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.

Gear ratios - which to choose

Discussion in 'The Garage' started by W7NB, Dec 14, 2006.

  1. W7NB

    W7NB 1/2 ton status

    Joined:
    Nov 9, 2006
    Posts:
    716
    Likes Received:
    0
    Location:
    Springtucky, OR
    As the build saga continues...

    I have a chance to snag a set of axles out of an 88 'burb cheap - like 250 bucks. Got to drive the rig and nothing seems out of place - he's just doing a major (like 2 1/2 ton Rockwells) upgrade and wants to get rid of these axles. Only issue is the axles have 3.73 gear sets. I run 32's and am trying to dump my 3.08 gears. I don't plan to go any bigger tires on this rig as I tow with it and would like to keep it fairly civil.

    Here are the calculations I ran comparing the 3.08's to 3.73 and 4.11. These are based on a 700R4 OD Ratio of .7:1

    3.08 with 33's 2100 rpm= 66 mph
    3.73 with 33's 2100 rpm= 54.5 mph
    4.11 with 33's 2100 rpm= 49.5 mph

    Or I can convert that to RPM@Speed

    3.08 @ 65mph= 2070 RPM
    3.73 @ 65mph= 2507 RPM
    4.11 @ 65mph= 2762 RPM

    Highway crusing and towing would be a bigger consideration for me then grunt. If I'm running a trail I'll put her in 4 low and have at it.

    My only problem is that I don't know where the ideal crusing rpm is for a TBI 350. I seem to remember somthing about 2400 RPM but I'm not sure - input here would be very helpful!
     
  2. AussieK5

    AussieK5 1/2 ton status

    Joined:
    Jun 17, 2001
    Posts:
    316
    Likes Received:
    0
    Location:
    A land girt by sea
    Check your calcs 32-33's on 3.73's with a lock up 700 will be approx 1800 @ 65.

    You need to look at 4.56 or 5.13's

    Jas
     
  3. pauly383

    pauly383 Daddy383 Staff Member Moderator

    Joined:
    Jul 22, 2001
    Posts:
    16,217
    Likes Received:
    1
    Location:
    Mesa , Arizona USA
    The charts and calculators don't take into account many real world factors , like wind , elevation , and lock up etc .

    In my own use , I have run 31 , 32 , 33, and 35's . And I have had 3.08 , 3.73 , and 4.56 gears . I also have a good working lock up 700r4 .

    With 3.08's and 33's , I could cruise 65-70 on flat ground and never see 2100 rpm .

    With 3.73's and 33's at the same speed I could tickle 2100 rpm .

    Now I have 35's and 4.56 , and its close to stock . I do 65 mph and maybe 2100 rpm at 65 mph.
     
  4. W7NB

    W7NB 1/2 ton status

    Joined:
    Nov 9, 2006
    Posts:
    716
    Likes Received:
    0
    Location:
    Springtucky, OR
    Problem is, I can't find any dino charts to determine what ideal cruise RPM would be. Was 2100 fairly decent for fuel economy and power, or should it be closer to 2400? I guess thats the missing element in all of this.
     
  5. pauly383

    pauly383 Daddy383 Staff Member Moderator

    Joined:
    Jul 22, 2001
    Posts:
    16,217
    Likes Received:
    1
    Location:
    Mesa , Arizona USA
    Well , I have a 305 with higher compression LE9 , and I make peak torque around 2000 rpm anyway .

    Yours is what , a LO5 ( tbi 300 ft lbs ) ? I think those were rated for peak torque under 2500 too .
     
  6. iwaxmyjimmy

    iwaxmyjimmy College web wheeler Premium Member

    Joined:
    Nov 3, 2006
    Posts:
    2,802
    Likes Received:
    34
    Location:
    Greenville NC
    what formula are yall using to come up w/ these rpms? b/c ive got 33s and i blew my pinion gear in my rear axle and ive got a 4.88 ring and pinion but i dont know how bad its gonna be rpm wise. i plan on putting 38s on it ASAP maybe around feb. and also ima swap in some one tones around then well hopefully.
     
  7. pauly383

    pauly383 Daddy383 Staff Member Moderator

    Joined:
    Jul 22, 2001
    Posts:
    16,217
    Likes Received:
    1
    Location:
    Mesa , Arizona USA
    Speedometer , and tachometer . The charts are a rough guide .
     
  8. iwaxmyjimmy

    iwaxmyjimmy College web wheeler Premium Member

    Joined:
    Nov 3, 2006
    Posts:
    2,802
    Likes Received:
    34
    Location:
    Greenville NC
    my jimmy didnt come w/ a factory tach and its not that high on my list of things to do ya know. i was just tryin to get a ball park figure for where my cruising RPMs will be
     
  9. W7NB

    W7NB 1/2 ton status

    Joined:
    Nov 9, 2006
    Posts:
    716
    Likes Received:
    0
    Location:
    Springtucky, OR
  10. blazinzuk

    blazinzuk Buzzbox voodoo Premium Member GMOTM Winner

    Joined:
    Dec 20, 2002
    Posts:
    17,412
    Likes Received:
    3,799
    Location:
    Afton / Star Valley Wyoming
    From what I have heard most poeple say your cruising speed should be as close as possible to where your engine developes its peak torque. I am not sure what that is for your engine. Also remember in doing your calcs you need to measure your tire from the center of the hub to the ground this will change your heigth calculations somewhat. By doing that you will get a truer rpm vs speed chart.
     
  11. iwaxmyjimmy

    iwaxmyjimmy College web wheeler Premium Member

    Joined:
    Nov 3, 2006
    Posts:
    2,802
    Likes Received:
    34
    Location:
    Greenville NC
    according to them caculators i should be fine running 4.88s. says ill only be turning 2000 rpm at 57 mph. dunno if its right but i guess that makes everyone i talk to a dumba$$ i had one guy tell me i was gonna be spinnin around 4K rpms at 65
     
  12. AussieK5

    AussieK5 1/2 ton status

    Joined:
    Jun 17, 2001
    Posts:
    316
    Likes Received:
    0
    Location:
    A land girt by sea
    I use this site http://www.4lo.com/ , its simple and has some good info on trans gearing plus a rudimentry guide for choosing rpm range. Its got crawl ratios etc etc

    I also use a neat excel sheet that I found last year that lets you model your rpm range Vs speed, I find it quite handy. PM if you want a copy.

    Jas
     
  13. AussieK5

    AussieK5 1/2 ton status

    Joined:
    Jun 17, 2001
    Posts:
    316
    Likes Received:
    0
    Location:
    A land girt by sea
    A couple of months ago I checked this out by doing an rolling diameter experiment with the 33's on my K20. At the end of the day I concluded that you should just measure across the tire, the effect is negligable.

    [​IMG]

    [​IMG]

    The figures on the pavement represent psi in the tire at each test. After crunching the numbers I came to the conclusion there would be as much variance due to tread wear over the life of the tire.

    The rolling diameter is only very midly affected by the ratio of vertical (to ground) and horizontal measurements.

    Jas
     

Share This Page