hi, this is a goood web site. i have a 1975 f-250 4wd with a 390 and a c-6 auto tranny. it gets 9 mpg with a load or without. i am thinking about putting a rebuilt 300 in it for gas mileage. can anyone advise the plur and minus of this? that should **** off some purists. also what aftermarket fuel injection source does any one like? this is a cherry old truck and worth the hassles. better than a 30000 dollar new 3/4 ton truck. thanks from camas wa. twr

 

You'd be spending an awful lot of gass money installing that 300. Engine mounts, throttle linkage, radiator, transmission, yadda, yadda,....

Nathan

 

t16, your problem is 4X4 setup. You most likely have short gears in the rears. You need taller gears and bigger tires, that will slow the engine rpms down say at 60 mph. 3/4 ton 4X4s just weren't built to be economical.

Put your engine in a 2X4 with 2.75 rears and it should get 16 - 18 mpg.

Sorry but it's just the beast in it, just think about how good it pulls. :-)

John
jowilker

66 F100s
In the still cool hours of the night, you can hear chevys rusting away.

 

On a personal note: I would more likely notice and admire your '75 4X4 than a 30,000 newer model.

 

I have to disagree with Jowilker on the 4x4 setup. My 73 f100 w/390/4 speed 2wd only got around 10 mpg loaded or emty. That sounds like typical 390 mileage to me

Trail Rider

 

TR did you have 2.75 rears behind your 390??


John
jowilker

66 F100s
In the still cool hours of the night, you can hear chevys rusting away.

 

I agree it's hard to complain if you get 9-12 mpg with a 360 or a 390. Loaded or unloaded ! Auto or manual tranny.

 

I agree it's hard to complain if you get 9-12 mpg with a 360 or a 390. Loaded or unloaded ! Auto or manual tranny.

 

I'm not a fan of 390's but if the motor is in good shape, why bother to change it? I have owned Ford pickups with every thing from a 240 six to a 460 V8 and they all got right around 10 mpg so changing your motor would be a major expense for very little,if any, gain in mileage. If you want better gas mileage try changing gears or adding an overdrive unit.

 

No, Mine had 3:08's.

Trail Rider

 

T16 I live near Camas and just went through the hassle of changing my 68 F100 Shortbox from a straight six to a 360, it was hard at some points, but not impossible by any means. I think just re-build the 390 with a good economical cam and performer intake and your better off, I get around 6 in town, but I think I've got 11 on the highway, but it has 4.10's and a posi in a Dana 60, it's kinda odd, a truck that was originally a 240 straight six with a 4 speed and 4.10's and a posi

 

Wnelson wrote "not a fan of 390\s"
tisk tisk tisk.
i'm dropping a GT into a mav/comet.
then ppl will be jealous of the old FE

 

The 390 in the trucks has a realy low compression ratio. I have a 75 F-250 2w/d Camper special with 3.71-1 gears. I rebuilt the engine with a 9.0-1 ratio and a small R.V. cam. It was huge improvement in milage and power and will still run regular gas (except when towing).

After the rebuild it was getting about 14 mpg. I then changed the stock holly 4 bbl. for a 600 cfm edelbrock carb. It now gets 15 to 16 on the open highway. Around town it's much less, but that's cause it forces me to hot rod it! (must be related to christine).

The 390 is an excelent motor. But it was designed for higher compression than 7.somthing-1. You could mill the heads if you don't want to rebuild right now.

 

What was the ratio before you rebuilt it?

John
jowilker

66 F100s
In the still cool hours of the night, you can hear chevys rusting away.

 

Actually in certain circumstances a 300 six might not get better gas mileage. With that heavy of a vehicle (assuming its probaly a highboy)it might work the 300 straight to death causing it to get worse gas mileage. Maybe not as bad as the 390 but not to far from it. I say this, my dad used to have Dodge W100 with a 318 (i know i know everbody gets wiser with age, he now has a ford)it had 3.23 gears and it got 8-10 mpg. The high gears were just working it to death(surprise) and lugging it.