5.13 gears

Discussion in 'OffRoad Design' started by adamcarp, Jan 28, 2001.

  1. adamcarp

    adamcarp Registered Member

    Jan 20, 2001
    Likes Received:
    sheridan ca (sacramento)
    I asked this on the other forum but would to here ORD's opinion. any experience with running real low gears
    like 5.13 or 4.88 I am swapping in a 14 bolt and want to regear at the same time have 3.73 now with 36"
    swampers. not low enough crawl. would like suggestions on rpm/fuel economy with these low gears.
    stock 350 engine 350 trans 203 case.

    78 blazer
  2. Stephen

    Stephen 1/2 ton status Moderator Vendor

    Sep 15, 2000
    Likes Received:
    Carbondale Colorado
    i'd be leery of the low diff gears with a TH350 trans and the 36's. you should find a tire size/gear ratio/RPM calculation formula or chart to figure this out. I have one that we may be able to make available on our web page that works really well.
    There are several factors you'll want to take into consideration: Max speed cruise, highest RPM you are comfortable running continuously at high speed cruise, power band of your motor, and gear strength with the lower gears. Also, are you going up in tire size later? going down? run more than one tire size, like smaller snow tires, etc.
    You'll want to decide what kind of high speed cruise you want to have, and this is where the TH350 trans will hurt you. If you want to be able to drive the thing 80 mph, you'll probably want a 4.10 gear, maybe a 4.56 at the most.
    Also along these lines, you'll need to run some rpms in a small block to be able to push the truck very fast. it takes a lot of power to make a k5 go maintain high speeds. A small block should be fine running 2800 rpms or more continuously. milage may not be great, but it will do it. I'm wishing I had deeper gears than the 4.56's I have now because I can't drop into overdrive very much with the 37's. A lot of this has to do with how your motor is built. If it has a gob of low end torque, take that into consideration.
    Gear strength: 5.13's should be fine in the 14bolt, but if you run a smaller front axle, you might want to consider staying with lower numbered gears for strength. Of course in a front, you'll probably break the ujoints before you damage the r&p set. With a D60 in front you should be fine with the 5.13's.
    You might also look at carrier swaps and things like that. You'll probably need a new carrier if you go to really deep gears, so if you buy a locker you need to really think it out.
    An overdrive changes everything, so if you're considering a th700 at all, you can go deeper. i'm going to 5.13's and with the th700 I figure I can run a 37 or 38" radial and have a good cruise on the highway if I want to, and may be able to drop it into the od gear even with the 42's. But with a 1:1 high gear this would not be something you could drive on the highway with less than a 40" tire, and still be able to maintain a safe speed.
    You can also use the Doubler to keep your diff gears reasonable. Basically you could keep the 3.73's, or go to 4.10's and still have a good cruise rpm, then use the doubler for the off road gearing. It could be cheaper and more effective to leave the diffs alone and put in a doubler. At that point the diff gears are only for street and highway driving and the t-case gears do the off road work.
    This is a lot of stuff to think about, but it does all come into play.

    Making the world better, one truck at a time.

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