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700r4, 5.13 or 5.38 42 inch tires

Discussion in 'The Garage' started by Big Truck, Apr 19, 2007.

  1. Big Truck

    Big Truck 1/2 ton status

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    I have an 84 blazer with 38.5sx's with 4.11 gears and can hardly use Od, I can trade a guy striaght across for 42 tsl's but I still want to use overdrive. I know I have to regear anyway with the 38.5s and 4.11s so might as well regear and change the tire size at the same time. My specs are a mild 350 engine/BTO700r4/np208/44/14bff. So what gears would you guys recomend 5.13 or 5.38, 42's would be the biggest that I would run.
     
  2. Xtremegmc80

    Xtremegmc80 1/2 ton status Premium Member

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    not too sure whats the best. a friend of mine has 38.5 and 700s but he has 5.13s.. has alot of power at that combo. so i would say run 5.38s
     
  3. Hossbaby50

    Hossbaby50 3/4 ton status

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    5.38's for sure with a 700R4. I would even say go deeper but you can't with a 14bff.

    Harley
     
  4. gmc4cw

    gmc4cw 1/2 ton status GMOTM Winner

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    do some research on the 5.38's for a 14 bolt. there is along thread over on Pirate. the way they are cut the pinion support is compromised. if it's trail only, who cares, but if you are running them on the street it may make a difference.

    you have the right carrier for it. they do not make a 5.38 gear for a 4.56 carrier. that and the pinion gouges will probably push me towards 5.13's.
     
  5. resurrected_jimmy

    resurrected_jimmy 1/2 ton status

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    5.13's..... Having read some of the 5.38 installs I don't think they are worth it
     
  6. desertrat67

    desertrat67 Hawk Driver

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    I had 4.56 and 39.5 Iroks and couldn't use overdrive much either. I am installing 5.13s right now and will be able to let you know by this weekend how it works out. I do have a diesel though, so it might not be an exact comparo.
     
  7. Copasetic

    Copasetic 1/2 ton status

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    I did a lot of homework on this as well and I'm going with 5.13's but I will be running 44's on mine. I'm not going to be doing much long highway driving, if any at all, so if I can't use my OD it's no big deal to me.
     
  8. Big Truck

    Big Truck 1/2 ton status

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    I have read the thread on pirate about the 5.38s But i'm hoping that they will get the problem fixed soon because I don't trailer my rig when I go wheelin, and it would sure help to have that extra gear.

    desertrat67 if you could let me know how those gears work it would be much appreciated.


    Anyone else running 42s and an overdrive tranny with 5.13s
     
  9. Greg72

    Greg72 "Might As Well..." Staff Member Super Moderator

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    Here's what CrawlCAD has to say about your potential gear swap:

    [​IMG]

    The upper set of numbers is your "current" configuration, the lower set is the hypothetical swap to 42's and 5.13s...

    1378RPMs at 55MPH would certainly explain why your truck doesn't want to stay in OD on the highway! :yikes:

    The gear/tire swap will bring the rpms up a bit but maybe still not enough to keep your tranny in OD. The "effect" will be very similar to people who are running 4.56s and 38s (since the highway rpms are almost identical to that combo) so you can ask around and see if anyone runs that combo too)

    As a last resort, you could regear to 5.13s and run the 38.5s on the highway....that would probably be about right (~1750rpm @ 55).



    :usaflag:



    The upper set of numbers is your "current" configuration, the lower set is the hypothetical swap to 42's and 5.13s...
     
  10. desertrat67

    desertrat67 Hawk Driver

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    As soon as I get them in and driven Iwill let you know.

    Gregg, that program you designed is great. It looks like with the 6.2 it will put me about where I need to be to keep overdrive usefull with 5.13s and 39.5s(interpolating the mid point between 42s and 38.5s).
     
  11. 6.2Blazer

    6.2Blazer 1/2 ton status

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    5.13 and 5.38 gears are really close to each other (less than 5% difference) so don't spend too much time trying to choose between them. With all other things equal I would go with the 5.38's, but if the manufacturing issues are still present, or if availability or price of the 5.38's is bad then I would not hesitate to go with 5.13's.

    In any case I think you still might be dogging the engine with the lower gears and 42's because 1,500-1,600 rpm is still pretty low considering the weight of rolling resistance of a 42" tire.
     
  12. smalltowncowboy

    smalltowncowboy 1/2 ton status

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    well if you would fix your truck or buy a truck to pull your blazer you could. :haha: :haha: :haha:
     
  13. muddybuddy

    muddybuddy 3/4 ton status

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    5.13s unless you run a d70, then run 5.86's
     
  14. desertrat67

    desertrat67 Hawk Driver

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    I took the sub for the first drive tonight with the 5.13s and am very pleased. Overdrive is usable above 50-55 or so and it has a pretty good kick at the bottom end. My tires and gearing must match the final ratio of the stock tires and gearing because my speedo isn't off by much either. I passed one of those "your speed is" signs reading 30 on my speedo and it read 27. I figure subtract 10% and i'll be good. It held 65 indicated on the highway until I hit a hill then it downshifted. I figure not bad for a 17 year old 6.2 diesel. Hope this helps.

    ryan
     
  15. Big Truck

    Big Truck 1/2 ton status

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    Desertrat67 thanks for your reply, I think that's a good combo and I will probrably get 5.13s.


    smalltowncowboy I just picked up a 350 to put in my truck to replace the 305, no more coolant in the oil so hopefully it'll be running sooner than later.
     
  16. muddybuddy

    muddybuddy 3/4 ton status

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    glad to hear it desertrat. i was going to run the 5.13s with my diesel as well, but i sold those axles and am going to run 4.56's so ill be in the 1800-1900rpm range at 65mph.
     
  17. desertrat67

    desertrat67 Hawk Driver

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    I had 4.56s in before and overdrive was pretty much useless, but since you have a 400 you should be sitting pretty at speed, just a little high geared for low stuff.
     
  18. muddybuddy

    muddybuddy 3/4 ton status

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    well im actually swappin in a 4L80E for overdrive, but its .75 instead of the .70 with the 700r4. according to the charts, ill be at about 1850rpms at 65mph, which right where i want to be for peak mpg :)
     
  19. desertrat67

    desertrat67 Hawk Driver

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    well, maybe 4L80 someday if I keep it on the street, but not now. If I ever get a trailer and tow only, i'll go TH400 with a lower first( I believe a 3:1 is avail) and add a black box.
     
  20. SkysTheLimit

    SkysTheLimit 1/2 ton status

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    X2. That's the exact gear ratio I'll be running when I'm finished & on 38"-40" tires.

    It just works out so nicely. nice deep crawl ratio and good powerband on the freeway. The speed limit is 70 (here) so you can't go no faster, might as well have a blast getting there...
     

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