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Old 10-17-2011, 07:59 PM   #1
79broncoowner
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Bronco Info: 1979 bronco 390 4 speed
390 fe gas mileage

I have a 390 in my 79 bronco. I get 8 miles per gallon maybe 9 on a good day but no better. I have been told that it should get 12-13. I have my timing set correct, I have a 600 edlebrock carb with headers and an rv cam. The vacuum advance is hook up and working right. No brakes dragging. I have 33 inch tires with 3.50 gears and my speedometer is correct. Any idea on how to improve the mileage. Also I have tried different grades of gas and it does best on 87 octane.
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Old 10-17-2011, 08:05 PM   #2
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Search around and you will find your answer. Also what trans do you have?
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Old 10-17-2011, 08:36 PM   #3
79broncoowner
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Bronco Info: 1979 bronco 390 4 speed
I have a 4speed. I have searched everywhere. Though maybe someone had a 390 that got better mileage that could help
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Old 10-17-2011, 08:51 PM   #4
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My 400 never got better than 11 so I find it difficult to believe a 390 would be any better.
None of the FE engined trucks I have owned ever got better than 14 and that was a half ton 2wd
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Old 10-17-2011, 08:51 PM   #5
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my 360 wouldnt get much better then that. id still take it over the modifieds that came in the broncos though
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Old 10-17-2011, 08:58 PM   #6
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Quote:
Originally Posted by 79broncoowner View Post
I have a 4speed. I have searched everywhere. Though maybe someone had a 390 that got better mileage that could help
but what about the compresion of that 390.
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Old 10-17-2011, 09:48 PM   #7
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Lot of good info here:

http://fullsizebronco.com/forum/showthread.php?t=114950
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Old 10-18-2011, 01:31 AM   #8
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Quote:
Originally Posted by 79broncoowner View Post
I have a 390 in my 79 bronco.
I get 8 miles per gallon maybe 9 on a good day but no better.
I have been told that it should get 12-13.
Sounds right on the money to me, and in my experience...
that's what you should be getting, easy.

I get >12mpg on my '75 F150
360FE+T18 -original- 2100 carburetor
3.50 gear, 31.5" 235/85-16E's
Comp 252/252 straight up
Stock manifolds, opened-up stock Y-pipe
Straight through 2+1/2" muffler and pipe
...and about 95k on the rebuild too!
Over 13mpg when it was first rebuilt. :)

My son is getting 12 to 13 mpg on a rebuilt 360FE
Unknown cam and miles!
We call it "The Farm Truck" it has a flat bed and really was a farm truck.
It was given to me by a cowboy buddy for scrap and parts. :)
Stock manifolds with dual exhaust.
It pulls stronger than mine is all we know about the engine.
'75 F250 360FE+T18 stock 2100 carburetor
3.73 gear 235/85-16E's

His -was- getting 8 to 9mpg before we rebuilt the 2100 carburetor
using -used- parts I had on hand. :)

It was a blown power valve messing up his mileage I believe..

The trick to good gas mileage is in the carburetor adjustment IME.
You know how to do that? :)

Quote:
timing set correct,
600 edlebrock
headers
rv cam
vacuum advance is hook up and working right.
No brakes dragging.
33 inch tires with 3.50 gears
my speedometer is correct.
it does best on 87 octane.
Since there is for sure something wrong with it, then
-at least- one of those things in that list is a wrong.

Start over on your list of things "that are right"...
1) What's the actual timing?
a) At idle?
b) Full mechanical at what rpm?
c) Mechanical plus vacuum advance at what rpm?

2) Carter AFB? ;)
a) Who adjusted it?

3) "RV cam" covers a lot of ground.
a) Which cam is it -exactly-?
b) Is it timed straight up or retarded?

4) Cool that your speedometer is right. ;)
a) What about the stinkin' odometer tho... Is it right? ;)

5) It's cool to hear it's running good on 87 octane. :)
...and for-sure, it ain't pinging -any-? ;)

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Old 10-18-2011, 07:50 AM   #9
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Bronco Info: 1979 bronco 390 4 speed
No pinging and I had all the timing readings a while back but can remember where I put them. I tuned the carb with a vacuum gauge. I also tried to go with smaller jets but it was running way to lean so I put the stock jets back in the edelbrock. I think at idle the timing is set at 12 degrees. I have also put a new dist. in not long ago. The compression is good also. The engine has about 70000 miles on a professional rebuild. It is bored 60 over also.
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Old 10-18-2011, 09:00 AM   #10
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the 360/390's were gradually phased out in favor of the 429/460's.....the FE's were heavier and not as efficient.they are good motors, but not as good as the 385 series.
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Old 10-18-2011, 09:28 AM   #11
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Assuming a good tune and an engine in good condition, it is probably your driving style. Do not expect 12 mpg unless you are spending the majority of your time driving on the highway.
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Old 10-18-2011, 10:27 AM   #12
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Quote:
Originally Posted by BigWheelz View Post
Assuming a good tune and an engine in good condition, it is probably your driving style. Do not expect 12 mpg unless you are spending the majority of your time driving on the highway.

This is what I'm thinking. You'd top out at 12mpg's if you only drove hwy miles. That aside, you said your speedo was correct...so you swapped the driven gear in the t-case? If not, with the larger tires and stock gear ratio...your speedo/odo aren't correct.
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Old 10-18-2011, 02:53 PM   #13
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I don't even count MPG in my 460; is really even worth it? I just fill her up when she is thirsty. I can't imagine the 390 doing better than 12 unless you were pushing it down the road.
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Old 10-18-2011, 03:10 PM   #14
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Originally Posted by 78Bronko View Post
I don't even count MPG in my 460; is really even worth it?
Yes because changes can be detected with that information,
like a blown power valve in the carburetor for example.

---------------------

OP,
Don't listen to the naysayers, there's something wrong with
your engine or figures or something. Many guys get used to
bad mileage and just accept it and take it lying down. ;) You
can do that too or get to the bottom of what's going on with
your rig by digging into it like you mean business.

Your call. ;)

Just don't whine at me about your FE getting less than 11mpg
in a light weight Bronco.

If it's a "not running right" problem then after you get it running
right it'll have more power. At that point don't brag on it here
-too- much because not everyone can own an FE. ;)

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Old 10-18-2011, 03:25 PM   #15
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[quote=Alvin in AZ;2731596]Yes because changes can be detected with that information,
like a blown power valve in the carburetor for example.[quote]

---------------------


I would THINK one would notice a bad powervalve; especially if everything else is "correct" as the OP stated. What we Don't know is the age, milage, and condition of the engine, which obviously was not original. Too many variables not known. a simple carb rebuild is the first place to start, set timing AFTER a carb rebuild, then try try try again. The 600 edelbrock should be a good carb for that engine, I wish I had one on my 460 :(
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Old 10-18-2011, 05:15 PM   #16
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Quote:
Originally Posted by Alvin in AZ View Post
Just don't whine at me about your FE getting less than 11mpg
in a light weight Bronco.
light weight?it's a '79
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Old 10-18-2011, 06:29 PM   #17
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Bronco Info: 1979 bronco 390 4 speed
I have owned this bronco for about 10 years. I had the engine rebuild by a machine shop. I know everything about this bronco. No one works on it but me I know every bolt and nut. I have drove this bronco many ways trying to get better mileage I would be thrilled to get 11 I never have. All I know is several people on different fourms have said that a 390 would get around 11 or 12 miles per gallon. My speedometer is very close. I have 3.50 gears with 33 inch tires and use a 16 tooth speedometer gear in the transfer. Also I would put my 390 up against any 460 any day. I had a truck 2wd with a 460 auto wasent impressed.
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Old 10-18-2011, 10:01 PM   #18
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Running 3.50 gears on 33's doesn't help much either unless you do nothing but highway driving. I run 4.10's on 33's and get about 11-12 with a 351M at 70 MPH. If I run at 55-60, my MPG goes up as well which should be true of any vehicle. You have to remember that from the mid 70's to the late 80's, the national speed limit was 55 MPH so that is where you'll get your best MPG.
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Old 10-19-2011, 02:51 AM   #19
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Quote:
Originally Posted by 79broncoowner View Post
I have owned this bronco for about 10 years. I had the engine rebuild by a
machine shop. I know everything about this bronco. No one works on it but
me I know every bolt and nut. I have drove this bronco many ways trying
to get better mileage I would be thrilled to get 11 I never have. All I know is
several people on different forums have said that a 390 would get around
11 or 12 miles per gallon.
Cool and x2 on all that! :)

So what cam did you pick for the engine when it was rebuilt?
(brand, type and size and was it timed "straight up"?)

Quote:
My speedometer is very close.
So what? :/

You don't use the speedometer to check mileage. ;)
How close to "right" is your odometer?
Have you ever bothered to measured it? ;)
Answer the questions in post #8.

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Old 10-19-2011, 03:05 AM   #20
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Quote:
Originally Posted by Robs79BBbronco View Post
light weight? a '79
Hmmm... yeah... I guess it isn't really any lighter than an empty pickup bed huh? :)

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