My '68 Fleetwood would get 14-15 on the highway back in the 70's and 80's when the father-in-law took it on road trips. Of course that was in the pre ethanol days... The car has been parked for the last 2 1/2 years (has maybe 200 miles in the last 15 years) and the gas just gets old. Probably be lucky to brake 10 mpg with out a carb rebuild and a tune up. That 3.00 to 1 posi rear end makes the big 472 seem like it is just idling along at 60 mph.

 

You have to remove the base mount plate from the carb. There is a thread here, somewhere, that illustrates the wells from this position, and an illustration of the epoxy application. If you remove the base mount plate, the caps to the wells will be evident. Rebop is a rebuild.

 

alright, thank you deville33, I'll do that and check it out. The funny thing is that there's no vapor after driving it but I notice that the Air cleaner gets a little wet with gas every now and then. I can smell it after driving it but I can't see it. Thank you again for the input!

 

This thread is depressing. I get about 15 in my 96 and I drive mainly highway...

 

Are you heavy on the gas? I don't understand the variation among cars on this thread

 

I'm having the carburetor rebuilt as for right now. I also notice an increase on idle speed, it was like the engine was missing and surging every 4 seconds. I believe it was like that for a while, but not as hard. I disconnected the vacuum line on the Idle Stepper Motor and the problem went away, I'm replacing that part too. I'll get back to you guys when I get the caddy on the road and measure the gas mileage again. Thanks!!

 

Ok, there may be another reason you are smelling fuel. These systems use a fuel recovery system wherein the excess fuel and the fuel vapors from the carb return to the fuel tank. On the top of the carb is a port for the fuel vapor to be directed to the charcoal canister and then back to the system. The draw to the canister is created by vacuum. Check the vacuum line to the canister to ensure you have vacuum and at the port to ensure there is draw to the canister. It could be the charcoal is clogged and there is no draw. When you shut the engine off, there should be some vacuum in the canister to draw off the fuel vapors that are present on shut down. The moisture you see could be the result of the fuel falling out of this vapor and not drawn to the canister.

 

alright Deville33, I might have found the problem with the carb and the smell of gas. The bottom part of the carb was pretty much falling apart, the screws were all loose, and somebody had put some kind of silicone around the gaskets!!! hahaha Took care of that. Now I don't smell the gas anymore, funny how we can break our heads trying to find a problem that is not there or it's something we never imagine. Thank s a lot for all the replies and for hanging with me here and helping me. I really appreciate it! Still not getting over 15 mpg highway though hahahaha

 

One other possibility could be that there is a leak in the fuel return line. The fuel return line runs from the fuel pump back to the fuel tank. The line runs down the passenger side frame rail. Check around the clips that secure the line to the frame. A leak on this line will cause deficient fuel to the carb.

 

Finally, I got the caddy to get 17.5 mpg at 60 mph on the highway, but I had to use premium gas lol!

 

I don't think the premium fuel would make a difference. Perhaps you had a tailwind that day.

I believe the only way to improve mileage by switching fuel is to go with Ethanol-free.
Octane shouldn't make a difference.

 

That would make zero difference.

 

Calculating the cost per mile I found out that it costs me 11 cents per mile to run on regular gas doing 15 mpg and 12 cents to run on premium doing 17 mpg. Or as I say, Same difference hahahaha

 

1969 Cadillac Fleetwood Brougham (472 engine)... 9-13 mpg

1976 Cadillac Fleetwood Seventy-Five Limousine (500 engine)... 9-12.5 mpg

 

My 87 Brougham gets me 13 ish in the city and 18 ish on pure highway.

It was as low as 9 when 6 people were in my car and a trunk full of presents. (A Wedding)

I too use premium fuel, no real reason besides its a Cadillac

 

My 86 FWB 307 OLDS gets me 14-15mpg avg w/50-60% city driving.

Used it one week 5 years ago purely for commuting to work and averaged 20mpg!, yes 20 on back roads at 55 mph with no traffic and minimal street lights, and just myself in the car.

 

That's what I was looking for. So in order to get 20 mpg we have to drive them at 55 mph on the highway... Impossible around large cities.

 

In August we drove the '96 Fleetwood Brougham from central Texas to the Sagenuay region of Quebec and back, 5200 miles all told. Mileage averaged around 25 mpg for the trip. Best mileage was going up when we had an additional couple hundred pounds of paper materials in the trunk and once obtained 26.2 mpg for one tank-full from Ohio into New York. The trip was a lot of interstate driving from gas stop to gas stop with little town driving. Maximum speed limits were adhered to whether it be the 75 mph in Texas to the 100 kmph (62 mph) in Canada.

 

For my 1995 Fleetwood, the best fuel economy on a few-hour highway drive was 23.1 MPG. It's usually 20-21 MPG. Around a town/city with lots of traffic lights, it can drop to 15.5 MPG.

 

Sitting in traffic every day going to and from work the '86 FWB gets nearly 14 MPG, 13.8 ish if I remember right, but if its a long highway trip the big 'ol boat can get a stunning 23 mpg...In a nearly 4000 pound 30 year old car....Explain that one to me hahaha