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Thread: Lookin for gears.....

  1. #1
    Join Date
    Dec 2003
    Location
    Norco CA
    Posts
    51

    Post

    Heres what I got.......

    '97 GMC 4X4, 6.5 Diesel. With 33' tires. Towing 32' 5th wheel. Toal trailer weight, 12k.

    My speedo is off. When it shows 60, GPS says 65. My RPM's are at about 1800. She is a dog now.

    Before the tires I was pulling a little smaller trailer, and she was great. With the new tires slowed way down. New trialer, slowed a little more. I saw to look at the glove box for the codes, but what do I look at? The VIN? Or the other numbers that are there?

    What gears do yo suggest I run?

    Thanks for all the info. I have Gauges on the way and gonna get a chip too.
    97, GMC 2500, 4x4. <br />32\' 5th Wheel toybox.

  2. #2
    Join Date
    Feb 2001
    Location
    Collegeville, Pa
    Posts
    1,479

    Post

    Well I assume you have a 3:73 axle ratio, and it should be 4:11, or even 4:56. But you have two differentials to change out, which can be a little expensive. Your other option would be a "GearVendors" or "US Gear"underdrive gear box behind your tailshaft, but it will only work in 2 wheel drive. Now your looking at about $3000.00 till your done with cutting the drive shaft etc. Do you have a lift kit that changed the driveline angle? That would be a problem when adding the "GearVendors" You could look to swap differentials with a salvage yard, or someone looking for better mpg. Lastly, you could go back to 245 tires , But I guess you wouldn't want that.
    Tom McCauley (DP Member #513)
    "Tankers-ToyII" Loaded 05 D/A K-3500 CC, SRW. Deep pan on Allison w/Transyn. Edge Juice w/attitude, 4" Kennedy exhaust, 98 Gal. Transfer-Flow cross bed fuel tank, Leer 100XQ bed cap. Reese 14,000lb class V hitch w/ dual cam HP sway control. Tow 34ft. 32FKD Holiday Rambler travel trailer. GCW 20,360lbs

  3. #3
    Join Date
    Dec 2003
    Location
    Norco CA
    Posts
    51

    Post

    Where exactly in the glove box do I find the current gear size?

    I wont be going back to smaller tires, I go in sand, I need the big foot print to get my big trailer through the sand. No lift on the truck.

    Plus, its accually the wifes truck, she like the big tires. [img]tongue.gif[/img]
    97, GMC 2500, 4x4. <br />32\' 5th Wheel toybox.

  4. #4
    Join Date
    Apr 2001
    Location
    CA
    Posts
    13,577

    Arrow

    Have a look at this link:
    RPO Codes

    It should have what you are looking for. Just match up the code.

  5. #5
    Heartbeat Hauler Guest

    Post

    There is a SPID label, usually on the door of the glove box that has all of the RPO codes for your truck. Here are few codes to look for:
    GH0 : AXLE REAR, 3.54 RATIO
    GH2 : AXLE REAR, 2.29 RATIO
    GH3 : AXLE REAR, 2.77 RATIO
    GH4 : AXLE REAR, 2.92 RATIO
    GK9 : AXLE REAR, 4.63 RATIO
    GL3 : AXLE REAR, 6.17 RATIO
    GM1 : AXLE REAR, 2.59 RATIO
    GM3 : AXLE REAR, 3.45 RATIO
    GM8 : AXLE REAR, 2.56 RATIO
    GS4 : AXLE REAR, 3.70 RATIO
    GT4 : AXLE REAR, 3.73 RATIO
    GT5 : AXLE REAR, 4.10 RATIO
    GT7 : AXLE REAR, 3.33 RATIO
    GT8 : AXLE REAR, 4.10 RATIO
    GU1 : AXLE REAR, 2.41 RATIO
    GU2 : AXLE REAR, 2.73 RATIO
    GU4 : AXLE REAR, 3.08 RATIO
    GU5 : AXLE REAR, 3.23 RATIO
    GU6 : AXLE REAR, 3.42 RATIO
    GV2 : AXLE REAR, 5.83 RATIO
    GV4 : AXLE REAR, 3.36 RATIO
    GV8 : AXLE REAR, 2.72 RATIO
    GW4 : AXLE REAR, 3.31 RATIO
    GW6 : AXLE REAR, 3.27 RATIO
    GW9 : AXLE REAR, 2.93 RATIO
    You will notice there a several codes for a particular gear ratio. This is because my list includes PU trucks, cars, and med. duty trucks. Hope it helps.
    JP

  6. #6
    Join Date
    Jul 2003
    Location
    Western PA
    Posts
    4

    Post

    danhercules,

    What size tires were stock on your truck? What rpm's were you running before the switch to the 33's? It sounds as though you have 3.73's, in which case 4.10's would put you back in the ballpark of the same rpm's-for-speed. You might already have 4.10's though, which would mean you might need 4.56's
    Then again, if you do a lot of towing of that trailer with the truck, you might want to go to 4.56's even if you currently have 3.73's If you ever go back to a stock-size tire though, you will not like traveling down the highway with the 4.56's

    The following is assuming stock 4L80E ratio of .75:1 for OD. For towing in Drive, add approximately 33% to the rpms:

    33 inch tires with 4.56:1 gears:
    approximately 2275rpm at 65mph

    30 inch tires with 4.56:1 gears:
    approximately 2500rpm at 65mph

    33 inch tires with 4.10:1 gears:
    approximately 2050rpm at 65mph

    33 inch tires with 3.73:1 gears:
    approximately 1850rpm at 65mph


    I tow with a total combined weight of 14,000 lb, which is about 5,000 lb less than you. My truck had 4.10's, and I went to 3.42's I haven't realized any noticeable drop in power, however my towing mileage went from around 9mpg to 12+mpg. Of course, I don't have as tall as tires as yours, mine are 235/85's, or 31.7" This gives me about 1775rpm at 65mph

  7. #7
    Marty Lau Guest

    Post

    Originally posted by Scottpearce:
    danhercules,

    I tow with a total combined weight of 14,000 lb, which is about 5,000 lb less than you. My truck had 4.10's, and I went to 3.42's I haven't realized any noticeable drop in power, however my towing mileage went from around 9mpg to 12+mpg. Of course, I don't have as tall as tires as yours, mine are 235/85's, or 31.7" This gives me about 1775rpm at 65mph
    Scott; Did you run the stock tires size 245-75-16 before going to the 235-85-16's ? If so how much differece did the change in tire size make with the 3.42 gears? (i went to those also)
    Thanks.

  8. #8
    Join Date
    Jul 2003
    Location
    Western PA
    Posts
    4

    Post

    Scott; Did you run the stock tires size 245-75-16 before going to the 235-85-16's ? If so how much differece did the change in tire size make with the 3.42 gears? (i went to those also)
    Thanks.

    16ga SxS,
    When I bought the truck, it already had the 235/85's on it, although it was still running 4.10 gears. The stock size tires (according to my highly accurate GVWR sticker ) for my truck were 225/75-R16's however, not 245's? Maybe because it is a two-wheel drive? The 225's should be about 29.3 inches, and the 245's should be about 30.5 inches.

    The rpm difference with the 225's would be about 7.5% (higher for any given speed in OD), and the rpm difference with the 245's would be about 3.8% (also higher for any given speed in OD)

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