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Thread: Max fuel economy for the 6.5TD

  1. #101
    Join Date
    Apr 2001
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    Newberg Oregon
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    Your Tahoe is very similar in size to a K5 Blazer and they would do 25 mpg hwy with the 6.2 n/a engine.

    The 6.5 td has never been a miser on fuel.

    Our 94 Burb got 20 mpg once on a trip, but usually it get 17

    Missy
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2007 Chevy Avalanche
    THIS IS BOW TIE COUNTRY

  2. #102
    Join Date
    Apr 2010
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    359

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    Quote Originally Posted by dragonriot View Post
    So this thread was necro'd about a year ago, but after reading everything I thought I would chime in with my situation as well.

    My truck is very stock on the outside... no Chicken Lights, stock mirrors, no bug shield, and nothing else that would increase the Aerodrag on the truck. I have fairly large tires, about 32x10x16.5 in US measurements, and hardly ever tow anything larger than a 150lb boat on a Sunfish Trailer. (speaking of which, I need a new sunfish trailer, if anyone has one laying around) My mods include a Turbo-Master set to Heath Specs, a performance chip from Heath Diesel, newer glowplugs, new PMD on a FSDCooler, removed Air Intake Ring, and don't have anything blocking the radiator or bumper holes in the winter (though I know I should.)

    At 65mph in Overdrive, I'm running at ~1950RPM, and in Summer I get about 21mpg highway and 18 in the city. In Winter, with Primrose Winter Power fuel additive, the best I've seen is 17MPG on the highway and 15 in the city. I don't know my final gear ratio, but I think it calculates out to about a 3.42 rear end, as I got a 3.10 calculation from my Overdrive RPM. When I checked it without overdrive, I was hitting about 2400RPM at 65 in third gear, and it just didn't sound very nice so I have only run that way once for the purpose of testing.

    I would like to get up to 24mpg without spending a crapton of money on the mods, as the truck already has 221,000 miles on it. I still have thoughts of doing a Cummins 4BT swap eventually for the magical 35MPH they are advertised to get in these trucks, but I'm pretty sure it would be cheaper in the long run - even with increased repair costs - to buy an older mercedes 300D or VW TDI when my engine decides to go the way of the dodo. I know that's blasphemous to say in here... but our trucks are becoming more rare, and harder to find parts and replace big ticket items.

    Actually, if you`re getting 21 MPG right now, you`re probably getting better than 75% of the people running a similar truck.

    Most are somewhere in the 14-17 mpg range.

    Looking at your specs about all I can suggest is giving Bill a call and see what can be done to your tune. Most calibrations are for performance and mileage is often a happy byproduct. Bill might be able to squeeze a few more MPG out of your truck, but I would be skeptical of a 220,000K 6.5 TD getting to much closer to 24.

    Good luck with your quest.

    1998 k2500

  3. #103
    Join Date
    Oct 2011
    Location
    Collingswood NJ
    Posts
    25

    Default 1999 2500 Suburban 4x4 mpg

    I just had my 4:10 ratio axle replaced with 3:42 after much reading and thought about it. I also had the factory posi replaced with an Eton unit.

    Thanksgiving of 2011 I towed my old 91 Montero 4x4 to Maine on a U Haul trailer and got around 15 mpg from NJ. I didn’t go past 55 mph for the trip.
    On the trip home I got 18 mpg but had to keep the speed under 60 mph as I didn’t want to go past 2000 rpm for my 6.5. It was tuff having every one pass me for the whole trip home! So it was bitter sweet, good mpg but a very slow drive.

    This Sub was very well taken care of by the pervious owner. It has 1149000 miles on the clock and runs and starts well.
    The only mod I can see is the FSD has been replaced and relocated in the front bumper. Also the exhaust system is 4”. I replaced the straight through type muffler with a 4” reveres flow muff to lower the noise level inside.

    I am planning another trip to Maine in July and will let you know what mpg I get with the new gearing.

    So far I like the 3:42 gearing MUCH better than the 4:10
    I have no plans to do any heavy hauling with my Burb. I bought it for long trips and hauling big stuff inside with the 3rd seat removed. (a 4x8 sheet of dry wall or plywood fits right in!
    So far:
    Less engine and exhaust noise (no one talks about that)
    Less engine wear per mile
    Hopefully better mpg

    Some one said the having 3:42 gearing makes your truck or Sub a dog! I disagree! It’s a diesel and now the rpm’s are 1200 to 1500 for everyday driving around my NJ suburbs right where it should be, in my opinion.

    The Trany Shop offered to recalibrate the vehicle speed sensor buffer but I felt I could do it after reading about it in the Vol 1 Book about the 6.5.
    Has any one out there done that mod them selves and or installed the 7-position dip switch instead of the jumper settings?

    PS: FYI I wanted to install a overdrive unit but found out from the mfg that the “Auto” type transfer case that is in my Burb is a one piece unit and can not be split apart for the install, other wise I would have gone with the overdrive unit…
    The manual transfer case will work with the overdrive unit I looked at.

  4. #104
    Join Date
    Jul 2003
    Location
    Allentown, PA
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    1,355

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    VSSB re-cal... yes, have done it. I bought a DIP switch (after I did the recal) with the anticipation of soldering it in for future use but never did. Either way, it's a pretty easy-peasy thing.
    1998 K2500 Suburban 6.5L TD 3.73 rear, Ron Schoolcraft 18:1, Kennedy ECM & IC, Timing gears, Splayed main caps, 3.5" Kennedy Exhuast/No Cat, K&N Filter, Boost/Tranny Temp/EGT(Pre Turbo), Ceramic-coated Manifolds, 195 Stat's, 265/75's (VSSB Adjusted) 7,000lbs (on a scale) Remote Mount Oil Filter, Remote Oil Pressure Sensor

  5. #105
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    Oct 2011
    Location
    Collingswood NJ
    Posts
    25

    Default Vssb

    Can you tell me where you bought the dip switch?
    Radio Shack no longer sells the one shown in the Article written in the Turbo Diesel Vol 1 Page 51
    Also I can not fig out how you translate the divide code into ones and zeros.

  6. #106
    Join Date
    Apr 2010
    Posts
    359

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    1998 K2500, ECLB, Auto, 4:10, 265 75 16.

    Latest results are 18 MPG combined.

    Combined is roughly 60% highway, 40% city. Highway speeds not above 100 Kph, windows up, AC on.

    Just switched rims to a set of PY0's from a later 2500HD. Requires far less throttle for same rate of acceleration and lighter touch highway to maintain speed.

    Weights:
    steel wheels scaled at 34 lbs each
    PY0's scaled at 17 lbs each

    That's 50% savings at each wheel in rolling mass.

    Hoping for a small return on the less rolling mass. Would be overjoyed with even 1 mpg increase.
    1998 k2500

  7. #107
    Join Date
    Sep 2001
    Location
    Ft. Wayne, In.
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    501

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    Wow!
    While 68 lbs are a lot go haul down the road, I'd be surprised if you gained 1 mpg.
    Keep us updated on how much you gain!
    Dave, N9LOV
    Member #242
    Dave's Diesels:
    Sold June, 07 '82 1/2 ton 4X4;340k miles
    '97 2 Dr Tahoe, Intercooled,
    Kennedy ECM, 4" Exhaust
    '02 GMC

  8. #108
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    Feb 2000
    Location
    Montana
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    11,118

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    I've been thinking lately about the questions posed here for improving fuel economy in the 6.5.

    My thinking includes a trans swap to a TH700R4 which is more efficient (higher OD and less rotational resistance) than the 4L80-E, a mechanical DB2 (possibly a 6.2L pump) and a Holset HX-35 for a more free flowing turbo.

    I know that that combo in a 6.2L diesel pickup I owned in the 1990s delivered 24-25 mpg. That truck was geared 3.42, and ran a Banks Sidewinder turbo system. The 6.2L engine itself, DB2 fuel injection pump and matching injectors were all pretty fresh.

    By comparison, the 1994 Blazer we have (6.5TD/4L80-E, DS4 and 3.73 gearing) has to be coaxed to produce 17 highway...

    Just thinking out loud...

  9. #109
    Join Date
    Feb 2000
    Location
    Michigan
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    Yeah my 1995 with a CKO-HX40W-II turbo, DS4, and 4L80-E is very similar in MPG to your 94

    Most of my miles are local stop/go in town and I am averaging about 15 MPG
    2016 GMC SLT 2500 CC 4x4 Duramax All Terrain 59K
    1995 GMC 2500 EC 4x4 6.5TD,236K,Custom Tuning,HX40W-II turbo,Amsoil Bypass Filter,KD Exhaust,FSD Cooler,KD Headlight booster,Hi-cap Cooling
    TDP member #14

  10. #110
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    Feb 2000
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    Montana
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    Quote Originally Posted by ToddMeister View Post
    Yeah my 1995 with a CKO-HX40W-II turbo, DS4, and 4L80-E is very similar in MPG to your 94

    Most of my miles are local stop/go in town and I am averaging about 15 MPG
    The Blazer produces 15-16 locally as well. Doesn't make sense when compared to that GMC I owned with a Banks 6.2L. The Blazer's engine is equipped with the small port cylinder heads, which don't seem to make a lot of difference - or there are other factors that make more of a difference.

    Our 6.5TD Power Project engine ran with the large port cylinder heads, 4L80-E, 4.10 gearing, GM-8 factory turbo, a mechanical DB2 marine pump and matching injectors. It would deliver about 2-mpg better than the 1994 Blazer (we owned them together in 2000).

    So... it looks like the transmission and the turbo make up the rest of the difference in MPG. I do know that when I swapped in the OD equipped TH700R4 for the TH350 3-speed non-OD automatic that that 6.2L GMC diesel K1500 pickup had in it, I saw a jump in fuel economy of 5-7 mpg before the Banks system was installed. Adding the Banks Sidewinder made no difference I could measure in the 65-mph fuel economy (50 miles per day commuting speeds).

  11. #111
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    Apr 2001
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    Newberg Oregon
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    Just my opinion here...

    Having been into several 700R4 OR Currently called the 4L60E

    The 700R is a wimpy gear box.....Even the later renditions are nothing to write home to momma about.

    I have opened up several of these that were behind 6.2 diesels....Literally shook to pieces.
    The clutches are tiny, the sprags are also tiny.

    The planetary gear sets are small....a 5 planet upgrade helps....

    The factory sun shell is a POS..."The beast" (After market unit) is much better

    The input shaft is pressed into the aluminum clutch housing.
    The housings break or the splines rip out.
    There is a hardened steel piece the presses over the aluminum hub to help it out.

    Several other issues.....
    The 700R or 4L60 is OK behind the V6 and the 305 V8.....

    The factory did not use the 700R / 4L60 behind the 6.5 for a good reason....The N/A 6.2 could break them....The 4L80E is a good durable box and will hold the 6.5....

    The cost of doing the swap, plus all the parts that need to be massaged....Drive shafts and such....

    IMHO This swap would be a really bad jump.....And I am not sure you will get much if any gains that will ever pay off before the little gear box that could ...grenades...

    When the 700R goes bang....It's very ugly, and usually spreads parts all over the street.
    I trashed 3 of the 700R in my K5 Blazer......Then dropped in a TH400 with a nice tight RV coverter.....No more issues

    The 4L80 does have a tad more parasitic drag due to it's size....Size ....as far as bigger parts is a good thing.
    The TH400 and the 4L80 share some internal parts....Bigger is better..


    Ma general likely knew what they were doing....

    Just sayin....
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2007 Chevy Avalanche
    THIS IS BOW TIE COUNTRY

  12. #112
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    Jul 2020
    Location
    Montana
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    Quote Originally Posted by More Power View Post
    I've been thinking lately about the questions posed here for improving fuel economy in the 6.5.

    My thinking includes a trans swap to a TH700R4 which is more efficient (higher OD and less rotational resistance) than the 4L80-E, a mechanical DB2 (possibly a 6.2L pump) and a Holset HX-35 for a more free flowing turbo.

    I know that that combo in a 6.2L diesel pickup I owned in the 1990s delivered 24-25 mpg. That truck was geared 3.42, and ran a Banks Sidewinder turbo system. The 6.2L engine itself, DB2 fuel injection pump and matching injectors were all pretty fresh.

    By comparison, the 1994 Blazer we have (6.5TD/4L80-E, DS4 and 3.73 gearing) has to be coaxed to produce 17 highway...

    Just thinking out loud...
    After doing my 700r4 swap behind the 6.5 TD Optimizer with 4.56 gears, GM1 turbo (Soon HX35w), it for sure gets better MPG in town and on Hwy compared to the Th400. Even better Mpg in town then my Stock LBZ Duramax. I have not crunched all the number yet except Hwy mountain grades where it gets about 15mpg empty at 75mph and about 10mpg towing a heavy 2200 pound pop up camper. I have not been conservative. So far 3,000 miles with the built 700r4 and it is still going strong with my heavy lead foot! That 1st low gear is amazing!

  13. #113
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    Feb 2000
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    Montana
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    I put 40K on a "diesel built" TH700R4 behind my 6.2L Banks turbo. No problems. I know that the 4L80-E is more robust, being an evolutionary advancement beyond the TH400 3-speed non-OD.

    A fellow east coast member came to a couple of our first Rendezvous here in Montana. He drove a late 1980's 1500 Suburban powered by a Banks turbocharged 6.2L diesel. That Suburban had a TH700R4, and he towed a 36' triple axle Airstream all over the country.

    We've produced two different articles on building a reliable 700 for the 6.2/6.5. Gotta use the right parts... use the right TC and do it all with the right case.

  14. #114
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    Newberg Oregon
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    Towing a big trailer is a scary thought with a 700 R handling the gears..
    Yeah...I am very much down on the 700R
    After breaking several in the 80's/90's it soured me.

    Then being into them with bunches of hands on reinforced my dislike....

    YES....RIGHT PARTS, TOP NOTCH SHIFT KIT (SET ON KILL)

    A top notch converter with furnace brazed fins on the Turbine, pump and stator.
    Better sprag in the stator....A much better converter clutch.

    The converter is small in these and with the clutch in there it takes up space that needs to be used for pump and turbine.

    An oil pump with more vanes in the rotor.......More oil volume available.

    Big azz cooler out front

    Transgo Dash2 shift kit installed on the stoutest settings.

    The Input shaft reinforcing sleeve on the drum.....

    5 planet carriers....The beast sun shell....Super duty 2-4 band.
    Corvette servo......

    A few other goodies.....The late units with the late style case (4L60E) with the tow haul option.....This will really help when working it hard.

    One thing to remember....These boxes have a nasty little thing they do....

    THE 2-3 and 3-2 SHIFT CLUNK (Light to mid throttle)

    The 2-4 band has to be released....and the 3-4 clutch applied to make 3rd gear....


    During the shift the output shaft goes into a reverse torque condition when the band is released and then the 3-4 clutch is applied.

    The slack in the drive train (Especially the 4x4 rigs) is all taken up CLUNK....Very annoying and will scare the hell out of the unknowing owner....

    Another thing

    The T case in your Blazer/Tahoe has a 32 spline input shaft at the coupler with the tranny.

    The 4l60 uses a 27 spline....Either swap the shaft or scare up a different T case...

    The T case adapter is different too....Not sure about the frame cross member and rear mount.

    Drive shafts will be different length due to the different tranny.

    All doable.

    The ECM PCM will likely need some massaging.
    Not sure about the wiring harness that connects to the tranny.....I think these are different too.

    Scaring up a similar year rig with a 5.7 gasser for a parts goat....might be a good ideal...
    Somebody offers a kit to do this swap....

    A bunch tougher than in the old days.....Build the gear box, bolt it in and swap drive shafts...Good to go

    Probably have to massage the T CASE shifter too.....

    The 700R converter has a smaller bolt pattern...The 4l80 has the large pattern.

    Most 6.5 flex plates have 6 bolts.

    Since the 6.5 never had a 700R the holes may need to be added to allow the small converter to bolt up.

    Fun stuff.....
    Last edited by Robyn; 06-02-2022 at 16:55.
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2007 Chevy Avalanche
    THIS IS BOW TIE COUNTRY

  15. #115
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    Feb 2000
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    Montana
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    Quote Originally Posted by Robyn View Post
    ... clip...

    Probably have to massage the T CASE shifter too.....

    The 700R converter has a smaller bolt pattern...The 4l80 has the large pattern.

    Most 6.5 flex plates have 6 bolts.

    Since the 6.5 never had a 700R the holes may need to be added to allow the small converter to bolt up.

    Fun stuff.....
    The swap from a 700 to a 350 was so popular back in the 1980's that the aftermarket made a trans/txcase adapter just for that swap, that made it really easy. The adapter allowed joining the TH350 to the original transfer case used behind the 6.2. Didn't have to change the position of the transfer case or cross-member.

    Swapping to a 350 during the 1980's didn't have a lot of downsides - mostly upsides... The national speed limit, even on Interstates at that time was a snail's pace 55-mph. I swapped back to a 700 when the national 55-mph speed limit was lifted. The TH350 made the 6.2 unusable for anything other than back roads... because 70-mph meant 3000-rpm in a 4.10 geared truck. Our speed limits were even higher - they are 80-mph now. The TH700R4 had a 0.71 OD. In my 3.42 geared truck, that TH700R4 OD produced about 1800-rpm at 65-mph, which was heavenly after 200,000 miles of having no overdrive.

    Another big advantage of the OD was that the diesel rattle practically disappeared at 65-mph and over. Then, there was the fuel economy advantage... I saw a change from 17 to 24-mpg during my 50-mile commute. All combined, I was very happy with the TH700R4.

    Going to a 4L80-E in a non-computerized truck means buying a computer and wiring harness (about $1200) in addition to the cost of the transmission. It also means engine sensors (TPS - ESS). I installed a 4L80-E and computer and wiring harness and engine sensors in a non-computerized truck. I did it for the power handling capability.

    Someday, if the DS4 or existing 4L80-E in Sarah's Blazer ever give up, I'll be highly temped to swap in a TH700R4 and mechanical DB2. There's no reason this engine can't deliver low to mid 20's in fuel economy instead of the 15-17 it does now.

    There are two flexplate options for the 6.2/6.5 - "light duty" and "heavy duty". The only difference is TC bolt circle - both use 6 bolt TCs. The TH350 and TH700R4 share the same smaller bolt circle, and the TH400 and 4L80-E share the same tiny bit larger bolt circle. The difference isn't very much. From a short distance away, I doubt many could tell the difference in size between a pair of TCs or flexplates made for each transmission.

    Here's a pic of Howard's 1987 1500 Suburban and his 36' Airstream... There's a TH700R4 in that Banks 6.2 powered Suburban. He was from New Jersey. That pic was taken here in western Montana.
    Attached Images Attached Images

  16. #116
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    Apr 2001
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    Newberg Oregon
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    That Airstream is no lightweight.....I am very surprised he was able to keep the little 700R alive and kicking.....

    Yeah.
    The good old days... when life was a simple bolt in option to make just about anything fit anything else....

    Yes
    The small bolt pattern flex plate and the large pattern.....The 6.2 with the 700R (Or TH350) got those and the 6.2/6.5 with the TH400 OR 4L80 had the large 6 bolt plate.....

    If you go to the DB2 Pump and a hydraulically controlled 700R/4L60 Then the electronics really means ZIP......It will all work fine.....

    Trying to get everything to run with the DS4 becomes an issue when you start swapping goodies.

    Not impossible...but annoying at times....
    (1) 1995 Suburban 2500 4x4
    (1) 1997 Astro
    (1) 2007 Chevy Avalanche
    THIS IS BOW TIE COUNTRY

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