Gas mileage help!!!

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Oct 4, 2014 | 08:49 AM

78fordbeast

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From: Harrisburg

Gas mileage help!!!

Hey guys I have a 78 Ford F-150 which I recently put a mildly built 400. I know that the motor has only about 2000 miles on it and has the Holley truck avenger carb 670cfm on it with an edelbrock intake. I've been calculating my gas mileage and I'm getting 5.5mpg!!!! Like with my old 351m I used to get 10mpg but I didn't expect this low of gas mileage. I have a c6 transmission that was freshly rebuilt and I have the dana 44 and the ford 9 in rear and I think they are just the stock gears(I don't know what ratio) and I am running 35x12.5 BFg all terrain kos. I really need some advice on this, I used to go mudding a lot with my truck before but I stopped doing that a while ago after I got my body work done to my truck.(so I have never seen or changed the oil in the axles) Also another factor that I think might be playing into this is that my front axle will sometimes make this whining and grinding sound for no reason?!? Like the 4wd is disengaged and the hubs aren't locked. Maybe the trasfer case is stuck in 4wd partly and maybe that's why I'm getting bad gas mileage? I don't know but please help me!! Thank you!!!

Oct 4, 2014 | 11:36 AM

dustyroad

Fleet Mechanic

Joined: Jul 2013

Posts: 1,997

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From: NE Ohio

One giant wall of text, clean it up some. ppl might help more.
But you think if you never changed the oil in the axles you might want to do that? Geez maybe that's the whine :P
I mean it isn't rocket science, you have to change fluids.
As for you mileage,. it could just be thats what you get now.
You don't tell us a thing about what you did to the engine. Do you even know the size jets in your carb?

Oct 4, 2014 | 11:43 AM

NumberDummy

Ford Parts Specialist

Joined: Nov 2006

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From: Simi Valley, CA

AXLE code stamped on the 2nd line-far right of the Warranty Plate, located on the left door face below the latch.

Oct 4, 2014 | 01:08 PM

77&79F250

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Joined: Feb 2007

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From: S/W Missouri, Polk county

Quote:

Originally Posted by 78fordbeast

mildly built 400.
carb 670cfm on it with an edelbrock intake.
stock gears( 35x12.5 BFg all terrain kos.

(so I have never seen or changed the oil in the axles) Also another factor that I think might be playing into this is that my front axle will sometimes make this whining and grinding sound for no reason?!?

Maybe the trasfer case is stuck in 4wd partly and maybe that's why I'm getting bad gas mileage?

I read all the answers to your questions/issues in your post. 35" tires and stock gears, what RPm are you running?

Xfer case stuck in 4wd....grinding and whining...never seen or changed fluid.

Oct 4, 2014 | 01:12 PM

phillips91

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Joined: Apr 2008

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From: Rogersville, TN

Are you calculating the mileage while having fun with the new engine? I have a built 351w in my mustang and I can get 15 mpgs if I just cruise at 55 mph but when I am playing around and driving it like I stole it I have gotten in the 5 mpg range for an entire tank.

Oct 4, 2014 | 02:59 PM

gatorfor88

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From: Panama City

If you are runung 35 inch tires and using stock speedo gear you are way off on calculations.

Oct 5, 2014 | 07:46 AM

NMFirstF2504X4

Senior User

Joined: Sep 2012

Posts: 372

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From: New Mexico

I used my iPhone (and a check with Google Earth) to find the calibration for my odometer running 35" tires. Also, from my recent adventure in tuning my 351M, tweaking the carb and timing could really help improve your mileage. If the carb is stock, it's likely on the rich side as I found with my 600 Holley.
https://www.ford-trucks.com/forums/1...g-project.html