correctly.  The secondary air valves on Q-jets utilize spring tension for correct operation.  The spring tension is adjustable (see the shop manual).  Incorrect adjustment of the Q-jet secondary air valve spring tension was so prevalent that the carburetor was maligned with the appellation "quadra-bog".

If adjusting the carburetor provides no or little improvement, you may want to consider a professional rebuild by a competent rebuilder.  A good rebuilder will want to know the number on your carburetor, so be sure your carb's number indicates that the carb is for a '67 Calais.

I think the next most important things to check are ignition timing/advance, tire pressure, and driving habits.  Make sure your timing is adjusted properly and that the mechanical and vacuum advance mechanisms are working correctly (see the shop manual).

The correct tire pressure for your car is the one that minimizes tire wear and maximizes handling/braking performance while enhancing ride comfort - inherently, somewhat of a compromise.  Modern (since 1979) tires are sold with a UTQG (Uniform Tire Quality Grade) rating.  If you familiarize yourself with the UTQG rating, you may be better able to establish the optimal air pressure for your tires.

Driving like grandma or Don Garlits will also affect mileage.  Your driving habits may be unlike grandma's and Don's, but there may be some driving habits you have that might be impinging on your mileage.  If you concentrate on how you drive, with an awareness for things you do that might reduce MPG, then you can begin to make changes in your driving style.  An effective tool to help with this is a vacuum gauge installed in a prominent place on the dash.

I have not driven my '67 Sedan DeVille far enough to determine my MPG.  I am still working on rebuilding all the systems on the car.  When all is done, my expectation is that I will get about 12 MPG.

If any Swedish Cadillac Club member is getting 24 MPG from his carbureted 429 equipped Cadillac, I will fly to Stockholm and buy him/her a steak dinner .  Good luck with your car.

Respectfully submitted,

Christopher Winter

[bcroe]

Get the engine right if it is not already.

I don't know how your driving goes, but a car does 0 miles per gallon
sitting at a red light, which does not average out well.  The amount of
this can vary so much, and it isn't where I pile on the miles, so I don't
pay much attention to city mileage. 

What I rarely hear, is that every time you use the brake, you are throwing
away gasoline.  So actively driving to MINIMIZE brake use will help. 

Often said is people in a hurry to get up to speed are wasting gas.  Well
I say actually those people tend to be the ones using the brake all the
time.  Getting a car from rest to 60 mph requires a specific amount of
energy, whether it is done in 10 seconds or 25.  Slow acceleration is
not the key to economy.  Note how rockets are launched. 

That 429 car is only 10% heavier than my 77 Olds, and has only 5%
more displacement.  We have identical switch pitch TH400 transmissions. 
But my Olds gets very consistent 19 mpg cruising 70 mph in favorable
conditions (not mid winter, not climbing mountains).  That, even with
the disadvantage of a low compression ratio.  The factor that makes it
possible is a 2.41:1 axle ratio, I think you have a 3.21:1.  My odometer
is carefully calibrated, we can't compare unless that is done. 

My 2.41 can seem to be sluggish getting started, but I pretty much
fixed that with an electronic control to optimize the operation of the
switch pitch transmission.  Bruce Roe

[richardbergquist]

Haha I will let the Swedes know they have a steak dinner coming up. I think Greg is probably right that 10 has to be considered alright mileage when mixed city and hwy. 10 is what I’ve been getting lately after having adjusted the screw for the rear ports on the carb that Christopher made we aware that one could do.

I must say that I never realized exactly how thirsty the American 60s cars were. Of course growing up I would hear that they would consume twice as much as our cars, which in Sweden in the 80s would be up to 2 liters per Swedish mile or 20 liters/100 km. 1 liter was considered the norm back then. Now new Volvo’s claim to get 0,38  it I’ve never gotten anything like that driving the new Volvo’s. 1 liter per Swedish mole would be 23,5 mpg and 2 liters would be 11,75 mpg. I didn’t think it could get up into 3 liters or anywhere near that, which is what a new 60 passenger bus in Sweden would be consuming on long hauls. 3 being 7,83 mpg.