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Thread: Is it worth changing gears for better MPG's???

  1. #1
    16gaSxS is offline Moderator, Have Shotgun & dogs will travel
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    Question Is it worth changing gears for better MPG's???

    Time too time the topic of switching rear end ratio's comes up to save fuel and there for $$. Here is a little though and calculation I came up with on this topic. The conclusions will vary based on your cost and if you have 2 wheel drive vs 4 wheel drive. The calculations are based on 4x4.

    If you pickup 2 mpg going from a 4.10 to a 3.73 ratio which assume you average 16 mpg and then would average 18 mpg. 16 MPG would take 62.5 gallons to go 1,000 miles 18 MPG would take 55.5 gallons per 1,000 miles. That is a savings of 7 gals per 1K miles or $21 per 1k at $3 per gallon. So if it cost $1800 to swap gears ($800-900 per axle) $1800/$21=85.7k miles or 85,700 miles to break even. Now unless you tow heavy and a lot say over 7,000 lbs I would consider going up too 3.42 ratio the cost would be about the same and if you pick up an extra 1 MPG that would mean it would take 52.6 gallons per 1K miles or 9.9 gallons less or a saving of $29.70/1k miles. $1800/$29.7=60.6k miles or 60,600 miles to break even.
    So anything over 61,000 miles and your ahead of the game. The key is HOW LONG do you plan on keep your truck or really how many miles do you plan on keeping your truck??? If you keep the truck for about another 100,000 miles you would save ~ $1200 in fuel cost @ $3/gal and 3 mpg increase.

    Gear changing was something More Power talked about more in the early days at The Page.
    I did this calculations back when I had about 30,000 miles on my truck and did the swap at 38,000 miles and figured it at $1.35 per gallon for Diesel. It worked out that I need to keep the truck at least 60,000 miles to break even. MY goals/was to drive the truck too at least 300,000 miles. With about 110,000 miles past break even on my truck and with fuel cost well above the $1.35 I used I am way ahead.

    So. I believe it is feasible to change axel ratio's out and save enough to pay for it self PROVIDED you drive the truck enough.

    So if you only keep the truck for 20 or 30,000 miles after changing gears your only wasting money and helping the next owner.

    A couple other thoughts about changing rear end gearing, More Power said in another thread when he did it on his old K-5 Blazer it made the truck much more pleasant too drive.I found this true also, lower engine and drive train noise. Jim in the early days talked about extending engine life by going to 3.42's from 4.10's by an estimated 100,000 miles and maybe 50-75,000 miles going from 3.73's (my old ratio) too 3.42's. At the time with a New 6.5 this was important too me. Maybe it's not as important to folks who buy a now well used 6.5TD as it was then. I do think there maybe few folks like myself who buya truck for a long time and want to keep their long term cost and headaches to a minium.

    In conclusion I think changing gears is a good option for some but not all 6.5TD owners. If your considering this consider your towing profile, type of usage, and expected ownership time frame.
    95, 6.5, C-2500 extended cab shortbox, Auto transmixer 3.42 diff, Jardine exhaust system
    FSD Cooler replaced w/ Heath PMD Isolator
    Heath Turbo-Master Boost Controller, Max E Tork chip '97 cooling upgrades, Kennedy Fan clutch,
    Glow Plug over ride,DeeZee 38.5 gallon Aux fuel Tank 225K miles and rolling

    1994, 6.5 Blazer, 3:42 diff, 4 inch exhaust, Heath PMD Isolator, Heath Turbo-Master Boost Controller Max E Tork, Heath SB intake filter system.
    221K miles and rolling.
    Diesel Page member #81

  2. #2
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    I just wanna go faster!

    Didn't like the 75mph interstate traffic with me driving 70 and getting looks from other drivers like that was the fastest my rig could go. It could go faster but at the cost of fuel economy and my ears.

    6.5 4.10 gears..

    Planning on the 4.10 to 3.42 swap, doesn't make sense to go 3.73 since I don't pull too often and too heavy anyways (just the weight of another vehicle occasionally).

    This way, all RJ has to do is follow the black cloud

    J
    1982 C10 SCSWB 6.5TD, mods too extensive to list. (13.69 1/4 mile @94.6 MPH) RACE TRUCK
    1982 C10 SCSWB 6.2NA, 2.73 700
    1986 C10 SCLWB 6.2TD 3.73 700
    1989 V20 SUB 6.2NA, 3.73 400
    1994 G20 VAN 6.5NA, 3.42 60E
    1994 K20LD ECSWB 6.5TD, 3.42 80E
    1995 K20 SUB 6.5TD, Wrecked, ran into by stupid teen.
    1995 C3500HD DRW 6.5TD, 12' Flatbed 5.13 80E
    1995 C3500HD DRW 6.5TD, 18' Rollback Wrecker 4.63 80E
    1994 C20HD ECLWB 6.5TD 3.73 80E Wifes Truck.
    1995 C20LD ECSWB 6.5TD 3.73 80E
    1995 K20LD SCLWB 6.5TD 3.73 80E
    1996 K30 DRW 6.5TD 4.10 80E
    1997 C10 Tahoe 2Door 2WD 5.7L to 6.5 Conversion Underway

  3. #3
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    Arrow

    I agree with John & Marty... Switching gears from 4.10 to 3.73 might not be worth the investment. If your use of the truck is primarily non-towing, then 3.42 is your best choice.

    I encourage people to look at how they use their truck to determine whether they should invest in a gear swap. If you use your truck to tow heavy, then stick with 4.10's. You need the gear reduction. Otherwise, you'll notice a big improvement in driving pleasure as well as a bump in fuel economy when going from 4.10's to 3.42's for a non-towing or light towing use.

    The cost of a gear swap is highly variable - depending on whether you can do the work or if you hire the work out. The parts aren't really all that expensive, but getting the gears set up correctly is vital for a long and noise-free gearset life.

    To get an idea of what the driving experience will be like when going from 4.10's to 3.42's, take your truck up to 65 mph out on the highway, then put the trans in neutral with the engine idling. After switiching to 3.42's and while running in OD, the engine noise almost completely goes away....

    Back in the mid 90's I added an OD auto (it had a non-OD auto) and a Banks turbo to a 6.2 GMC K1500. If I were forced to choose just one of these two upgrades, I would choose the OD auto - just for how nice it made the truck to drive on the highway - regardless of how it affected mpg (although it added about 5 mpg). The truck already had 3.42 gears.

    Jim

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    Default If I did it again!

    It would be 3.73 for me the next time around because of the autotrac. What would be nicer is a stand alone tranny controller so I could stay w/3.42's.
    99 GMC SUB, 2500, 4x4, 6.5 TD (F) #H32 enhanced blk, Phaser timing set, B&D IC, 3" ATS exh, K&N w/prefilter, 4WD NVG 246, 4L80E kevlar Transgo off road/tow mods/B&M supercooler, 14 FF MagTec w/locker 3.42:1, 9.25" IFS frt/diff 3.42:1 w/ARB locker, AutoMeter Boost & pre-turbo pyro meter, 12,000 lb hydro winch, Warn classic bumper w/brushguard & diesel scoop, Max E-torq Stage 3, hi pop inj, oil bypass sys, on board air comp for front ARB locker & tire inflation, DynaView driving/auto-cornering lts, DZ nerfs, A/T 285/75/16 SilentArmor 3750 lb rated on 3750 lb rated Alcoa, 3 IP drivers 2 r spares.

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    [font=times][size=+]Good Day!

    I picked up ~ 2 mpg going from ~ 3.80 effective ratio (4.10 gears w/ 8% oversized tires) to 3.08 effective ratio (4% oversize tires + Gear Vendors Aux. OD).

    To figure out when a modification will pay for itself, put this in your spreadsheet program (do NOT input the quotes
    82 6.2NA K15 4X4 pickup, 4spd man w/ OD, 335K+ "In Rust We Trust" (parked)
    95 6.5TD 2500 4X4 pickup, Gear Vendors Aux. OD, > ¼ million miles - gone
    95 6.5TD 1500 4X4 3/4T Suburban, Kennedy exhaust, > ¼ million miles
    93 6.5TD 3500 4X4 1T crew cab LB pickup, 230k miles

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    How about the 5sp manual trannys. They about the same ratio as the automatics?

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    Good info!

    What's funny is that my 4.10 geared powerhouse 6.2L turbo truck gets about the same mileage as my 3.42 geared 6.5L family car...around 16-17 mpg.

    However, the Tahoe will cruise at 75 mph all day long without breaking a sweat and without much noise. The truck struggles to get to that speed, and the noise is quite annoying. Both vehicles have overdrive transmissions.

    The truck is designed to tow trailers, and does a marvelous job at it. But it sure doesn't like long-distance interstate driving.

    Casey
    1995 K1500 Tahoe 2 door, 6.5LTD, 4L80E, NP241, 3.42's, 285/75R16 BFG K02's; 1997 506 block; Kennedy OPS harness, gauges, Quick Heat plugs, and TD-Max chip; Dtech FSD on FSD Cooler; vacuum pump deleted, HX35 turbo, Turbo Master, 3.5" Kennedy exhaust, F code intake; dual t/stats, HO water pump, Champion radiator; Racor fuel filter

  8. #8
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    Quote Originally Posted by bmp34
    How about the 5sp manual trannys. They about the same ratio as the automatics?
    IIRC, the 4L80E overdrive ratio is 0.74:1 and the NV4500 is 0.71:1
    The Constitution needs to be re-read, not re-written!

    If you can't handle Dr. Seuss, how will you handle real life?

    Current oil burners: MB GLK250 BlueTEC, John Deere X758
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    I just got my truck, 1996 2500 6.5td, 5spd, xcab 4x4, at the end of May. The guy I bought the truck from didn't know the diff. ratio. How would I find out what mine is? I pressume it's stock because everything else was. My buddy thinks they're 3.73, but I don't have a clue. The only hint I have is that, unless I have a load on, and am stopped on a hill, I don't use 1st gear. Cheers! Brett

  10. #10
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    I thought the OD ratio's for the auto vs manual were opposite of what John wrote, ie the auto was a .71 vs tha manual at .74. Where the auto lose's, is the first gear effective ratio. The manual depending upon yr, is 5.64 for later NV4500's or 6.34 for the early IIRC pre 94 NV4500's or ANY of the dodge tranny 4500's. Where as the auto first gear effective ratio is about 4-1. A 1.64-1 difference, quite abit when it comes to take off! You can have 3.08's with a manual, and still have better take off than a auto with 4.10s, and if you can drive in OD, what should be great MPG's!

    If you tow, be carefull about changeing out gears, you may lose all you take off ability if you go too tall.
    MartyB
    05 DW3500 D/A crewcab
    2000 C2500 reg cab 350/auto
    mem# 2825

  11. #11
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    what is lowest ring and pinion you can get for a 4x4? The lowest i have seen is 3.42.
    93 chevy ext cab long box,
    6.5 turbo diesel-4x4-5sp manual.
    186k miles. Cold air intake,
    4in exhaust.

  12. #12
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    Quote Originally Posted by DA BIG ONE
    It would be 3.73 for me the next time around because of the autotrac. What would be nicer is a stand alone tranny controller so I could stay w/3.42's.
    You and I have similar trucks. Why is the auto track a problem with 3.42
    Art Paltz
    1999 Suburban K2500 6.5TD (stock)
    2000 Undercover Dragster, 468 BBC, 7.74@173MPH, waiting on new 622 aluminum BBC to be finished.
    1992 Tube Chassis Camaro, 468 BBC, 8.54@157MPH (SOLD)
    1987 Buick Grand National, 11.8@114, pump gas (for sale)
    1969 Camaro SS/RS 396-350HP, stock restoration, it never leaves the garage...

  13. #13
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    Default Gear?

    [QUOTE=TurboDiverArt]You and I have similar trucks. Why is the auto track a problem with 3.42
    99 GMC SUB, 2500, 4x4, 6.5 TD (F) #H32 enhanced blk, Phaser timing set, B&D IC, 3" ATS exh, K&N w/prefilter, 4WD NVG 246, 4L80E kevlar Transgo off road/tow mods/B&M supercooler, 14 FF MagTec w/locker 3.42:1, 9.25" IFS frt/diff 3.42:1 w/ARB locker, AutoMeter Boost & pre-turbo pyro meter, 12,000 lb hydro winch, Warn classic bumper w/brushguard & diesel scoop, Max E-torq Stage 3, hi pop inj, oil bypass sys, on board air comp for front ARB locker & tire inflation, DynaView driving/auto-cornering lts, DZ nerfs, A/T 285/75/16 SilentArmor 3750 lb rated on 3750 lb rated Alcoa, 3 IP drivers 2 r spares.

  14. #14
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    Quote Originally Posted by DA BIG ONE
    I'm thinking as 4x4 2500 burb is raised catching more air the 3.42 is ok w/stock, even slightly larger sized tires, after this I think the drivetrain gets loaded "height/weight/speed" and defeats any would be gains milage wise. However, the tires I used were 285/75/16 GoodYear MT/R's so that may be the problem, now using 285 SilentArmor which is more highway friendly, so flip a coin now. The forged alcoa's 16" x 7"w/285 silentarmor weigh about the same as the factory 16" x 6.5" riveted steel wheel w/stock 245 tires.

    I'm thinking, stay w/the gears you have now and go for larger diameter tires that would equate to 3.42 +- & don't use a too aggressive tread pattern. You'll be looking at od/lockup @ 60mph +-.

    As for towing? With my mods, I don't see much of a loss except from a stop but I don't really tow too much.
    That's sort of my feeling. I really don't stomp on it much from a stand still so I don't expect to see much difference. For towing, based on the last rendezvous pull-off, the stock pickup with 3.42 gears pulled the heavy tow trailer in about the same times as a stock truck with lower stock gears so I don't think towing is too much effected.

    When you have the 3.42's and the VSS hacked so speed is corrected, is the OD-converter lockup speed raised from 48 to 60 MPH? I guess I just assumed the lockup parameters would stay the same if the VSS were properly set.

    Art.
    Art Paltz
    1999 Suburban K2500 6.5TD (stock)
    2000 Undercover Dragster, 468 BBC, 7.74@173MPH, waiting on new 622 aluminum BBC to be finished.
    1992 Tube Chassis Camaro, 468 BBC, 8.54@157MPH (SOLD)
    1987 Buick Grand National, 11.8@114, pump gas (for sale)
    1969 Camaro SS/RS 396-350HP, stock restoration, it never leaves the garage...

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    Default VSS Hack

    Quote Originally Posted by TurboDiverArt
    That's sort of my feeling. I really don't stomp on it much from a stand still so I don't expect to see much difference. For towing, based on the last rendezvous pull-off, the stock pickup with 3.42 gears pulled the heavy tow trailer in about the same times as a stock truck with lower stock gears so I don't think towing is too much effected.

    When you have the 3.42's and the VSS hacked so speed is corrected, is the OD-converter lockup speed raised from 48 to 60 MPH? I guess I just assumed the lockup parameters would stay the same if the VSS were properly set.

    Art.
    Yes, hacking the VSSB only gives od/lockup @ 60+-.

    I have hacked the signal to the vssb from the VSS and the shift points for the 4.10 gearset remained even though gears are now 3.42. However, I get some banging of gears on occasion, I'm thinking this is because I have a shift kit (transgo towing/off road), and the PCM is upgraded with shift mods too. It shifts fast when driven normal, holds gears a little longer when I stomp it.
    Looking now at hacking vssb for 3.42 gearset, then modifing the signal from vss for tire changes, this will keep the od/lockup at 55 mph+-.
    Keep in mind I have a full tilt 4L80e w/all updates so it may take more of a beating than say a tranny w/shift kit. So, proceed w/caution when hacking just the signal from vss to vssb.
    99 GMC SUB, 2500, 4x4, 6.5 TD (F) #H32 enhanced blk, Phaser timing set, B&D IC, 3" ATS exh, K&N w/prefilter, 4WD NVG 246, 4L80E kevlar Transgo off road/tow mods/B&M supercooler, 14 FF MagTec w/locker 3.42:1, 9.25" IFS frt/diff 3.42:1 w/ARB locker, AutoMeter Boost & pre-turbo pyro meter, 12,000 lb hydro winch, Warn classic bumper w/brushguard & diesel scoop, Max E-torq Stage 3, hi pop inj, oil bypass sys, on board air comp for front ARB locker & tire inflation, DynaView driving/auto-cornering lts, DZ nerfs, A/T 285/75/16 SilentArmor 3750 lb rated on 3750 lb rated Alcoa, 3 IP drivers 2 r spares.

  16. #16
    16gaSxS is offline Moderator, Have Shotgun & dogs will travel
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    Quote Originally Posted by TurboDiverArt
    That's sort of my feeling. I really don't stomp on it much from a stand still so I don't expect to see much difference. For towing, based on the last rendezvous pull-off, the stock pickup with 3.42 gears pulled the heavy tow trailer in about the same times as a stock truck with lower stock gears so I don't think towing is too much effected.

    Art.
    Art;

    That was my truck at the Pull Off your talking about. And I tow a lot about 5,000 lbs. I have towed bumper pull Travel Trailers from Yuma Arizona to Great FAlls Montana right up through the middle of several Mountain ranges. The largest TT I have pulled was 27 foot and my truck handled it fine and that was before the Heath MaxiTorq EPROM. This is always in January or February how ever. I have had the 3.42 gearing for about 170,000 miles and I feel that with my set up I can two 7-8,000 lbs with no worries. If I was towing more than that on a regular basis I would have the 3.73 gears put back in. Also it is not just weight but Aerodynamic drag has big affect. The last TT I brought back was a "18" foot but did include the hitch so it was about a 22 foot if you include the hitch to bumper. The weight wasn't bad empty like 4,700 lbs but I got the worse fuel economy pulling this trailer than any I have pulled. I pulled the passes fine but the thing was very Areodynamic dirty. Frontal area like a 2 4x8 sheets of plywood, high clearance axels, A/C and storage locker on the roof and TV anenta, and awning, crank down jacks hanging dwon on all the corners. This TTweights about the same as mine but mine sits lower too the ground, sloped front end, not much on the roof no A/C storage locker or jacks hanging down. I get 12 to 13 MPG towing it were the Dirty trailer I averaged 10 MPG. The difference between 10 & 12 MPG 1s 20% and the 10 MPG TT takes 17 gallons more fuel on a 1,000 trip. Thats a $50 bill at $3 per gallon. So to wrap this up I find the 3.42's do a good job towing in the lighter and middle weights for towing. I don't mind towing heavier once in awhile.
    95, 6.5, C-2500 extended cab shortbox, Auto transmixer 3.42 diff, Jardine exhaust system
    FSD Cooler replaced w/ Heath PMD Isolator
    Heath Turbo-Master Boost Controller, Max E Tork chip '97 cooling upgrades, Kennedy Fan clutch,
    Glow Plug over ride,DeeZee 38.5 gallon Aux fuel Tank 225K miles and rolling

    1994, 6.5 Blazer, 3:42 diff, 4 inch exhaust, Heath PMD Isolator, Heath Turbo-Master Boost Controller Max E Tork, Heath SB intake filter system.
    221K miles and rolling.
    Diesel Page member #81

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    Default 3.42:1

    ART, Just a tire size change to 285/75/16 and your 3.73's will give you a final of 3.42+-
    99 GMC SUB, 2500, 4x4, 6.5 TD (F) #H32 enhanced blk, Phaser timing set, B&D IC, 3" ATS exh, K&N w/prefilter, 4WD NVG 246, 4L80E kevlar Transgo off road/tow mods/B&M supercooler, 14 FF MagTec w/locker 3.42:1, 9.25" IFS frt/diff 3.42:1 w/ARB locker, AutoMeter Boost & pre-turbo pyro meter, 12,000 lb hydro winch, Warn classic bumper w/brushguard & diesel scoop, Max E-torq Stage 3, hi pop inj, oil bypass sys, on board air comp for front ARB locker & tire inflation, DynaView driving/auto-cornering lts, DZ nerfs, A/T 285/75/16 SilentArmor 3750 lb rated on 3750 lb rated Alcoa, 3 IP drivers 2 r spares.

  18. #18
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    Quote Originally Posted by DA BIG ONE
    ART, Just a tire size change to 285/75/16 and your 3.73's will give you a final of 3.42+-
    I wasn't intending on making a big change in tread width. I think increasing the cross section of the tire from 9.6" to 11.2" will probably have a pretty big adverse effect on MPG. Additionally I think it'll be much louder.

    If I kept the stock height I was planning on using a 245/70-17 tire.

    If I went up I was thinking of using a 255/75-17. I think this brings me pretty close. Might not be down to 3.42's but definitely at least 3.55's. Increases the height of the tire from 30.46
    Art Paltz
    1999 Suburban K2500 6.5TD (stock)
    2000 Undercover Dragster, 468 BBC, 7.74@173MPH, waiting on new 622 aluminum BBC to be finished.
    1992 Tube Chassis Camaro, 468 BBC, 8.54@157MPH (SOLD)
    1987 Buick Grand National, 11.8@114, pump gas (for sale)
    1969 Camaro SS/RS 396-350HP, stock restoration, it never leaves the garage...

  19. #19
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    Default Hack

    Quote Originally Posted by TurboDiverArt
    I wasn't intending on making a big change in tread width. I think increasing the cross section of the tire from 9.6" to 11.2" will probably have a pretty big adverse effect on MPG. Additionally I think it'll be much louder.

    If I kept the stock height I was planning on using a 245/70-17 tire.

    If I went up I was thinking of using a 255/75-17. I think this brings me pretty close. Might not be down to 3.42's but definitely at least 3.55's. Increases the height of the tire from 30.46” to 32.06” or 1.6”. I think the ideal height change is probably closer to 2” to get it real correct. Don’t know how precise you can be with correcting the speed sensor.

    Not having the converter lock up until 60 MPH is a little bit of a concern for me though. That means that on most state routes around here where the speed limit is 50 MPH I probably won't get the converter to lock up as I usually don't go more than 10 MPH over the speed limit as there are just too many police waiting to give me a ticket. It’s nice that at 50 MPH the converter will lockup. Did know that the 1500 Suburban’s with stock 3.42’s locked up at 60. Does this mean that a truck with 4.10's locks up before 48 mph?

    Can someone point me to the article regarding hacking the VSS/VSSB?

    In your post above you mention modifying the signal from the VSS (at trans?) as well as modifying the VSSB? I guess I didn’t think that there were two things in play here that could be modified. My thought was to hack the sensor signal (resistor or something) under the dash. Thought doing this would correct for the gearing so that based on output shaft speed the gauges and computer would read speed correctly. Again, I didn't know that this had an effect on how the PCM instructed the transmission to lockup. Does it also have an effect on shift point? I think I'm confused on how this works. My assumption was that you were correcting the speed but the computer still functioned the lockup at the same speed regardless of gearing, 48MPH for 3.42’s through 4.10’s.

    Art.
    Art, I'm thinking closer to 45mph for od/lockup w/4.10's. I also think using cruse control locks converter too.

    Tire width does effect milage, but tread pattern plays bigger role the MT/R's I used made mpg suffer, the SilentArmor is more road friendly and quieter so not as bad.

    Your vss is on the t-case along w/2 other speed sensors, 1 for the front drive shaft, 1 for the rear driveshaft. I just discovered the "vss" hack works a little while ago.

    Hacking the speed signal from the vss to the vssb will correct the speedo only and leave the 3.73 shift/lockpoints you have now.

    Hacking the vssb you modify speedo, shift/lockup to higher mph setting, abs, cruse control. TheDieselPage sells the book showing the hack of VSSB.

    I used a commercial VSS re-calibrator to modify vss signal only.

    Getting accurate on speedo calibration comes w/hack either way, especially if you measure tire diameter and use this measurement in re-cal.
    99 GMC SUB, 2500, 4x4, 6.5 TD (F) #H32 enhanced blk, Phaser timing set, B&D IC, 3" ATS exh, K&N w/prefilter, 4WD NVG 246, 4L80E kevlar Transgo off road/tow mods/B&M supercooler, 14 FF MagTec w/locker 3.42:1, 9.25" IFS frt/diff 3.42:1 w/ARB locker, AutoMeter Boost & pre-turbo pyro meter, 12,000 lb hydro winch, Warn classic bumper w/brushguard & diesel scoop, Max E-torq Stage 3, hi pop inj, oil bypass sys, on board air comp for front ARB locker & tire inflation, DynaView driving/auto-cornering lts, DZ nerfs, A/T 285/75/16 SilentArmor 3750 lb rated on 3750 lb rated Alcoa, 3 IP drivers 2 r spares.

  20. #20
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    Who's a good source now days for the gear sets?

    Greg
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