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Gear ratio and mpg

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by achuskies69, Mar 12, 2001.

  1. achuskies69

    achuskies69 1/2 ton status

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    <font color=green>Well even though i dropped in the new 350 and bolted on a set of Flowtech headers, im still getting crappy gas mileage. ok im running my stock weak ass 10bolts still and there geared 3.08, and im pulling 33" Cooper Discoverer Radials. Are the gears my problem? I planned on going to 3.73's b/c i bought a D44 and GM 12 for cheap and i think they are geared 3.73. But i wanna go to 35's later, so what should i do to keep good gas mileage. Its getting about 9mpg now. I know Blazer Boy gets 16 mpg and he is running 4.56's and 36" tires under a 7" lift.
    thanks for your help....

    <font color=green> I love cowboy hats
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  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    I'd say the gears are hurting your mileage right now. Lugging an engine below its normal operating range kills power (duh!) and mileage.
    I'd go through everything else as well. Usually after headers are installed some carb adjustments need to be made. Cap and rotor good? Wires and plugs? Timed properly? All these things will make a difference. Is the new 350 broke in yet?

    Rene

    [​IMG]<A target="_blank" HREF=http://jules.coloradok5.com>http://jules.coloradok5.com</A>
     
  3. achuskies69

    achuskies69 1/2 ton status

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    <font color=green> Well the motor has about 500 miles on it, has a new set of plug wires, and i guess i need to fine tune the carb a little. There is a new cap and the rotor is good, and we timed it after we got the motor put in.

    <font color=green> I love cowboy hats
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  4. CaptCrunch

    CaptCrunch 1/2 ton status

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    Well the axcle swap will help big time now... I have 3.08's and only get about 11 BTW. 3.73's should be prefect for 33's... might want to change to 4.10's or 4.56's when you go to 35's. although the 3.73's wouldn't be that bad.

    1987 Chevy K5 Blazer- 350 TBI
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    It depends on where you are measuring mileage..I don't think anyone here would claim to get 16-20 city and freeway combined..I doubt its possible for these heavy trucks. As for the gears, my dad, with a 454 and 4.10's, gets 9MPG city OR freeway. Low gears (numerically high)won't hurt you as much in the city as they do freeway. With my K5, 3.08, 305, 700R4,31" tires, I was pulling 17 freeway, and 10-13 city depending if I was easy on takeoffs. I just pulled a 12.5 out of my K5 with 350/SM465/3.42/33's city/freeway without tuning the carb. Lugging can be a problem, but freeway, on relatively level ground, I had no problems with the 3.08's. 33" tires from 31" tires, might make a 500RPM difference, but that still won't be a major issue on flat ground at 60MPH.

    Dorian
    My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
     
  6. loudnowlouder

    loudnowlouder 1/2 ton status

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    your engine still has a good 1500 miles till complete break in. if you have a new carb with it, it's going to take some readjusting in the first few thousand miles to get it operating properly.
    i just dropped a 350HO Deluxe in mine and have about 300 miles on it, the holley on it would take attention everyday if i had the time...but its all brand new so you have to expect low milesage. but i agree that your gears are hurting you. i have 3.73s and wish i had 4.56s...currently have 33s getting 35s.

    --adam H
     
  7. 6.2Blazer

    6.2Blazer 1/2 ton status

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    I agree that the 3.08 gears/33" tire combo is probably hurting the gas mileage some, but don't expect to jump from 9 to 16 mpg with just a gear change. I would think that you would gain maybe a couple of miles per gallon when going to 3.73 gears.
    As for average gas mileage for a Blazer, I would think that something like 16 mpg would be considered exceptional mileage even if it's all freeway driving. I could maybe squeeze maybe 17-18 mpg out of my 6.2 diesel (known for much better mileage than the gas engines) when it had 32" tires and 3.73 gears (O/D trans) and running a lot of freeway. Even the '98 Toyota Tacoma 4x4 with a 4 cylinder and 5-speed I used to have would only get 16-17 mpg, so keep that in mind when looking at the mileage of a carbed 350 in a heavy Blazer with the aerodynamics of a barn door.
     
  8. Hayduke

    Hayduke 1/2 ton status

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    i just put my open K&N air filter back on b/c it's not as cold here anymore and just with that i get almost 2 extra mpg....
    HAYDUKE

    Insured By COLT, Policy #.357M

    <A target="_blank" HREF=http://haydukesk5.coloradok5.com>http://haydukesk5.coloradok5.com</A>
     
  9. Innerpiece

    Innerpiece 1/2 ton status

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    I've got a stock 87 silverado blazer with slightly larger tires (32" instead of 31") and I'm getting about 12 on the highway ... I do have problems staying in OD going up even relatively slight grades (if they're 1/2 mile or so) would lower gears help this? I do need to replace the cap and rotor and prolly spark plugs, but its got new plug wires (it's amazing how strong the 305 was on 7 cylinders ... 8's even better, really helps the gas mileage, and stops the annoying exhaust too rich codes). any other tips for mileage?
     
  10. dyeager535

    dyeager535 1 ton status Premium Member

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    I'd INSPECT the cap and rotor, not necessarily replace it. If its defective, replace it. It's pretty easy to tell visually if they are bad. Plugs most definitely if they haven't been changed in awhile. With the 3.08's, you WILL bog down in OD going up a hill. Solution is to drop it into 3rd.

    O2 sensor a lot of times is cause of bad mileage, but is an expensive item to replace if its not broken. Should be able to check voltage on the 02 sensor to make sure its working while still on the truck.

    Dorian
    My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
     
  11. achuskies69

    achuskies69 1/2 ton status

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    <font color=green> This is a brand new 350, not rebuilt, and it has a new cap and the rotor off of my 305, and it was in good shape, and all new plugs and a new set of plug wires. Im in the Appalachain Mountains, so i drive in 3rd all the time cause of it bogging around in OD, i only use that on the interstate going to Charlotte to see my sisters. I think htat the O2 sensor was replaced when i installed the motor and headers too.

    <font color=green> I love cowboy hats
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  12. dyeager535

    dyeager535 1 ton status Premium Member

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    Hmm...I don't know why i assumed you had a newer truck. I don't even think you HAVE an O2 sensor! I wonder if you aren't having some distributor problems, as in advance? Your vacuum advance hooked up? Are the mechanical advance weights free to move? Just went through this on my dads truck, and the weight bushings were gone, and the weights were binding. the vac advance will affect your cruise MPG. Is it hooked up correctly? (timed port, not full manifold vacuum)

    That carb should not need to be retuned if it was running ok on the 305, since the bigger motor will now just be running leaner at cruise. How does it idle? How does the exhaust smell at idle?



    Dorian
    My K5 and Chev/Olds tech/links page: <A target="_blank" HREF=http://www.dorianyeager.com/index2.html>http://www.dorianyeager.com/index2.html</A>
     
  13. muddin4fun

    muddin4fun 3/4 ton status

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    Let the motor break in...they usually get bad milage untill they break in. Sometimes they even smoke a little untill the rings seat. Let the motor break in and then start messing with it.

    [​IMG] Keep your feet dry [​IMG]
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  14. achuskies69

    achuskies69 1/2 ton status

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    <font color=green> yeah i noticed today as i was leaving school it was putting out some black smoke so i guess i have a few hundred more miles before its broke in good...

    <font color=green> 82 K5 - 350, 4" lift, 33's 3.08's
    I love cowboy hats
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  15. muddin4fun

    muddin4fun 3/4 ton status

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    black smoke means the carb is running rich. It'll do that if you really mash on it but if you're driving it normally then it shouldn't. Blue smoke is what i was talking about. A good rule of thumb is to turn the mixture screws on your carb all the way down and then back them out one and a quarter to one and a half turns. That's for an edlebrock or a carter. If you have a quadre-junk, uh..i mean quadra-jet then I don't know how to adjust those or even if you can adjust those. I'm not sure about holleys either. I'm sure someone on this site knows. Hope this helps.

    [​IMG] Keep your feet dry [​IMG]
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  16. achuskies69

    achuskies69 1/2 ton status

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    <font color=green> yeah im running an Edelbrock Performer 600cfm carb, and i didnt readjust it from when we swapped it over. guess ill have to do it this weekend...

    <font color=green> 82 K5 - 350, 4" lift, 33's 3.08's
    I love cowboy hats
    [​IMG]
     
  17. muddin4fun

    muddin4fun 3/4 ton status

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    Cool! That's what I'm runnin'! That's the way I adjusted it and it's doing great.

    [​IMG] Keep your feet dry [​IMG]
    <A target="_blank" HREF=http://muddin4fun.coloradok5.com>http://muddin4fun.coloradok5.com</A>
     
  18. achuskies69

    achuskies69 1/2 ton status

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    <font color=green> what kind of mpg are you getting?
    id be happy if i could squeeze out around 11-12.


    <font color=green> 82 K5 - 350, 4" lift, 33's 3.08's
    I love cowboy hats
    [​IMG]
     
  19. kenbrown

    kenbrown Registered Member

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    im getting a good 15 mpg out of my k5. i am running a 4" lift and 33 12.5's bfg's at's i have 3:73 gears in my ten bolts. they are going the way as i speak, a 14 bolt sf with 4:56's are taking the place of the 3:73's oh, and its fuel injected which helps a ton in the gas milege area.

    ken with the blue ox (89 k5 silverado)
     

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