Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

going from 3.08 gears to 5.13 gears, MPG???

Discussion in 'The Garage' started by 85mudblazin, Oct 5, 2004.

  1. 85mudblazin

    85mudblazin 1/2 ton status

    Joined:
    Mar 26, 2004
    Posts:
    3,951
    Likes Received:
    0
    Location:
    Austin,TX
    I will be going from the 3.08 gears in my 10bs to the 5.13's in the 14bff, what can I expect my MPG to do?? The tires will get going easier so less gas but in the upper sppeds I might loose some, What do yall think???
    Thanks
    -Chris
     
  2. 84_Chevy_K10

    84_Chevy_K10 Banned

    Joined:
    May 30, 2001
    Posts:
    17,669
    Likes Received:
    0
    Location:
    IL, USA
    With 35s? I'd say you gain 2-3 mpg at least, especially around town.
     
  3. 85mudblazin

    85mudblazin 1/2 ton status

    Joined:
    Mar 26, 2004
    Posts:
    3,951
    Likes Received:
    0
    Location:
    Austin,TX
    really I would GAIN MPG?? that would be awesome /forums/images/graemlins/thumb.gif
     
  4. RustBuket

    RustBuket 1/2 ton status

    Joined:
    Feb 21, 2003
    Posts:
    1,618
    Likes Received:
    0
    Location:
    Saskatchewan, Canada
    I think around town would improve lots but on the highway gearcalc says you'd go from 1349RPM on the highway....pretty low to 2240RPM in OD. Hmmm that doesn't sound so bad afterall. I'd do it.
     
  5. 85mudblazin

    85mudblazin 1/2 ton status

    Joined:
    Mar 26, 2004
    Posts:
    3,951
    Likes Received:
    0
    Location:
    Austin,TX
    those calculators suck, at about 65 I am running at about 2000RPM right know, so I am thinking around 2500?? after the swap?? That would still be good, would have more power at the upper rpms.
     
  6. mofugly13

    mofugly13 1 ton bucket of rust Premium Member

    Joined:
    Oct 6, 2002
    Posts:
    2,813
    Likes Received:
    2
    Location:
    San Francisco
    It kinda makes sence and it kinda doesn't. I will be going from 3.08 w/ 33" BFG's to 4.1 w/ 35" BFG's. I get 8-10 MPG now, will my MPG's go UP when I change? I understood the reason a lot of our trucks/K5's got the high gears in the first place was for fuel economy? How can lower gears be better? Just 'cause the motor doesnt have to work so hard?Sorry for the hijack, any enlightenment would be appreciated.
     
  7. RustBuket

    RustBuket 1/2 ton status

    Joined:
    Feb 21, 2003
    Posts:
    1,618
    Likes Received:
    0
    Location:
    Saskatchewan, Canada
    Yeah I think it would be a good gear in general but I think highway mileage might suffer a little.
     
  8. 85mudblazin

    85mudblazin 1/2 ton status

    Joined:
    Mar 26, 2004
    Posts:
    3,951
    Likes Received:
    0
    Location:
    Austin,TX
    about 85% of my driving is just around town so im not very concerned about highway mileage. If I could pick up a few in town that would be sweet, not to mention the put you in the back of the seat feeling /forums/images/graemlins/peace.gif
     
  9. smprider112

    smprider112 1/2 ton status

    Joined:
    May 1, 2004
    Posts:
    624
    Likes Received:
    0
    Location:
    Beaverton,Oregon
    ya round town lower gears work better...cuz the engine doesnt have to work as hard...also yer tranny will like it better...i'd imagine with the 3.08 gears and 33" tires W/ an overdrive tranny...it probably would BARELY shift into OD...now it should and cruise pretty well /forums/images/graemlins/thumb.gif
     
  10. 84_Chevy_K10

    84_Chevy_K10 Banned

    Joined:
    May 30, 2001
    Posts:
    17,669
    Likes Received:
    0
    Location:
    IL, USA
    [ QUOTE ]
    It kinda makes sence and it kinda doesn't. I will be going from 3.08 w/ 33" BFG's to 4.1 w/ 35" BFG's. I get 8-10 MPG now, will my MPG's go UP when I change? I understood the reason a lot of our trucks/K5's got the high gears in the first place was for fuel economy? How can lower gears be better? Just 'cause the motor doesnt have to work so hard?Sorry for the hijack, any enlightenment would be appreciated.

    [/ QUOTE ]

    Yes.

    Everyone thinks that low rpms = fuel economy.

    That's pretty much all wrong when it comes to trucks.

    Want MPG? Maximize vacuum so you hardly have to touch the gas to make a truck go. My 4.10 geared, non-overdrive C30 with a well-worn TBI 350 gets 14 mpg empty even at 3,000+ RPM @ 65-70 MPH.
     
  11. fad2blk99

    fad2blk99 1/2 ton status

    Joined:
    Feb 19, 2001
    Posts:
    860
    Likes Received:
    0
    Location:
    Charleston, SC
    I didn't notice a significant change in MPG when I went from 3.08s and 32s to 5.13s and 38s. Never ran the 3.08s with the 38s though.
     
  12. wayne

    wayne 3/4 ton status

    Joined:
    Feb 1, 2001
    Posts:
    6,178
    Likes Received:
    0
    Location:
    Carlisle, Pa
    [ QUOTE ]

    Everyone thinks that low rpms = fuel economy.

    That's pretty much all wrong when it comes to trucks.


    [/ QUOTE ]

    That is not totally true. On my Burb I picked up 2mpg running it at lower rpm's on the highway. Grim gave me some advice on it and I tried it. Keeping the bigblock below 3k rpm's really makes a difference. I went from just under 10mpg to 12mpg. My old Jimmy when it was stock it had 350tbi/700r4/3.42's and 31's sucked gas and after the swap to 4:56's and 35's I got a lot better milage. Never checked the actual amount but I know a tank went quite a bit farther then before the swap.
     
  13. 83ZZ502_Jimmy

    83ZZ502_Jimmy 1/2 ton status

    Joined:
    Feb 7, 2001
    Posts:
    3,103
    Likes Received:
    0
    Location:
    Richmond, VA
    And all of this MPG info is based on you speedometer/odometer. So if its off you will not be able to calculate the acutal miles driven, PERIOD.

    I often wonder if everyone that complains of bad milieage or someone who is gettting "great" mileage, has their speedo dead on right.

    The only real way to know how fast and how far is with a GPS unit.

    Just my 02,
    John
     
  14. hawkeye649

    hawkeye649 1/2 ton status

    Joined:
    Mar 26, 2004
    Posts:
    848
    Likes Received:
    0
    Location:
    New Hamp-shi-ah
    Don't you guys have measured miles out there?
    Mine goes about .99 miles on the odo for every 1 mile in the real world. That means my 16MPG in my 1990 GMC F/S Jimmy 350TBI TH700R4 3.73 31x10.5x16's, is actually a little higher. /forums/images/graemlins/thumb.gif
     
  15. 6.2Blazer

    6.2Blazer 1/2 ton status

    Joined:
    Feb 24, 2000
    Posts:
    4,675
    Likes Received:
    26
    Location:
    Ohio
    [ QUOTE ]
    those calculators suck, at about 65 I am running at about 2000RPM right know, so I am thinking around 2500?? after the swap?? That would still be good, would have more power at the upper rpms.

    [/ QUOTE ]

    You are running 2,000 rpm at 65 mph, in OD, with 35" tires and 3.08 gears??????........that's seems like a lot of RPM for that comibination. Maybe if it was in 3rd gear and not OD /forums/images/graemlins/dunno.gif
     
  16. 6.2Blazer

    6.2Blazer 1/2 ton status

    Joined:
    Feb 24, 2000
    Posts:
    4,675
    Likes Received:
    26
    Location:
    Ohio
    [ QUOTE ]
    That is not totally true. On my Burb I picked up 2mpg running it at lower rpm's on the highway. Grim gave me some advice on it and I tried it. Keeping the bigblock below 3k rpm's really makes a difference. I went from just under 10mpg to 12mpg.

    [/ QUOTE ]

    That sounds like a different set of circumstances. You didn't mention changing gears, so the only other way to lower rpms is to slow down......slower speed equals less wind resistance which equals better mileage.

    In any case it's making the particuliar engine combination run in the correct rpm band in order to get the best mileage. If the rpm's are too low than the engine is lugging and working harder to keep the truck moving (equals more fuel), too high of an rpm and you are just pumping more fuel into the engine to let if spin that high though the extra power is not needed.
     
  17. metalbonde

    metalbonde Registered Member

    Joined:
    Jun 28, 2003
    Posts:
    7
    Likes Received:
    0
    Location:
    Sweden
    so basicly what you want is : high rpms in town (2-3 K)? low on highway (1500-2000)?

    my mpg is not that good .. but with 2:73:s and 32"s i get ok on the highway 14-15 ,but hey with tbi and everything shouldnt 17-18 be possible going slow , 50-55 mph or so?
     
  18. 84_Chevy_K10

    84_Chevy_K10 Banned

    Joined:
    May 30, 2001
    Posts:
    17,669
    Likes Received:
    0
    Location:
    IL, USA
    Basically what you want is low load, not low RPM necessarily. You need to be geared to an RPM range that will pull the truck down the road with minimal throttle input.
     
  19. tecton

    tecton 1/2 ton status

    Joined:
    Dec 1, 2002
    Posts:
    1,553
    Likes Received:
    0
    Location:
    WV
    my understanding that the more turns the engine is making the more gas its using

    IE...higher the RPM, higher the gas usage

    keep rpms low, keep mpg low, rpm's high, mpg high
     
  20. nvrenuf

    nvrenuf NONE shall pass! Premium Member

    Joined:
    Jan 7, 2002
    Posts:
    13,081
    Likes Received:
    248
    Location:
    Mobile, Al.
    It's not that simple, the workload pays a factor as well.
     

Share This Page