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400 cid Fuel Mileage

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Old 02-28-2011, 08:34 PM
hunterlee15 hunterlee15 is offline
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400 cid Fuel Mileage

I'm looking at a 1979 F150 4x4 for my 16 year old. It only has 47,000 original miles on it, but it has a 400 with a 4 spd and I am concerned about the mileage it will get. Would swapping the 400 for a 351 or 302 improve the mileage noticably? Or is there maybe another route to go to improve the mileage, i.e. changing trans? When he's older and can afford the gas, we could always go back to the 400. Thanks for any suggestions.
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Old 02-28-2011, 08:37 PM
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Originally Posted by hunterlee15 View Post
I'm looking at a 1979 F150 4x4 for my 16 year old. It only has 47,000 original miles on it, but it has a 400 with a 4 spd and I am concerned about the mileage it will get. Would swapping the 400 for a 351 or 302 improve the mileage noticably? Or is there maybe another route to go to improve the mileage, i.e. changing trans? When he's older and can afford the gas, we could always go back to the 400. Thanks for any suggestions.

If your worried about fuel your buying the wrong type of vehicle for him.
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Old 02-28-2011, 08:55 PM
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I doubt a 302 or 351 would get better mileage in a 4x4. They would have to work harder. The 400 makes a ton of torque down low and that helps with mileage.
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Old 02-28-2011, 09:58 PM
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My truck has the 400, it was rebuilt, has about 12,000 miles on it... The best I've gotten was 12mpg on a 200 mile trip across Nebraska. No wind, it was summer time, and I stayed under 65mph. The engine is stock except for the edlebrock intake and 4bbl.

I don't keep track of the mpg's when I'm running around town.
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Old 03-01-2011, 07:49 AM
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My 78 F250 (400\4 speed\Edelbrock intake with Holley 4 barrel\4.10 gears) typically got 10-12 mpg around town or highway, with a load or without. The gas mileage dropped off at speeds higher than 60 or so. You will not likely improve the gas mileage by going to a 302 or swapping the trans to an AT.

I've recently swapped to a Q-jet on a rebuilt 400 with a different cam so I'm hoping for an increase in mpg but even then it may only be a couple mpg.

A high school kid with little income is going to have a hard time keeping that truck on the road.
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Old 03-01-2011, 08:06 AM
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If you put on a straight up timing set and headers with dual exhaust you'll see a mileage increase not sure by how much. What is your rear gear ratio? 3.54? 4.10? If you run a 4.10 run a big tire to lower the overall gear ratio. Or just swap to the 3.54. The more volumetric effiancy your engine gets the better mileage you will get since your engine is working less. swapping cams to one a little bit bigger then the stock one will help. Running like a little 600 cfm 4bbl vaccum secondary carb will help because you can adjust it better. And the vaccum secondary helps because you will run on the two front barrels till you stomp on the throttle. Lighten the truck up. For every 100 pounds you lose you gain 2% in fuel mileage.
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Old 03-01-2011, 08:12 AM
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Quote:
Originally Posted by JustBlaze View Post
I doubt a 302 or 351 would get better mileage in a 4x4. They would have to work harder. The 400 makes a ton of torque down low and that helps with mileage.

What he said. Assuming the engine runs smooth and clean you should be in the 10-12 mpg range.
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Old 03-01-2011, 11:37 AM
hunterlee15 hunterlee15 is offline
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400 cid Fuel Mileage

Thanks for all the feedback. Some pretty good food for thought. May leave this truck in the garage for a build myself later on!
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Old 04-15-2016, 06:54 PM
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unbelievable milage for a 400

I know this was years ago, but I just signed up. as for fuel mileage with a 400, I was just amazed at what I made mine get. I have a 78 f150 4x4 with granny gear powered by a true stock 400 with a 2150 2bbl. i decided to see what I could squeeze out for mileage with the motor running at optimal power (never go lean! I know that robs hp and mileage, not to mention harming the engine). I have opened the exhaust to 2-1/2" with mufflers as far back as possible, did away with most of the vacuum, set timing to around 12btdc initiallly and checked total timing to 34btdc and adjusted vacuum advance with the 1/8" allen inside to coincide with how fast it gets there ( I ended up with 15btdc at idle), carb overhaul and tuned by vacuum (which is about 21psi, I went back and forth with timing and vacuum). anyhow, I ended up getting 17.987 mpg!!!! and still have the horsepower to not use a chainsaw....just hook the chain to a tree and pull the whole f$#ker out!!! lol . one added thing, because it was a test, I know these engines loved leaded gas, so I used 91octane and added a bottle of 104+. so, if everyone keeps saying that you can ony get 8-10mpg, they havenet worked hard enough to get the 18mpg....even without the 10-4+, I will get over 15mpg....I LOVE MY TRUCK !!!!!
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