400 owners...
Moderator
Joined: Nov 2007
Posts: 22,415
Likes: 92
From: Denver Metro Area, CO
9-11 MPG
1981 F350 4x4
400 .040 over
TMI pistons
headers
Holley Truck Avenger (4180 I think)
Edelbrock dual-plane (Performer 400)
T-18
4.11 axles
31" tires
All else stock (or nearly so)
1981 F350 4x4
400 .040 over
TMI pistons
headers
Holley Truck Avenger (4180 I think)
Edelbrock dual-plane (Performer 400)
T-18
4.11 axles
31" tires
All else stock (or nearly so)
Freshman User
Joined: Feb 2006
Posts: 37
Likes: 1
Similar to the above. Had a 4900lb 77 351m 4x4-c6-3.50, got about 10mpg overall, 12mpg highway if lucky. Stock 351m, ie 8.0cr, hedman 1-3/4 headers and 2" dual turbosonics, weiand 4v intake and either stock 2v or Autolite 4300-4v carb.
Also had a 5400lb 79 400m 4x4-c6-3.50, got about 10mpg overall and 13mpg highway. That one had 9.6cr (premium only diet), 204-214 cam, hedmans, carb and intake as above.
In each case, making sure the cam was timed right (get a new chain and degree in the cam even if you keep the stocker) made a big diff in power and mileage. Plus basic tuning, etc. Pipes were always a nice light gray after a highway trip. If they are dark gray or black, it's too rich.
Correctly tuned, 20mpg is a good goal.
(I just had to work in that cartoon).
Also had a 5400lb 79 400m 4x4-c6-3.50, got about 10mpg overall and 13mpg highway. That one had 9.6cr (premium only diet), 204-214 cam, hedmans, carb and intake as above.
In each case, making sure the cam was timed right (get a new chain and degree in the cam even if you keep the stocker) made a big diff in power and mileage. Plus basic tuning, etc. Pipes were always a nice light gray after a highway trip. If they are dark gray or black, it's too rich.
Correctly tuned, 20mpg is a good goal.
(I just had to work in that cartoon).
Senior User
Joined: Nov 2009
Posts: 125
Likes: 0
From: TX
I got an average of 11 mpg driving from North Carolina to central Texas in my '71 F100. I have a 400/c6 setup from a '72 LTD connected to the stock truck 3.50 rear axle via a 1-piece driveshaft.
The motor is all stock with the exception of a set of Hooker Headers and a Comp Cams timing set with the 4* compensation.
The motor is all stock with the exception of a set of Hooker Headers and a Comp Cams timing set with the 4* compensation.
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Senior User
Joined: May 2010
Posts: 244
Likes: 0
From: Oregon
My goal is 14/15 MPG. I'd be happy with that.
I've rebuilt the carb and now I'm getting 12.
I'm going to do the whole engine rebuild this winter and dial it in best I can for fuel economy/best power curve. Should be fun, and a challenge.
I'm hearing some guys are putting in the Cummins and getting 21-24. They may be an alternative down the line.
I've rebuilt the carb and now I'm getting 12.
I'm going to do the whole engine rebuild this winter and dial it in best I can for fuel economy/best power curve. Should be fun, and a challenge.
I'm hearing some guys are putting in the Cummins and getting 21-24. They may be an alternative down the line.
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Postmaster
Joined: Aug 2001
Posts: 4,270
Likes: 5
From: Delaware
TMI 434 Stroker, 273/299 HR cam with 114 degree LSA, and 0.600 lift. 10.5:1 CR H-beam rods, forged pistons, roller rockers, CHI heads, EFI with 42 lb Injectors.
13 mpg Highway.
13 mpg Highway.
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Laughing Gas
Joined: Nov 2003
Posts: 898
Likes: 1
From: southeren Oh
The last time i checked mine was getting 14?? rural-- 17MPG Highway.
It has taken quite a bit of tuning to get there though. It has a NP435/ NP205 3.50 gear and 31 inch tall tires. Weighs only 4700 lbs.
It also has a Edlebrock performer intake and 600 carb.
The timing is really played with, (set by ear) I Run as much inital timing as it will stand , I also have and adjustable vacume advance canister, and have that can hooked to ported vacum on the carb.
this way it has plenty of timing at high vac/high RPM (no load)
And it has Lower advance at lower RPM and high load like a ton of gravel in the bed.
The timing thing I did my self, the internals of the engine are unknown as I've never had it apart. (I know its not stock, one head is a 2V clevland head)
It has taken quite a bit of tuning to get there though. It has a NP435/ NP205 3.50 gear and 31 inch tall tires. Weighs only 4700 lbs.
It also has a Edlebrock performer intake and 600 carb.
The timing is really played with, (set by ear) I Run as much inital timing as it will stand , I also have and adjustable vacume advance canister, and have that can hooked to ported vacum on the carb.
this way it has plenty of timing at high vac/high RPM (no load)
And it has Lower advance at lower RPM and high load like a ton of gravel in the bed.
The timing thing I did my self, the internals of the engine are unknown as I've never had it apart. (I know its not stock, one head is a 2V clevland head)
Senior User
Joined: Feb 2004
Posts: 295
Likes: 0
From: conway arkansas
got 16-17 out of mine in my 71 shortbed, running TMI pistons a crane cam and stock heads with roller rockers on em with headers and 2.5 inch exhaust with 3.5 gears had a 750 holley on it long as you stayed out of the secondaries it got decent fuel mileage for a boat anchor
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