Hi I am looking at the possibility to get better gas mileage by swapping out my 390 for a 300/6. I built one truck from two. I had a '68 f350 dually flat bed and a '72 f250 4x4. Now I have a 68-72 f350 dually flat bed 4x4. I installed a re-manufactured 390 and am getting about 6 mpg. It killing me not to drive my truck. I am looking for advise on replacing the 390 with a 300/6, what are the pros and cons, in and outs.

Thanks

 

You should move up to 10 or 12 with that much truck to move.

You will be down from mid to high 300s torque to mid 200s.

You will be down from 200 horse to about 120.

Of course these numbers are based on "average" stocker numbers. I doubt you are going to put an EFI 300 in there, and your 390 was a pickup spec motor, right?

Bell to block patterns are different also, so a different automatic or a different bell for a MT are required.

 

radonich, I'd say the mileage is about right for a F350 4WD. That thing is a hauler/crawler not a cruiser. I hope you didn't build it for a daily driver.

10-12 mpg is about right for a 2wd F100.


John

 

1968/72 F350 4WD is a Frankenstein Monster with parts swapped from lawd only knows what.

How many F350's came with factory installed 4WD prior to 1979? None.

How many F350's with 4WD had dual rear wheels prior to 1983? None.

You can spend a grand or ten grand in an effort to improve MPG, but all you'll be doing is throwing your money away.

These trucks have the aerodynamics of a brick. Back then...a gallon of regular gas was about 25 cents and no one gave a damn about MPG

It amazes me that people buy full sized pickups, then expect to get decent MPG. And this includes the new piles as well.

I had a 2011 F150 Lariat 5.0L 2WD Stupor Crew, on-board computer's best overall MPG reading was 15.3 and this was after a trip to Santa Barbara and back.

Otherwise the average overall MPG was between 14.7 & 14.9.

 

Sooooo, got any F350 parts left?

 

You all should be humored by this passage in the wikipedia discussion of the Ford 240/300 Straight Six Engines:

"Both the 240 and the 300, no matter the application, used a single barrel Autolite 1100/1101 (or Carter YF/A) carburetors until the introduction of Electronic Fuel Injection. With proper gearing, many F-trucks and Broncos are able to achieve over 30 mpg.[citation needed] with these carburetors, when properly tuned. This was heavily used by Ford's advertising campaign (some television advertisements and written literature even claimed 30 mpg), since the V8 engines in these trucks rarely achieved over 14 mpg. Modern rebuilds are possible to reach 40 mpg in some cases.[citation needed]

The fuel economy of the 300 makes the engine a popular choice amongst truck enthusiasts that want both power and economy. The addition of performance parts (such as intake and exhaust manifolds with a four-barrel carburetor) place the engine power output near the same levels as the stock HO (high output) version of the optional 351 V8, with little or no change in economy. There are claims of 300s that have had the power output doubled, or even tripled, with less than a single mpg drop in fuel economy.[citation needed]"

 

I think the true limit of these trucks is based on weight and their inherent aerodynamics of a billboard in a cross wind. 20MPG is a lofty goal.

 

Total BS!

Anyone that believes what wiki wiki claims...needs to take a course in Reality 101!

And...no Bronco was available w/a 240, the 300 wasn't available until 1980.

From the end of WWII until the first Arab Oil Embargo occurred in 1973/74, most people could care less about MPG.

 

1st let me say Welcome to FTE!
But Son, you did everything against fuel economy except put in 4.56's. You didn't, did you? Anything you can do to make it easier on the engine is all you can do now for MPG. High flow air cleaner,Headers,High flow exhaust, all new tune parts if you didn't with the reman motor,etc.

 

When I had the stock 300 I-6 I got 16 mpg as long as I kept it no faster then 50-55mph with 31.5 x11.5 tires with 4:10 gearing. in my 5,3500-Lbs brick wall back in 1978.

Added a cab over camper gas mileage dropped 12mpg @ 50-55 mph

Added a rebuilt 390 out of the 67 /4 door wagon that was getting 19 mpg at 55 mph.

The rebuild was all stock replacement parts with a 0.030 over bore.
Gas mileage after a 1000 mile break in then mileage went up to 10-11 at 50-55 mph
Now, I could not run any longer with stock carb 2barrle jetting that had #48 jets.
I had to go up to 56 for it to no ping still having the timing backed off to *2-4 ATC. Also had to plug off the dizzy advance line. this had 9.3+ cr and had to run the highest octane gas, I could get while having the camper fully stocked for camping..
8-11 mpg at best even with adding long tube headers and a Accel 50,000 volts big yellow coil with points then..
orich

 

Thank you JEFFFAFA for the welcome. orich, I am interested in your reply. In your experience with a 300-6, did you think you had enough power? I pull a trailer part of the time and I can't consider a swap if the engine won't be strong enough. I understand some of the other responses 85e150six4mtod listed some of the differences, but I can't put any practical experience to the numbers. NumberDummy thinks I built a monster ( he may be right) and I should expect nothing more, but 6 mpg is just not going to work. I was told ( maybe incorrectly) that I could expect 10 mpg. If there is not a big enough difference in the two engines economy the I might consider what JEFFFAFA has to say about increasing airflow. Can anyone tell me if I can expect much difference by adding headers and exhaust and a high flow air cleaner.
Thanks for your input

 

Rasputin53 what parts are you in need of?

 

I have run a few 300s. The first was built to a pretty high level, and in an 84 F250 with straight axle swap, 6" lift and 35s, the mileage was terrible. Towing I got 6, and stopped keeping good track of it.
My current truck as stock had a 300, C4, and 4.10 gears. Going 50-55 it would turn high teens to very low 20s, but push it on the highway and mid teens was the top. The engine was fairly worn, but even after a stock rebuild and electronic ignition upgrade it was the same.
The most recent one was an upgraded stock build in a 78 E150 with a C6 and 2.75 gears. The block I started with was garbage, it didn't clean up until .080 over, so I went with .030 over 360 flat top pistons, a mid range torque cam, but stock intake and exhaust. It was about 9:1 compression, and had a very nice flat torque curve. The worst mileage I ever got was 13 running into a 25mph headwind at 80, with room to spare on the throttle. Typical was mid teens just running around.
The last gas engine I ran in my truck was a built 360, and while it was decently powerful it only turned a tops of 12mpg in a 2wd truck. Towing it got 7-7.5, but would tow at 75 all day as long as there were enough gas stations on the route.
The truck's current setup gets 22+ empty, and the lowest I have seen is 14.5 towing a loaded 20ft gooseneck trailer. It is running a 7.3IDI and a ZF five speed manual.
Diesel is the only way you are going to see the mileage you are after.

 

Well my truck is 4wd Highboy and heavy to start with. In 1978 gas was about $0.65-0.75 cents a gal. where, I was living then so was not hurting when going to the 390 for more power. Mainly on hills as 40mph 3rd gear was about tops unless, If, I got slowed down by a big rig passing one other on a long hill grade. If, I slowed down enough then, I'd have to drop back to 2th gear and it was hard to build up enough speed too get and around a big trucks with 45' trailer or doubles going about 12-15mph to pass and get into 3rd. So I was stuck until I topped the hill grade.

Now with todays high cost of poor grade of low octane gas is $4.20 a gal around the SF Bay areas, I'd go with the 300 I-6 and as the cost of fuel is a fun killer now days for a lot of people.
Some day a lot of us will all be priced out of daily driving these old classics or forced to just sell them for better mileage vehicles.
orich

 

What gearing and transmission does the truck have? If it doesn't have a trans with an overdrive ratio that would be the best addition IMO.