: 425 4bbl MPG



ferrisworld
09-08-11, 05:27 PM
I have a Ford Focus, and it gets great mileage, so lets get that out of the way.

Getting good mileage in my Caddy is just a hobby, and something that's been on my mind a lot lately since I always think about these things when I can't afford them (no job right now).

Now, this website claims that the carb version of the 425 could get 20mpg highway according to 1979 EPA estimates: http://www.AboutAutomobile.com/Fuel/1979/Cadillac/DeVille

The revised number being 19mpg. That doesn't seem that bad! But I tend to average about 16, even with mostly highway driving. Even more interesting is that the EFI version gets worse mileage according to that site, although it would make sense that those cars probably had a shorter ratio final drive too. I've done a bit of online research regarding Q-Jet tuning, and a lot of sites say they're one of the best carbs there is, and are capable of getting pretty good mileage. I'd be willing to bet then, that with very careful driving and lots of tuning, that the carbed 425 could get even better than 20 mpg. Anyone ever done it?

77CDV
09-08-11, 10:53 PM
Even in a federal emissions car with an absolutely dialed in Q-jet, the most economical rear axle, totally flat terrain, low rolling resistance tires, zero headwind, and holding it at a steady 55 with the AC off and the windows up (basically, totally unrealistic conditions), you'd have a very tough time cracking 18mpg.

sven914
09-08-11, 11:28 PM
EPA Estimate fuel economy is rarely an accurate measure. Many times the vehicle is never physically tested for economy (especially back then) and if it was, the test did not factor in wind resistance or road-forces. Sometimes, instead of testing the entire fleet of cars, with all available options, the EPA would test one car, made by a company, that had that engine. The tests may not have reflected it, but there are difference between a Coupe de Ville and a Fleetwood Brougham, which may not have been considered.

Bro-Ham
09-09-11, 03:40 PM
These are 15 mpg cars. I think you have too much time on your hands if you're spending your life's precious moments trying to figure out such a nominal fuel economy increase. :) Get a job, do it well, get rich, drive your Cadillac and be happy. :) :) :)

Bro-Ham
09-09-11, 03:44 PM
The tests may not have reflected it, but there are difference between a Coupe de Ville and a Fleetwood Brougham, which may not have been considered.

Sven, sometimes I wonder how you come up with so much misinformation and heresay. I have the original 1979 Cadillac EPA mileage estimate sheet; mileage for the deVille and Fleetwood Brougham are the SAME. The Limousine model is listed and mileage is lower however. :)

Fleetwood70
09-09-11, 11:04 PM
Sven, sometimes I wonder how you come up with so much misinformation and heresay. I have the original 1979 Cadillac EPA mileage estimate sheet; mileage for the deVille and Fleetwood Brougham are the SAME. The Limousine model is listed and mileage is lower however. :)
That's what he's talking about, Deville and Fleetwood should have different mileage, EPA says it's the same. Did I get that right sven?

sven914
09-10-11, 12:56 AM
^Exactly... They probably tested a low optioned de Ville, which is lighter (and less luxurious) and used the numbers as universal figures for all de Vills and Fleetwood Broughams, equipped with the carburetted 425. The Fleetwood Limo is obviously a different vehicle, so like the Eldorado and Seville, required a separate test and/or adjusted figures when estimating fuel economy.

ferrisworld
09-10-11, 04:17 AM
Haha, I know it's a waste of time, still, it's fun to dream. My Deville is actually pretty low on options. It has the sunroof, 8 track player, and leather. I guess the full vinyl is an option too. The sunroof adds a bit of weight I'm sure. No trunk pull down or release or even factory cruise, and I've yet to see another without it.

Bro-Ham
09-10-11, 10:30 AM
A power astroroof in a 79 Cad was relatively rare and I think it's cool that your car has it! What color is it?

I had a 78 Sedan deVille d'Elegance several years ago, it had all kinds of options - - wire wheel covers, lighted vanity mirror, cruise, power trunk pull down, vinyl top, and the fancy d'Elegance package - - but it didn't have tilt/telescope wheel! My 78 Fleetwood Limousine I had about 10 years ago had tilt/tele but no cruise or power trunk release/pull down.

That was the classic and fun thing about these cars from the pre-option package days: dealers and customers could order the cars exactly the way they wanted them. :) Also, you could get a million different interior colors. The luxury car manufacturer that brings back a myriad of choices for the buyer will win the game since it will be such a novel concept. :)

Bro-Ham
09-10-11, 10:38 AM
^Exactly... They probably tested a low optioned de Ville, which is lighter (and less luxurious) and used the numbers as universal figures for all de Vills and Fleetwood Broughams, equipped with the carburetted 425. The Fleetwood Limo is obviously a different vehicle, so like the Eldorado and Seville, required a separate test and/or adjusted figures when estimating fuel economy.

According to the 1979 Cadillac brochure, here are the weights of the models in question:

Fleetwood Brougham 4250
Coupe deVille 4143
Sedan deVille 4212

Can a 107 pound weight difference make any measurable change in fuel economy? I can't imagine how it would. Also, I don't know if it matters, but a 1979 Fleetwood Brougham used a larger tire size than the deVille; 225/75-15 on Fleetwood Brougham compared to 215/75-15 on deVille.

See the link to the specification page of the 79 Cad brochure here to see more:

http://www.lov2xlr8.no/brochures/cadillac/79cadillac/33.html

:)

sven914
09-10-11, 11:21 AM
People around here strip their cars bare of everything except the driver's seat, just to save a few dollars at the pump. They seem not to realize that though the seats and plastic trim may save 200lbs, their 400lb ass cancels out the savings.

ferrisworld
09-10-11, 01:27 PM
@Bro-Ham, mine is the same color as yours if your avatar is yours. Light yellow paint, top, and interior. I'm thinking of selling it since I don't really have a job right now to take care of it. So far I've gotten two offers. One for $1000, another for an 89 Sable wagon in trade.

Bro-Ham
09-10-11, 04:13 PM
@Bro-Ham, mine is the same color as yours if your avatar is yours. Light yellow paint, top, and interior. I'm thinking of selling it since I don't really have a job right now to take care of it. So far I've gotten two offers. One for $1000, another for an 89 Sable wagon in trade.

Car in avatar is indeed mine. Mine is Colonial Yellow with matching vinyl top although my interior is Burnished Gold d'Elegance cloth, a pretty highly optioned car although I do not have the astroroof.

Unless you're totally broke why not just keep your car if it's only worth $1k?! :)

deVille33
09-14-11, 09:28 AM
I have several '77 coupes and your gas mileage will be dependent on how well you set things up and pay attention. The one I'm presently driving gets around 15.5 mpg and I haven't messed with it that much. My blue '77 gets 16.5 mpg. This car, I ensured that there were no vacuum leaks by replacing any questionable hoses and following the vacumm lines diagram. I checked and lubed the centrifical advance and I had gone through the carb, cleaning out the fuel bowl and setting all the carb settings and replacing the dashpots that had failed. The other blue coupe I had got similar gas mileage.
Bruce Roe has a fuel injected version of this car and through his mechanical wizardry he has closed in on close to 20 mpg.

Bro-Ham
09-14-11, 01:12 PM
I remember reading online about a company called MTS, they build custom Cadillac engines, I think I read that they fit a cam (or some such thing) to a 425 or 500 that enables it to get mileage around 20. Here, I found the website:

http://www.500cid.com/

This is the webpage that talks about the "mileage cam":

http://www.500cid.com/cams.htm

outsider
09-14-11, 01:30 PM
lots of people swear by MTS. I know a few forum members here have engaged them when rebuilding their 500's (or engines in that family)

The Ape Man
10-02-11, 05:14 PM
Highway fuel economy back in the day took 55 MPH national speed limit into account.

Back in 1985 I owned a 77 CDV carb equipped completely factory except cat converter emptied and idle screw access punched out and idle mixture slightly enriched .

Took it on a road trip leading the way for another family member who had an 81 CDV completely factory with working 8/6/4.

The whole trip was highway driving. When refueling I personally made sure the gas was topped all the way to the filler neck on both cars.

The 77 got a best of 19.4 MPG on and the 81 got a little over 20 MPG. Might have been close to 21. I do not remember.

Took my 77 on the same trip into Canada up and down the QEW. Speed limit there was higher and I was able to cruise at 70 MPH. Fuel economy went down to 14 MPG on this stretch.

A properly working EGR valve goes a long way to highway economy on there cars.

When I first bought my present 1980 Fleetwood 2 door in 2004 we took it to Florida from NJ. Ran 70 MPH all the way down. Got over 19 MPG with it. I could not believe it. Best I've ever done at that speed with a Cadillac.

All my Cadillacs get lousy around town mileage : (

ferrisworld
10-06-11, 04:47 PM
So with the MTS #3 cam, they claim the record is 22 mpg in 1976 Sedan Deville with a 500. So why couldn't a 77-79 beat that with the same cam in a 425? Was the 76 fuel injected or carbed?

CBodyFan
10-06-11, 09:44 PM
1976 500 was carbureted with fuel injection optional.

ferrisworld
10-08-11, 04:03 AM
Right, but I was asking if anyone knew which one the deville with the #3 that got 22mpg had.

Bro-Ham
10-08-11, 09:51 AM
Right, but I was asking if anyone knew which one the deville with the #3 that got 22mpg had.

Just e-mail MTS and ask them...

ferrisworld
10-20-11, 10:14 PM
I think what I might do now is swap in a t-56 behind the 425. I know that one guy from Sweden posted about how he put a 700r4 in, and it got worse mileage in OD than in 3rd on the highway, but I still think it would be the way to go. I like a stick, and I'd be willing to bet that in some situations running the engine at 1000 rpms on the highway would help mileage. I'd start there, do a good tune on the carb, and then maybe down the road swap to efi and a megasquirt.