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ive decided to replace the 300 i6 in my 1991 f150 with a 4bt the factory power ratings seem about the same as if not better than the stock gasser and the suspension shouldnt cause any trouble as the cummins weighs about the same as a 460 v8 the plan is to mate the 4bt to a five speed stick from a mid 90's cummins ram 2wd and ive found a 2.83:1 ford rear end the truck as it sits gets 18-23 mpg on the highway depending on road conditions my goal is 30+mpg on highway i need some info on which variation of the 4 cylinder cummins i should be looking for how to tune for the best balance of power drive-ability and mileage and if there are any flaws in my plan
 

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Sound like a reasonable plan. Getting to 30 mpg is everone's goal but you must remember you're driving a brick. I own a 90 f250 which is the same body as your's. It isn't very aerodynamic. If you don't have all you're parts yet you might want to think on a Ford ZF S5-42 transmission and use a Cummins SB Ford adapter/flywheel set up. However, the Dodge set up may be cheaper in the long run. The 2.83 gear should be plenty high enough. Wouldn't plan on any heavy towing with that. What you want to do is calculate your engine speed based on your mph. Should be somewhere between 1800-2000 rpm. Too low and you're lugging the engine and too high it uses more fuel. Although 30 mpg isn't impossible, I would guess mid to upper 20's is more realistic. Much will depend on you driving habits and type of road.

One item many people overlook is the economics of their plan. I've been driving diesels for 33 yrs. If I were buying a new vehicle today I'd buy gas. You pay a huge premium for the engine and the fuel is $.40 more expensive. You have a Ford 300-6 which is a great motor. With a few mods you might get another couple mpg's out of it. Those things have been known to go 300,000 miles or more without a rebuild. Got to remember those were even used in dump trucks. Sound kinda like a diesel. If your engine is shot then I'd consider the diesel. If not that would be a tough decision. Remember if you get to 30 mpg you'll only be saving about $.02 per mile. Takes a lot of miles to pay for a conversion.
 

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Discussion Starter #3
thanks for your feed back i have also found in my research that going from the 4.10:1 towing rear end to the 3.73:1 posi in my parts truck and applying a low pressure exhaust back turbo from sts along with some other tweaks will give me around 28 mpgs for around $1500 in terms of hauling its my camping truck so the most it ever sees is about 3000Lbs combined weight between the camperete quad boat and trailer i just though powered by cummins would look really cool on my front fenders but i guess staying with the 300 is cheaper and easyer

thanks
 

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ive decided to replace the 300 i6 in my 1991 f150 with a 4bt the factory power ratings seem about the same as if not better than the stock gasser and the suspension shouldnt cause any trouble as the cummins weighs about the same as a 460 v8 the plan is to mate the 4bt to a five speed stick from a mid 90's cummins ram 2wd and ive found a 2.83:1 ford rear end the truck as it sits gets 18-23 mpg on the highway depending on road conditions my goal is 30+mpg on highway i need some info on which variation of the 4 cylinder cummins i should be looking for how to tune for the best balance of power drive-ability and mileage and if there are any flaws in my plan
Go with a stock VE pumped 4bt and use factory settings. 2.83 rear end? Sure not 2.73?
 

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I would say it prob has something to do with the year.
 

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That's what I was told plus my captain at the fire house ( that I bought it from ) built it 30 over & installed a Rv cam!
Maybe cam timing is out too? I don't think the carb versions of the 300 I6's got as good fuel economy as the newer efi engines.
 

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My 75 ford 4x4 with a 300 six and the stock single barrel bested 9.3 mpg average town and highway. That was with 36" tires and 3.70 gears though. My 68 short wide bested 10.1 with a 240 six and stock size 15" tires. I could never get either truck to get decent mileage. I loved driving the 75 so much that something had to be done. Out with the 300 in with the 4bt, dont know what mileage is yet, but it is a blast to drive.
 

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Discussion Starter #15
hows your 75 4x4 to drive the kybosh on the 91 due to severe rust on the cab mounts which caused the rear of the cab to collapse i recently bought a 1978 f150 4x4 in pretty good shape its got a 400v8 which i understand likes to throw con rods im still interested in doing a diesel swap id like to hear more about your truck
 

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What transmission is in your 75 with the 400? If it's manual it would be a bit more economical to convert. 73-79 were essentially the same except the front grille. I had a 73 F100 4x4 with a 360 and it got 13.7 mpg city or highway. I did everything in the book to impove it and it never budged. That series of truck are good candidates for a swap. Lot of room under the hood.
 

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Maybe cam timing is out too? I don't think the carb versions of the 300 I6's got as good fuel economy as the newer efi engines.
odd, my '83 F150 4X4 carbed 300-6, 3.55 gears, np435 4spd(no OD) with 31x10.50x15's would pull 16mpg. at the same time my dad had an '87 F150 4X4, EFI 300-6, 3.08 gears, np435 with 30x9.50x15's and it would do 17mpg. pretty close in my book. certainly somthing is very wrong if you can't even pull double digits.
 
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