: What kind of gas mileage could I expect out of a LT1 Fleetwood Brougham?

12-31-06, 12:03 AM
One of the cars I'm considering purchasing next is a LT1 Brougham. I wonder what kind of mileage it will get? My LT1 Roadmaster consistently got around 12-13 city and 17-18 highway when I took it to North Dakota in the summer of '04. I took it on a highway trip around the cities a few months after that and got 22 MPG. Either way, it sucked. I think something was mechanically wrong wiht that Roadie, because my Grandpa would get 15-16 city in his TBI 350 Roadmaster, and consistently 25-26 on a trip out to Vegas in '96 or so. My uncle had a '96 Impala SS and he said his MPG NEVER dropped below 21, but all of his driving is highway, where mine is stop and go.

I want a car that will get atleast 17 city and 26 highway consitently, is a LT1 Brougham capable of that? They're rated 16/25 so I think it's plausable, if I keep my foot out of it. Otherwise, the L67 cars I like are all rated 18/28 or so, they'll all average 20-22 MPG in the sort of driving I do, and I've heard a range of 26-31 MPG on straight highway. And the Mark VIII's will get mileage in between the LT1 and L67...usually 16-17 city and 27-28 highway...they're rated at either 17/26 or 18/26..

12-31-06, 12:13 AM
I get 18-19 city/highway, (same route daily) and as high as 21-22 highway. Now, remember, mine is a factory towing package car too, with 3.42's and 210K miles.

If anyone is getting worse than that, they likely have something wrong.... I think the EPA city mileage is pretty accurate, EPA highway for me is 23, and mine never has seen it. I think it might with a fresh timing chain, as mine is likely pretty slack now.

12-31-06, 12:18 AM
Ok, so 21-22 highway with the V4P Package. How about with the stock 2.93 or 2.56 gears? I'm gonna take a guess and say 25 highway for the 2.93 Brougham and 26 for the 2.56 Fleetwood.

12-31-06, 12:52 AM
Probably reasonable to get.

My old 91 SDV with the 4.9L is rated 16/25 still got better mpg than my FW V4P rated at 17/23. I got 29 on the highway with the 4.9L.... And same driving I am doing now, 22-24 daily where the LT1 is getting me 18-19. But the LT1 is much much more fun!!!

12-31-06, 12:56 AM
29 mpg? How? The best I ever got was 28.3 and that was only once. I had the wind to my back, and the temp was about 65-70, tire pressures were at 32 for rears and 35 for fronts. And I meticulusly kept the speed from 70-72 MPH at all times and never opened 'er up at all. Drove like a granny! On the return trip the next day I got like 23 mpg.

12-31-06, 03:45 AM
When I was looking into a Roadmaster I was told 16/24-26 is what to expect.

As far as Mark VIII vs L67, my Mark gets 1-2 more MPG (city and highway) than my friends Park Ave Ultra. This was measured when we drove his car back from buying it out in Phoenix.

12-31-06, 08:33 AM
Got my 1995 FWB w/ the LT1 in June of this year.
Car had ~ 91k miles on the odometer.
Towing the PopUp RV we got about 16 mpg.
In town with a lot of idling in heavy traffic with A/C we got ~ 12
Most recent pure road trip with considerable idling to warm up (chilly morning) netted 23+ mpg.:)
driving 45 to 85 mph simply to stay with traffic.:thumbsup:
CAVEAT:The car has given better and better fuel mileage since I have started feeding it fuel injector cleaner every other tankful. (I use Mobil or Sunoco fuels, 89 octane almost exclusively and run Mobil1, 10W-30 Oil in the motor.)
My next stop will be to dive under the hood and go after the Mass AirFlow Sensor and clean it per the posts on this BB. (Either with a spell of warm weather or find a warm garage)

12-31-06, 10:29 AM
Stock you will get decent gas mileage with the 2.93.

Once you start modifying, it goes downhill. I get 13-14 in the city and maybe 20 on the highway on a good day.

12-31-06, 12:00 PM
In my 1994 Cadillac Full Line Brochure, the Fleetwood is rated at 17/25, any of the Northstar models are 16/25. Why would a bigger engine pushing a heavier car get better mileage in the city? Is it because of the 16V v. 32V setup? DOHC v. OHV?

My 4.9 deVille gets about 13-15 city, 23-25 highway on average. I've seen as low as 11 city, with lots of warm up time in the dead of winter and all short drives so the engine can't get to operating temp. And as high as 16 city for a couple of weeks back when I first got it. On highway, I've seen as low as 21 MPG at 75-80 mph in 30* temps and as high as 28 at 70-72 in 60* temps. Usually it's 23-25 though.

I just think it's odd that a car that weighs 800 lbs more than my deVille, with a motor 50 cid bigger will get better mileage in town, and similar mileage on the highway. Is the tranny suited that well to the motor? The 4.9 is one of those motors where a small increase in speed or driving habits will change the MPG drastically. I assume the LT1 isn't?

12-31-06, 12:41 PM
I'm still looking for the smoking gun for mpg. It ISN'T weight, but it is a factor, it ISN'T engine size, but that does make some difference, it ISN'T aerodynamics, but it is a factor.

My 4.9L Deville NEVER got lower than 16 except when I had the bad injector. I don't idle much at all, I start up, and go. Rarely ever sat idling. It was fairly sensitive to dropping mpg with cooler temps and really was affected by headwinds. I commonly saw 27-29 highway. My mom and dad's best was 33 when they had it, but that was only 2 times and never saw it again, but it did get in the 29-30 with them often. I think that if I could get in and tweak the VE tables in the 4.9L more mpg could be squeaked out of it. It really is only tuned at the part throttle cruise range, heavier throttle (not WOT) is runs lean and light throttle it runs rich. Details, brass post HEI cap, good wires, good plugs, AC Delco air filter, 10W30 synthetic were all I ran. I had around 13 degrees timing dialed in and I probably was running 93 octane non ethanol gas. I didn't have much winds at all, and was driving at night with little traffic. I featherfooted all my accelerations as much as I could and drove by the mpg montor to watch it carefully. Coasting really helped the 4.9L improve mileage.

My LT1 hasn't responded well to much for improving mpg. PCM tuning, second gear starting, entering OD at 29 mph, TCC in 2nd, TCC earlier, etc. Nothing. Only thing that helped the LT1 was dropping timing down at cruise seemed to help some and has got my mpg more consistent, that and 5W30 Mobil 1. air pressure hasn't even made much difference, some, but not drastic.

Idling is #1 mpg killer, next is heavy throttle starts.
Well tuned engine is #1 improver, and coasting as long as you can to a stop seems to help quite a bit. With 3.42's you aren't coasting like you do with 2.56's!!

The new Suburbans with the 5.3L with AFM are rated at 21 mpg. Not bad at all for a heavy Sub. And the new Escalade is as aero as our 93-96 Fleetwoods! With 400+ hp and 413 lb/ft torque to boot. And people wonder why I am looking at a 5.3L 2007 engine swap..... Time to rethink old school vs new school....

I have watched my MAF and my MAP sensor and the injector pulse widths seem to drop some when you keep it in taller gears as long as possible compared to running higher rpms and light loads. Heavier MAP loads and low rpms seem to use less fuel.

As for synthetic, well, it is rumored to seal rings better. But the key is the thinner cold start viscosity. And if you can have thinner hot viscosity, that can help too. I don't think running 5W20 is safe, but it should help mpg.

So, a stackup of these things will help mpg, engine tune and driving style I think are the biggest factors we can play with...

12-31-06, 12:57 PM
N0DIH, you've got this down to a T! :thumbsup:

12-31-06, 04:20 PM
I am sure with a T56 swap I could get well into the high 20s on the highway.. :stirpot:

12-31-06, 05:22 PM
Yeah, I am sick that way. I have 120 miles a day to ponder it, and that started in Nov 2002.....

T56 swap is something I would love to do! Drop in some 4.10's or 4.56's to boot.... even 4.56's makes it 2.28's in 6th!!! Sound familliar anyone?

Anyone got any thoughts share em!

N0DIH, you've got this down to a T! :thumbsup:

12-31-06, 07:09 PM
Well my '95 approaching 123k miles is bone stock--air intake and all--and isn't that great. The highest I think I've ever gotten, at least when I actually calculated it was 21 mpg and maybe just a smidge more other times when I didn't check. Most of the time, though, when it's not highway cruising it's pretty much faithful to the 16.5-17.5 range exactly. Not far off the city rating at all, so not too bad.

Still wish it was better, especially on the highway, but it's not terrible. Both L05 cars we have sip a bit slower, though, regardless of driving style--just a small point worth mentioning. Wouldn't give up the LT1 power and smoothness for much, though.

01-01-07, 04:31 PM
LT1 Broughams specifically may be pulling a tad more weight since they will have some extra gadgets like seat warmers/motors, vinyl top, etc. So weight can be a factor in resulting MPG.

But maintenance is key. And since these cars are all used you must consider prior maintenance records and mileage. This is usually a crapshoot if you don't know the previous owner.

Friction is another key factor. Wider tires increase friction (rolling resistance) with the road and therefore may translate into lower mileage. SO you may want to stick on some 225/75-15 tires instead of the stock 235/70-15s.
Proper oil weight can help as well to free up some power and reduce frictional losses. Thin oil is not always best for longevity, however, even though it may increase MPG.

Everything is always a compromise with cars. If you modify a car, most often MPG will go down. But if you keep specific goals in mind when choosing mods you can achieve those without affecting MPG negatively; in fact, MPG can be increased.
Opening up the air intake and the exhaust system are a great first start. If GM rated the car at 17/25 in stock trim, imagine what the results can be if you let the car breath better.

Also getting the stock PCM reprogrammed can help MPG since GM tends to richen the air/fuel mix (especially at high RPMs). If you lean this out then you are maximizing the car's potential.

Even rear end gears (bigger, numerically) can improve MPG contrary to popular belief. This depends on your driving routine, of course. A lot of in-town driving (many stoplights) can be benefitted by bigger gears since they allow you to get off the line quicker (less throttle is needed and therfore less gas). On the highway bigger gears will lower MPG, so it DOES depend on how you drive the car.

To sum up, I would say that gas mileage is NOT a strongsuit of these cars. You don't buy one of the all-time biggest modern day Caddies expecting good mileage, that is just counter-intuitive.
The LT1 does allow fair mileage and good power on 87 octane (cheaper gas), however, which is a step above its predacessors.

01-01-07, 07:04 PM
I wasn't expecting anything extremely good for mileage from a 4500 lb car, but considering the size and sheer mass that this baby has, 17/25 surely ain't anything to scoff at.

I never considered that doing a CAI and making the exhaust flow better would increase mileage, but it makes sense. Trouble is staying off the pedal with all the new additional power.

01-01-07, 08:33 PM
Considering a 1996 Toyota Avalon gets 20 city and 29 highway and a combined 23. The 96 Fleetwood is 17/26 (2.56's) and a combined 20 mpg.

I'll gladly pay 3 mpg loss to have a Fleetwood over a fat Camry....

01-01-07, 09:51 PM
Once more having a look at the goodies (and drooling over some of them) Elias has put on his FWB, I fail to see how his mileage isn't better than 20 on the road unless . . . his right foot is heavier than his left.
Ya think? :cool2:

01-01-07, 11:58 PM
I have that problem sometimes.... I think it is lead poisoning of the right foot...

01-02-07, 09:04 AM
Haha, well there are some other factors that come into play as well....

I have an exhaust leak at my H-pipe and a bad O2 sensor code that I keep clearing. Just haven't had time to fix them or else I might see better gas mileage.

That plus the fact my right foot pounds on the stock tranny so a CPT might replace it :crybaby:

01-03-07, 01:31 PM
I never considered that doing a CAI and making the exhaust flow better would increase mileage, but it makes sense. Trouble is staying off the pedal with all the new additional power.

You hit the nail on the head there! :bouncy:

01-03-07, 03:19 PM
I have that problem sometimes.... I think it is lead poisoning of the right foot...

I have that problem too! :yup:

I have been averaging 22mpg on the highway, but since I live in South Dakota, I have been idling a lot lately too in this cold, icy, crappy weather (nice snow storm on December 30th through the first of January; 1/2 inch of ice under 6-8 inches of snow. This next tank doesn't look good. I calculated it out and I think I might get 18 at best in the next tank. All I do is drive 35 miles to work and 35 miles back, every day, averaging 72 MPH. Sometimes a little faster because of that darn lead poisoning of the right foot. One thing I am surprised about, not to get off the subject, is how well my car handles in the snow and ice. I was shocked since my previous vehicles, two 4x4 pickups and an SUV (mistake, Jeep Grand Cherokee with loads of electrical problems), handled terrible in the snow and ice whereas my car handles pretty decent all things considered.

Oh, I forgot to add that my 95 FWB has 154k on the ODO and the only upgrade is freer flowing exhaust. Not the best either. Just like my harley, nice sounding, but not an upgrade by any means. Straight pipes with the resonators left on. Kind of sounds like a '53 Merc with lake pipes. :)


01-24-07, 12:32 PM
on both 95 (57k miles) and 96 (7k miles) mileage has been consistently close to 17 / 25 FL so no cold weather

01-24-07, 07:31 PM
I got 13-14 MPG city on my 138k LT1 Roadmaster and I drove it hard, I drive my 4.9L deVille normally and still get 13-14 city...I wonder, if I drove the Roadie sanely, I'd probably get 15-16 city.

01-27-07, 11:06 AM
I get 10-11 MPG in the city and 20 on the highway.

01-27-07, 11:21 AM
Either you drive very aggressively, or something's wrong. No LT1 should get 10-11 city.

01-27-07, 04:34 PM
Few things combine to improve mileage AND available power the way letting the engine breathe better do. A better air filter -Try K&N, they seem to work the best; and a freer-flowing exhaust will help 100% of the time. Do not just willy-nilly pull out the regular muffler; some engines require a degree of backpressure or they lose low-end torque while gaining high-end horsepower. This is an issue with the Toyota 4AGE motor I know for sure, I don't know if the LT1 has the issue but performance info on it is definitely in no short supply. A buddy of mine did both of these to his car and saw a total 7 MPG increase, 2 from the filter and 5 from the exhaust.

01-27-07, 06:13 PM
No engine NEEDS backpressure, but some emissions devices, say a positive backpressure EGR valve, would be out of calibration with backpressure changed.

Now, as we all know from the old days of carbs, changing the backpressure with headers, changed the fuel mixture. With EFI, this hoses up the VE tables significantly. The MAF can compensate some, but only within the limits of the PCM. Which is CAN adjust pretty far, but still is never ideal when it strays from BLM 128. As WOT fueling then becomes often richer than it should be as a safety margin.

Also not to forget, the cat converter overtemp can trigger extra fuel more times to compensate for a consistently leaner mixture. As cat converter overtemp is a calculation, not an actual measurement, if it feels the mix is leaner, it will put in a compensation of richer mixture to keep the converter cool, which is often 12:1 give or take. Which definately screwing up mpg.

Over a stock muffler and stock good air filter, I am dubious about 7 mpg. If you get that much improvement, then there was something majorly wrong in the first place. You won't be seeing that over stock EPA mpg estimates at all. For sure, else the factory would be doing those improvements!

01-27-07, 07:30 PM
NODIH you are correct that no engine "needs" backpressure but I essentially meant what you said; changing the backpressure can change the way certain components work and will in some cases negatively affect performance.

First of all my buddy is a fuel economy freak; he drives in overdrive all the time and does other stuff like shutting the car off and coasting into driveways, etc. Secondly no sane auto manufacturer would equip a car with the muffler that was fitted to this car; it drones like a swarm of bees from hell on the highway and you cannot carry on a conversation; the air filter alleviated this somewhat, but it is still quite loud under almost any load. It is about as loud, under normal operation, as my Corvette is under WOT.
And the air filter was probably clogged to shit to begin with as the car belonged to a man who never took care of it at all (he knew nothing of cars, really, the car's a Mazda wagon, that he knew nothing of cars should be emminently clear from that fact alone as there are only three ways to leave a mazda dealership: In a Miata, in an RX-8, or in the car you came there in).