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Old August 11th, 2011, 03:50 PM   #1
letsgomuddin
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K20 10 mpg?!

There has to be a way to get better than 10 mpg out of my truck without drastic mods, though i am not opposed to some mods. The specs are: 1984 Chevy K20, 350 with edelbrock carb and performer intake, headers with crappy duals, TH400, 4.10 gears, bald tires that equal about 31". No matter how good i drive, i cannot get better than 10 mpg with this thing, it's my 4th one of these trucks and I mannaged 16 mpg with one of my other trucks. Brakes don't bind, carb is adjusted very close to perfect, and I have very good driving habbits. Thinking maybe the crappy exhaust is costing me something since theres no backpressure and no crossover but cant imagine more than 1 mpg or so. The truck I got 16 mpg with was a '78 1 ton with a 350 set up the same way except for an rv cam and a TH350 tranny, same gears too. Any thoughts??
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Old August 11th, 2011, 04:00 PM   #2
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those 4:10's aren't helping at all. Got a big carb, like a 750 cfm or something? That is not helping much either, but those trucks are not built for economy. But you should at least be able to pull in another 3-4 mpg by addressing the above.
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Old August 11th, 2011, 04:02 PM   #3
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swap out the 410 for a 342 or 373 might help
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Old August 11th, 2011, 04:09 PM   #4
letsgomuddin
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Im using a 600 cfm carb, had considered the gear swap, though I cant understand why i get 6 mpg less than my old truck with the same equipment
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Old August 11th, 2011, 04:16 PM   #5
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Someone correct me if I'm wrong, but it takes significantly more power to turn a TH400 than it does a TH350. Just like the 4L80vs4L60.

An RV cam should probably pull better mileage than a stock cam too.

Don't feel bad, my 2000 with a 350, 4l80, and 4.10s only knocks out 12. My cousin's 78 k20 with the I6, sm465, and 3.73s only gets 8 or 9.
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Old August 11th, 2011, 04:17 PM   #6
letsgomuddin
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That answers a question i had about the I6 economy, geesh!
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Old August 11th, 2011, 04:21 PM   #7
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I heard that too about the 400, heck I cant stand the 400 it just happen to be in there. The whole not being able to kick down from 2nd is annoying at times
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Old August 11th, 2011, 04:26 PM   #8
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the 400 does not take quite that much power. But maybe like 1 mpg. what size tires do you have? and 10 isnt that terrible for an old truck like that.
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Old August 11th, 2011, 04:31 PM   #9
letsgomuddin
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tires equal 31's, How did I manage 16 mpg with my last one then? Could the combo perhaps of a TH350 instead of the 400, better exhaust and an rv cam make 6 mpg?
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Old August 11th, 2011, 04:52 PM   #10
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My 1988 Suburban with 4.11 gears, 33" tires, 350tbi, th400 trans got 10mpg also, it did not matter how I drove, cty, hyw, uphill, down hill, tail wind, head wind the thing got 10mpg, ran perfect and had no mechanical issues, I tried everything to squeak even 1mpg more out of it and still got 10mpg!
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Old August 11th, 2011, 07:27 PM   #11
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I have owned several of these over the years. the best one ever was my `84 burb. it got 18mpg and had a 350 w/rv cam th350/208 and 3:42 gears. did really well with 32 x10.50 15's on it. I sold it when the rear axle started making noise.

on the flip side I owned a `88 K30 dually crew 454/400/262/4:10 and it ate fuel like crazy. I knew something was wrong when it dropped from 6-7 down to 3.5mpg. ran perfect and every runnabilty guy in two dealers couldn't figure it out.
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Old August 11th, 2011, 07:51 PM   #12
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Are you correcting for tire sizes on this and our other trucks. I mean are we comparing 31s to 33s to 235s??
My last old truck was a 79 F150 with a 400/C6 and 33 combo and I was getting 11-12 with it. That was a 2 bbl motorcraft carb to boot. Usually a 4 bbl will do better in the economy range of you stay out of it.
Try some split fire plugs and maybe flip the lid of your air cleaner over, I hear that helps! Or maybe sell it, and buy a mid 90s 1500 with a 5.7 TBI.
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