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

  1. #61
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    Quote Originally Posted by ronniejoe
    My experience has shown that egt's run high under those conditions. I prefer to run in OD at higher boost levels where things stay cooler.
    I'll agree with that! RPM's and low boost equals high EGT's. Before the intercooler install, when the transmission would downshift out of OD and the RPM's would go up to the 3000 range the EGT's would climb dramatically and quickly. Higher boost helps to cool things down but isn't the real solution to the problem.

    That was why in a previous post I was suggesting hacking the VSSB to read 4.10 gears when mine were in fact a higher ratio. If setting it to 4.10 gears helps to keep the converter locked and/or keep it from dropping out of OD then I think it's a possible viable modification. I believe one poster above said that with the 4.10 gears it was very difficult to drop out of OD over 65-70 MPH. With 3.73 gears that's about 2200-2400 RPM's. That's a nice RPM range for a lot of boost when towing. On mine if it kicks down out of OD at 65-70 that means like 3100 RPM's and the trucks just not pulling anymore, only generating EGT's.

    Art.
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  2. #62
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    Default 3rd Only

    It has been suggested that I try and run my burb in 3rd only to see if mpg results get better.
    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.

  3. #63
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    Quote Originally Posted by DA BIG ONE
    It has been suggested that I try and run my burb in 3rd only to see if mpg results get better.
    You thinking you got too little gear? With the oversized tires you're probably at what, 3.08's? Probably real good at 50 MPH but maybe the engine's got to work too hard at highway speeds with wind resistance. What do you get at 65-70? On straight highway I'll get about 18.5 MPG and 14 towing on relatively flat highway.

    Art.
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    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...

  4. #64
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    WOW!! I just read through this thread to catch up and find myself wondering what turbo selection has to do with a gear swap for MPG??? Unless one has an interest beyond MPG.

    On my '93 I started out with stock 245/75/r16 tires and 4:10 gears. That would net a consistent 15MPG around town. When I swaped to 285/75/r16's I gained nothing... probably lost 1mpg truth be known. I always atributed it to added rolling resistance. I've always been under the impression that swaping out gears doesn't have that effect because rolling resistance is the same and the only difference is how long it takes to get rolling. That's what I got out of this thread thus far, am I somewhat correct?
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  5. #65
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    While I agree the turbo stuff drifted a little, it was originally on subject.

    The conversation had to do with engine speeds and ideal shift points for economy. There were some questions that ended up bringing the discussion around to performance as well. Then the subject of falling flat above 2800 rpm... That led to turbos... That led to discussions of power... That led back to economy...

    You get the drift.

  6. #66
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    Vicious circle ain't it...
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  7. #67
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    Quote Originally Posted by dieseldummy
    WOW!! I just read through this thread to catch up and find myself wondering what turbo selection has to do with a gear swap for MPG??? Unless one has an interest beyond MPG.

    On my '93 I started out with stock 245/75/r16 tires and 4:10 gears. That would net a consistent 15MPG around town. When I swaped to 285/75/r16's I gained nothing... probably lost 1mpg truth be known. I always atributed it to added rolling resistance. I've always been under the impression that swaping out gears doesn't have that effect because rolling resistance is the same and the only difference is how long it takes to get rolling. That's what I got out of this thread thus far, am I somewhat correct?
    I think as Ronniejoe said the subject drifted a little but did cover a lot of interrelated issues.

    I don't think any of the above topics concluded that a gear reduction has an adverse effect on mileage. I think yours is the only one that has stated that reducing the gearing from 4.10 to 3.73 via. tire change adversely effected mileage. I think DaBigOne's poorer mileage may be because he's reduced a little too far and was using an aggressive tread pattern. Tread pattern has a big effect as does the width of the tire. If you go bigger, any benefits may be negated if you use a really wide tire and/or aggressive tread pattern.

    I believe comparing stock trucks to stock trucks it's been proven that 3.42's get better mileage than 3.73
    Art Paltz
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  8. #68
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    I never corrected the VSSB/speedometer, but made the correction in my head. If I wanted to travel 65 mph I would put the speedometer at 60 mph. Whenever I filled up I corrected the number of miles on the odometer to make up for the larger tires. The tread pattern is not that aggressive, the tires are yokohama geolander at+2.
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  9. #69
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    Quote Originally Posted by dieseldummy
    I never corrected the VSSB/speedometer, but made the correction in my head. If I wanted to travel 65 mph I would put the speedometer at 60 mph. Whenever I filled up I corrected the number of miles on the odometer to make up for the larger tires. The tread pattern is not that aggressive, the tires are yokohama geolander at+2.
    Huuumm, strange I think. I would think you'd see a fairly significant difference going from 4.10's to an equivalent 3.73's if you were traveling at the same speed. I'd think with 4.10's you probably don't go long distances at over 65 MPH. You'd probably be screaming along at about 2300-2400 RPM's. I just wanted to make sure you were not calculating mileage at 60 MPH with the 4.10's and comparing it to 70 MPH with the 3.73's. Although the RPM's may be similar you are going to experience a significant amount of additional wind resistance at 70 that you would not have at 60.

    I
    Art Paltz
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    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...

  10. #70
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    Hi Diesel Dummy;

    I agree that things did "drift" of topic a bit. Any way my thoughts on the worse MPG for larger tires is that with wider tires you have besides more rolling resistance you also have more areodynamic drag with a larger tire face in the slip stream. The other with a "taller" stance on your truck with the taller tires you also increase the "drag" from the under carriage of the truck.
    This all adds up.
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  11. #71
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    Default Hot button

    Holy smokes! Go away for a couple of days and miss 4 pages of posts!

    The hot button is DaBigOne keeps saying his shift points changed. I'm saying the way they are computed is not based directly on road speed and they have not changed; the road speed is what changed. I never meant the factory settings were optimal. To each his own.

    On that subject, now that he's got them "back where they belong" he's thinking of driving in 3rd all the time. Anyone else see any irony here?

    Re optimal shift points, I always thought you wanted to be where the decending torque curve of he current gear crosses the ascending curve of the next gear. No?
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  12. #72
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    If you want to accelerate down a race track as quickly as possible, you want to shift where the power curves cross from gear to gear.

  13. #73
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    Quote Originally Posted by ronniejoe
    If you want to accelerate down a race track as quickly as possible, you want to shift where the power curves cross from gear to gear.
    I'm sure you are correct, but wouldn't the torque curve give you the strongest acceleration, hence the highest trap speed? (Bear in mind I've retired all my math skills past basic algebra... )
    The Constitution needs to be re-read, not re-written!

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

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  14. #74
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    Red face Road Speed Changed, or?

    Quote Originally Posted by JohnC
    The hot button is DaBigOne keeps saying his shift points changed. I'm saying the way they are computed is not based directly on road speed and they have not changed; the road speed is what changed. I never meant the factory settings were optimal. To each his own.?
    Perhaps, I'm just thick headed so bear with me will ya?

    I changed out to 3.42 gears from 4.10's frt/back, using stock sized tires 245/75/16.

    I hack the vssb to correct speedo and shift points/lockup go to higher rpm's and speed points and od/lockup goes from 45 w/4.10's to 55 mph w/3.42's.

    Now I only hack the vss only to correct the speedo and the shift points lockup, od/lockup all stay as they were w/the 4.10's, same rpm's same speeds.

    So, I guess I'll remain confused on this one until someone can drill it into my thick skull........................
    Last edited by DA BIG ONE; 07-06-2006 at 15:39.
    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.

  15. #75
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    For what it's worth, I'm with you DA BIG ONE!

    I'd like to try the same mod, but keeping the lockup at the same speed on the speedometer.

  16. #76
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    Default Power is the rate of doing work!

    Torque is basically work, or energy...they both use the same units.

    Power is the rate of doing work or rate of energy usage. To go down the track quickly, you want to do the work quickly.

  17. #77
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    You going to be racing that burb down the track Ron?

  18. #78
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    Red face Back to where I started, as in full circle.

    Biggest lesson learned with much $$$$$ in my gear changeout is that you get good mpg until you start going larger tire diameter & width, then you arrive at the same spot you paid so much to get away from in the first place.

    So, perhaps leaving the 4.10's and just doing the tire change I might have been happier.

    All, in all the burb is a keeper.
    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.

  19. #79
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    Hum, food for thought. I have a set of H2 rims on the way to me. I was planning to put slightly bigger tires on them and increase the diameter by about an inch and a half. This would not have reduced the gears down to 3.42's but probably to about 3.55's. The width would have increased by 0.4 inches. Now I'm wondering if it's worth it. If I
    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...

  20. #80
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    Default 3.42:1

    I'll add that my burb w/it's mods is happy w/3.42's and stocked sized tires. These 6.5 td's aren't like the old 6.2's in that the 6.2 4x4's were ok w/3.08 gears.
    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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