I have owned a 72 F250 truck for years. It is a nice truck, rust and dent free, and very heavy duty. Over the years I have upgraded the suspension with 1 ton components, and rebuilt the power disk brakes and power steering components.

It has a 390 engine and C6 tranny. The mileage has always been horrible - 8 to 10 empty, as low as 6 when towing. When the gas price hikes started I intended to sell the truck because of this, but somehow never got around to it.

Last summer I sized the engine. I was trying to run the engine with a lot of advance in the timing, and I think I burned a hole in the top of a piston. I was limping home when the engine sized up, and it has not run since.

This spring I am pulling the engine and transmission and replacing them with a 300 EFI inline six and C6 combo from a 86 F100. I could have rebuilt both the stock engine and tranny for about the same expense, but want better mileage.

The 300 six is a good industrial engine, not as fast or as powerful as the 390, but dependable and cheaper to operate. Low speed wise and up mountain roads I expect it will perform as good as the 390, and I no longer need or want high speed operation.

The truck is too heavy and rides too rough to be a daily driver - it is a work horse I will use only when I have a big load to haul or tow.

Any comments or suggestions? I will update this thread as I progress.

Please start a new thread and tell us about your spring and summer project.

 

*****,I own a 72 F-250 2wd 390 with a rod knocking after only 850,000 miles in this engine.I also own a 71 F-100 4x4 300 six and had the engine rebuilt about 4 years ago.I would not choose to use the 300 over the 390 for short time usage such as towing or hauling. When I built the 390 the first time I was getting between 14 to 16 miles to the gallon if I took it real easy but only 11 to 13 towing and or hauling with 354 gears and 12:50 16:5 33" tires and a C6.My 71 with the 300 gets 13 to 14 no matter what and I understand it is a 4 wheel drive and loses mileage to extra weight.I would not choose the 300 over the 390,the small amount I might save on gas would not offset the advantages of having the extra torque and horsepower available to the 390.I will rebuild my 390 and hope to get as good an engine as it was before. "bowties in the rearview mirror"

 

Thanks, Nod - it is certainly a decision I have been struggling over. I know what you are saying is true - I have also owned two other trucks with the 300 six in them. One was a 77 4wd F150, and its mileage was like you said. The other was a then new 1980 F100 shortbed - it often gave me 20 MPG running empty. It was a very light duty truck used as a daily driver.

I don't know why my 390 gets such poor mileage - about half the posters on the FE board get about the same, with the other half getting mileage in the range you do. My 72 has the same equipment yours does - 3.54 rear on 16.5 tires.

I would hate to go to the work and expense of rebuilding the 390 and the C6 to wind up with a 10 MPG truck.

I also have no experience with the fuel injected 300 six - the EFI should help it both on low end power and mileage. I originally bought this combination to put in my 59, but have since decided to either use the stock 223 six/4 speed or a 302 with aod in it.

Two things - I will dismantle the 390 and see if it is rebuildable when I get it out, and, even if I do put the 300 six in, I can always take it out and go back to stock.

I looked at your gallery - your meadow with all the Fords reminds me of home. The same vintage of Fords, except the home place is in Colorado so all the trucks except mine are 4 wd. My brothers all like the 3/4 ton highboy 4wd, which ride like dump trucks with square wheels.

Anyway, thanks for your thoughts, I will sure give it another look when I have things apart.

 

I would go with the 300 I6. It has plenty of low end torque, they are very hard to kill, should outlast the 390 in rebuilds two to one, and if Ford put them in F-600 trucks, you should be fine with it in a F-250.

Just don't expect any speed like the 390 can give you. Most people you talk to nowdays say that the 300 isn't made for work, go get a V8. Most of this thinking is because most people tend to drive a 300 like a V8 out of it's power RPM range, and because it's not fast at doing it's job.

You hook two equal trucks, T-18, 3.75 gears, same tires etc together, but one with the 300 and one with the 390, I'd say it was even money at the outcome if I'm not biased toward the 300 to win. My opinion.


As for the EFI 300s They tend to get less gas mileage than their carbed counterparts in the 80's, (due possibly to more smog control devices), but they have more power due to the more open intake and exaust. A EFI exaust manifold upgrade for the carbed 300, is a quite common bolt on performance upgrade.

Go to the inline six forum and ask some questions about the 300, and what they think will be good for you.

 

HaHa, then go to the FE forum and ask them too.....
I don't have much experience with either but I have a 390 in my truck that I love and my dad has the 300 straight 6 in his. He's got about 300,000 miles on his with all original parts, except the freeze plugs starting blowing out at some point, so I know those have been replaced. Either way, I think it's personal preference.