: Fuel economy on LT1 Fleetwoods...nightmare?

05-12-06, 10:07 AM
Hi guys,
something wrong with my ´96 Fleetwood Brougham: The fuel consumption is abnormally high - 15 MPG on higway. I bought this car last year with only 70k miles, after the car was 1 year not driven, but before excellent maintained.
I replaced the fuel pump (jamming) and adjusted throttle cables.
Last year, the milleage was approx 19,5 (50:50 highway/city). So I replaced the spark plugs & wires, fuel filter, air filter, oil (TropArtic 5W30 - every 3.000 miles), new tires (Yokohama Avid Touring 235/70/15), trans oil seems qood, chasiss lubricated...
The engine runs quiet, acceleration is excellent, no other problems (OK, the resistor pellet in the ignition key was broken).

BUT THE FUEL ECONOMY ??? 15 MPG on highway ???

Before this car, I drove a 94 Fleetwood, with 100k miles, and during the same trip was the mileage aprrox 24,5 MPG (the same tires, oil, condition, speed: 75 - 85 mph, the same route, 2 adult persons + 1 child...)

Even a ´95 Fleetwood Limousine 70 inches stretch make approx 19 - 20 MPG on higway, well, where´s the problem?

I have read some threads here, some guys have the same troubles, but I find not the solution - someone get it?

10 years before, I drove a ´88 Brougham 307 cui Olds engine, with the same initial problem - 13 mpg, but I found the reason soon - just one burned fuse under the dash was to replace, and my mileage increased to 24 MPG exact (highway)...

Someone knows a similar gimmick for the LT1 Fleetwood?

05-12-06, 01:21 PM
Hmmm... When I had my Roadmaster, I'd average 12/13 around town and I got 18 on the highway at about 70-75 MPH once.

05-12-06, 01:34 PM
I dunno, my mileage made the same drop, but I have a 3.42 rear end that got dropped in. Might it be the higher ethanol content?

05-12-06, 01:39 PM
The only thing that occurs to me given all you've already done is that perhaps the transmission isn't shifting into OD. If it's staying stuck in 3rd, that could explain low mileage despite all the obvious fixes you've done. Good luck.


05-12-06, 01:45 PM
The only thing that occurs to me given all you've already done is that perhaps the transmission isn't shifting into OD. If it's staying stuck in 3rd, that could explain low mileage despite all the obvious fixes you've done. Good luck.

Good point!

Also, how old are oxygen sensors? If still original, then they are overdue! Buy two new AC DELCO brand and should see an increase.

No check engine lights, I'm assuming?

I get 14-15 MPG combined driving (lots of city) and my car is modified, so I expect that poor mileage (heavy foot, as well). Never been on a road trip but should be above twenty, myself....

05-12-06, 02:42 PM
Oxygen sensors are original - now I will them replace soon.
No check engine light, right.
Transmission is shifting in OD, when I shift from "D" to "3" while driving, I can feel the transmission shifting, RPM goes up, the engine is slowing down the car, so I think the transmission is OK.

Night Wolf
05-12-06, 02:53 PM
My gosh, ya'll with the LT1 are getting what my 425 got.... 27 years old, 75cubic inches more, carburated, 3spd, no overdrive transmission

05-12-06, 03:39 PM
I am getting 15.5 in the city and 24-26 highway after the Borla :)

05-12-06, 04:54 PM
Sick isn't it!! My Olds Delta 88 350 Olds Rocket with 2.73's and a THM375 (THM400 minus 1 clutch pack) got 12.5-13.1 consistently driving for Domino's Pizza. My Turbo T/A with a 301 Turbo V8 got 14.5 on the same type driving. My 94 Fleetwood with the LT1 and factory 3.42's is more on par with 17-19 mpg city/highway and 21 highway only. I get 14.5-16.7 with E85 fuel.

I think it still takes a certain amount of power to get a certain amount of weight down the road. Gas mileage isn't tied so close to CID as people tend to think. It has an effect, but isn't the key or rosetta stone of mpg....

My gosh, ya'll with the LT1 are getting what my 425 got.... 27 years old, 125cubic inches more, carburated, 3spd, no overdrive transmission

Night Wolf
05-12-06, 05:46 PM
Nope, and in fact, a large displacement enigne dosn't have to work as hard to move a car as a small engine does... better mileage, and less wear.

Kinda like... how when you try to get a 4banger to make power output similar to a large V8... look what happens to the gas mileage..... look at a WRX STi or an EVO... turbo 4banger, really fast... getting about 16-22mpg! A Corvette get better gas mileage then them!

My Oldsmobile got 22mpg town, AC running, heavy foot... and 32mpg highway. That was a 3.8L V6 in a mid/full size car with nearly all the luxury features of a Cadillac.... my friends Civic, bare bones would get 30mpg town and 38mpg highway. Sorry, I'll be glad to give up 8/6 mpg if it means driving a much better car.

Public perception is a very funny thing.

05-12-06, 05:51 PM
Double the CID doens't mean 1/2 the mpg, as a matter of fact my 455 T/A with 3.08's and NO OD got 16 mpg highway. Spinning 3000 rpm at 70 mph with the stock non lockup converter. 16. So, if that little 4 popper has an engine 1/4" the size with a lighter body and less wind drag, why doesn't it get 60 mpg??? Or even 45??

I have always wanted to build a 455 Pontiac designed only for max mpg. Just to see what it would do. 301 heads ported heavily, OD, design cruise rpm around 1400 or 1200. TCC with tall gears. Wonder what it would be???

If a 5000 lb barge 1976 98 get 18 mpg hwy, why I can't do better with a lighter car and OD?

Night Wolf
05-13-06, 02:24 AM
I'd use an Olds 455.

The Cadillac V8-6-4 system is a direct bolt on for the Rocket..... cut the beast down to 4 cylinders and THEN do all you mentioned...

may very well be getting upper 20's with a big block in a barge....

05-13-06, 11:27 AM
Why is it that a 4x2 1/2 ton, V8 pickup truck will get such horrible highway mileage compared to a LT1 Brougham or even a 350 TBI Brougham? For example a 1995 C1500 Pickup with the 350 and a five speed manual is rated at 14 city/ 18 highway. The 1992 350 TBI powered Brougham is rated at 16/25 with the exact same motor as the C1500 AND another 400-450 lbs of weight. The Brougham also has a .49 coefficent of drag. Hardly an aerodynamic car. The '95 LT1 Brougham is rated at 17/25, and thats another ~200 lbs on top of the '92 Brougham, but it has a .36 coefficent of drag.

Is the 2wd truck's significant mileage drop due only to the different gearing?

05-13-06, 12:56 PM
I suspect aero has a fair amount of effect on that, no one has ever rated a truck for aero.... But they are in the .6 to .65 range.

And setup for more power mode most of the time.

We are picking up a K2500 Suburban, 350 with 4.10's. 4.10's are my first choice, but the K2500 is. So we will see what the same aero and shorter gears garners us. 2500-2700 rpm cruise rpm instead of 2250 with the 3.73's... Same engine except for HD emissions instead of light duty.

Compare a 91 Deville. 4.9L V8, 200hp, 3600 lbs, decent aero, and yet it is 16/25 mpg.

BUT, reality check also shows that my 94 LT1 factory 3.42's gets 17-19 best on my drive run, and my 91 SDV got 22-24 on the same run. So it does show the weakness of EPA ratings. My LT1 is rated 17/23.

05-13-06, 02:10 PM
Hmmmm.... Interesting.

I was riding in my friend's 02 F-150 Lariat Crewcab FX4 Edition last week, and I remember looking at her tachometer, and at 60MPH she was at 2000 RPM. At 60 MPH, I'm at 1580-1610 RPM. So that obviously plays a role, but it's probably not as big of a role as the huge difference in the coefficent of drag and the fact her truck is like 1300 lbs heavier than mine.

Another note, I was talking to Brian about the mileage his '90 Brougham 350 gets on the freeway, and he says its around 22MPG at 60MPH, and around 15 at 75 MPH and I was stunned! But then again his coefficent of drag is .49 and mine is around .38 or so. BTW, I got 23.4 MPG at 75MPH on a 160 mile trip, and 21MPG at 80 MPH.

05-13-06, 07:05 PM
I am trying to make some new theories on fuel economy on my Cad running with my datalogger.

Looking at MAF airflow, rpm, vehicle speed and vacuum (MAP pressure), I see is from 75 mph to 71 mph, I see over 10 gm/sec of airflow drop. But from 71 to 65 I only see around a 5 or so gm/sec drop. Vacuum didn't really change, rpm changed (obviously). So I think to properly monitor mpg, you really need to keep much closer tabs on airflow. Less airflow HAS to be less fuel, as there is a fairly strict ratio of 14.6:1 air/fuel ratio. So running at 71 mph might be more efficient for me to run at than 65 or 75. Maybe. Waay to early to make that assumption. But I think airflow is key to monitor.