At 1800 rpm a 300 pulls 156 cfm. At 2300 rpm it pulls 200 cfm. So the primaries on a 390 cfm square-bore 4 bbl ought to be able to feed it through most of that range, but not at the top. But as Gary said, where they actually will open depends on how it's set up.

 

That is what I have. Mine's a 1985 model, and it came with a 3.08 geared rear end and the tires were 215/75/15. The engine, tranny and Overdrive name plate are correct.

 

What MPG do you get?

 

well i did some lookinng in to it and the zf 542 has gears that are close to the np435 so if i can get it for the right price that will give me an od and i will get to keep my low first and revers.
i had been thinking what it would not pull the od with 2.75s but after what 86f50 said i might give it a try.
unfortunately when i got the truck it was down at least 2 cylinders so i could not drive it enough to see how it did. actually it not it on making it home was the big reason but hay i got it for 700 bucks and the body is in fantastic shape.

 

You can always shift down in those rare instances. If MPG is more important to YOU, then go for the gusto!!

 

I think you will find it is an S42. The ZF's came in S42, with 420 ft-lb capability, and S47, which is good for 470 ft-lbs. Anyway, I think you will like the ZF far more than an NP435 - I sure do. It gives you four very usable gears and then there's 1st when you want to pull stumps.

 

thats what i was thinking but went to the multiple junk yards this past weekend and could not find ether trans the zf im not surprised however i am usually tripping over np435s might try again this weekend

 

My daily driver is a 2wd '81 F150 with a 300 with stock Carter 1bbl and a 4 speed overdrive transmission. Not sure what transmission, but 4th is overdrive, no granny gear, and knee knocker reverse. 2.75 9 inch rear end.

I've got an older heavy tool box in the back with probably about 150lbs of stuff in it, stock wheels with street tires.

Most of my driving is on county roads around 40-55mph. I have a heavy foot and still get 17-19 average. I do drive down to Seattle frequently, 200 mile round trip, driving 65 and 75 half the time, I always get at least 19mpg, sometimes a little above 20mpg.

It's not a bad combination having the 2.75 rear end and a 4 speed, the only time it's annoying is driving in town. Otherwise it's not a problem, and I am still able to haul a good amount of weight when I need to.

Could anyone tell me were to find out which transmission I have? I'm curious about this one, and another 4 speed in my parts truck. I didn't see any tags or anything on either transmission...

 

They had two different OD 4 speed trannies. One was a modified toploader 4 speed where they changed the shifter arm coming out of the tranny to make 3rd gear 4th, and made 4th gear 3rd. So 3rd is now 1 to 1 and they changed the ratio of 4th(the old 3rd gear) to make it a overdrive gear. This tranny has the shifter arms exposed on the side of the tranny. I think I am right in saying you will see "RUG" on the side of the tranny somewhere.

The other one was a SROD or "single rail overdrive". The shifter mechanism is built inside the tranny, and this tranny is partially aluminum. It has no visible shifter arms on the side.

Do some internet searching on some of the terms I listed and make sure I am correct. I am going by my memory and it's not very good sometimes.

 

There is an old Ford truck sales promotion advertisment posted on U-Tube that claims 30 MPG with the 240 inline six.

Rememeber when in high school in 1965 , could get 102 octane supreme at Standard oil gas stations for 25 cents.

 

We didn't have Standard stations in Kansas, but I do remember high school and 1965 - I graduated that year.

 

The 240 died at the end of 74. The ad you mention is likely for the 300 and manual OD 4 speed. It was rather optimistic, since the only way to get 30 MPG results, was run the truck on rollers, like they did back then.

However, I'll bet I can get my truck to the 30 MPG mark when I'm done replacing the 300/C6 with a Cummins 6BT and NV4500. Just not sure if I am going to stick with the 2.75 rear axle ratio, or drop it to a 3.00 or 3.25. That will take a bit of testing.

 

I have decided the gas mileage decision should be how much highway driving one does.
I used to drive alot of freeway driving so my 300/4OD/3.08 combo worked exceptionally well....cruise at 1600 rpm at 60 mph.....21 mpg with some city driving included. Didn't have to downshift on upslopes much because the 300 is very torque-y at lower rpms. But if ones routine is driving around town, shifting alot, the rear end needs to be a higher number.