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Discussion Starter · #1 ·
Hey guys,

I have checked two tanks of fuel now and to my disappointment I am only getting around 12mpg. Given, this is most all around town driving and little time with the engine up to full temperature, but it is still lower than I expected. I suppose driving habits can play a good part on around town, but what would you guys think makes sense? The truck is an 87 GMC crew cab 4x4 with 35" tires and a 4L80E.

While I know this is about double what a gasser would get in a truck like this, I really thought when I went from the TH400 to the 4L80 I would see some increase. I have seen none for around town and I do cruise in OD around the loop going to work.

What are you other 6BT guys getting around town?
 

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Doesn't sound right to me. Not to insult your intelligence but sometimes simple stuff slips by: how's your brake adjustment? Any chance something's causing a drag?

Sure the speedo is right with those big tires? It would read far less miles than you've actually traveled if you didn't put a converter gearbox on speedo drive output. At least a 15 mph error at cruising speed, plus 480LE speedo gear is spec'd according to that vehicle's original tire size & gear ratio. If several factors are in error your figures could be way off.

Easiest way to tell is fill the tank, drive a known distance, top up tank and do the math on fuel used per miles. Use freeway markers to check speedo odometer to verify readings.
 

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and other factors like your rear axle gearing... Then all of those other factors. And smoke? Tons of it?
 

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Good point, Andre: I guess what a guy is smoking could have an effect on perceived mileage figures and actual mileage traveled. W- w-o-w-w....



M-a-n.......:rasta:
 

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Discussion Starter · #6 ·
Thanks for the fast replies guys.

To answer a few....

I have not checked the brakes, but I guess when I think of it I am not sure how other than to guage drag. The truck does not appear to have any unsual drag and can be pushed on flat ground(takes two of us), but I dont think anything is dragging.

The speedo is hooked to the transfer case on this truck, so the transmission did not effect that. It is off, but I compensated for that in the mileage. It only got 10mpg if you take out the inaccuracy of the speedo. I am planning to get a gear, but I have not gotten there yet. I have not used the known mileage method yet, but I think that is the next thing along with a road trip. I need to take a short trip on the highway, then fill right back up so I can check like that....good point .

The axle gears are 4:10, but with the 35's cruise RPM is pretty decent. It is about like a 3:54 with regular size tires. However, the weight of spinning the big suckers does have some effect. What about pressure? Maybe I should bump them up a little to reduce surface friction? They do howl pretty good.

The engine only smokes when I take off for a second or so until boost comes up. It cleans up and does not even haze unless you really stand on it where it goes to brownish grey. No real black smoke unless I floor it from a stand still....then it rolls coal, but I dont make a habit of that.
 

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1989 Jeep Wagoneer, 360v8, 727, stock for now,
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92 W250(4x4)6BTA, stock fueling, 5 speed, 4:10's & 35"s, 14-16 in town, 18 on the highway.
Your intown isn't to far off.
My wife's 94 4x4,CTD, auto 4:56's 38's (an excellent combo BTW) gets 18 intown and on the highway. She dosen't know the meaning of mash the throttle, balls to the wall.
Steve
 

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My '98 dodge 3500 12valve cummins, nv4500, 4.10 gears, gets around 15 in town and 18 @ 65mph. That being said I tend to be pretty hard on my engine. I have 3000 rpm springs and use them often. I'm also running a #10 plate and have the star wheel turned up pretty high so black smoke is pretty common. How much does your truck weigh? I know that can make a big difference but your mileage still sounds pretty low. My truck weighs in at 7400 lbs any given day of the week.
 

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Discussion Starter · #9 ·
I have never weighed the truck, but I bet is is 7K plus. It is a 3500, 4 door, 4x4, long bed on top of having the cummins and 4l80E which both weight more than factory trim 6.2 and TH400.
 

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how's the 4L80E behind the 6bt? other than mileage... shifting/power, can you push through it pretty easily?
 

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Discussion Starter · #11 ·
how's the 4L80E behind the 6bt? other than mileage... shifting/power, can you push through it pretty easily?
It holds great....at least right now. The converter will slip to a point, but it is pretty efficient and will load the engine very well. Full thottle shifts are rock solid. 1-2 shift will raise the front of the truck about 2 inches under full throttle and chirp the tires or break them loose on slick pavement. The full manual is nice because you can power through the gears and keep the engine loaded without ever letting off. Auto shifting autos are just about impossible to match perfectly. You have to be able to "feel" when to shift to make them work right. I have three "manualmatics" and I love 'em.
 

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good to hear! I've got a 4L80E sitting here just need something to put it in :D

How did you mount the shifter?
 

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Discussion Starter · #13 ·
good to hear! I've got a 4L80E sitting here just need something to put it in :D

How did you mount the shifter?
I am using a BM megashifter floor mounted. It just uses a cable. The transfer case shifter had to be cut and moved forward about 3 inches due to longer overall length once installed.
 

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sounds like a nice setup. I thought about doin it with the 4b, but the nv4500 wasn't much of a price difference.

4L80E's are supposed to suck mileage, i have never seen calculations on how much though...
 

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Discussion Starter · #15 ·
sounds like a nice setup. I thought about doin it with the 4b, but the nv4500 wasn't much of a price difference.

4L80E's are supposed to suck mileage, i have never seen calculations on how much though...
Yeah, after I started to do some research I read that the pumps can suck 80HP, which seems high but apparently they are hogs. Unfortunately, they are the only GM AOD that will hold alot of power.
 

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yeah thats garbage for fuel mileage i plan on doing a 6bt conversion to get better than the 14 i get in my 1 1/4 ton 6.2 diesel with a th400 and 456 gears.
 

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Discussion Starter · #18 ·
Interestingly enough, the 6.5T I pulled out got 12mpg as well. The PO claimed it would get 18, but I never got close to that.
 

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Hey guys,



What are you other 6BT guys getting around town?
i drive a 92 d250 5 speed 4x2.all stock.21 in town/highway. 17 freeway @ 75mph.24mpg @ 55 highway only
also drive a 66 patrol 4bt/1 ton drive train and 35's.5300lb. i dont know exactly what im getting yet.its only been on the road 5 weeks.but i can already tell its better than my 6bt gets.
 

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What are you other 6BT guys getting around town?
Well I have a 4BT not a 6, and I do not have an automatic, but my fuel mileage has been affected dramatically by the crap that comes out of the pump right now that is otherwise known as 'winter fuel'. I don't know if they change the fuel blend where you live or not, but I keep a spreadsheet of my mileage and can show you exactly how much it affects mine, combined with the colder temps here now.

9/8/2007 - 75 to 85 degree temps - Road trip to Pennsylvania with empty 20' trailer (Combined truck and empty trailer weight = 9200lbs.) - I-71 to I-76 to I-80: 426.4 miles/20.54 gal. = 20.76 mpg :smokin:

12/3/2007 - 30 to 35 degree temps - Another road trip to Pennsylvania w/ empty 20' trailer and the same highways (just slightly farther off I-80 to different pickup): 429.1 miles/23.13 gal. = 18.55 mpg :eek:

1/16/2008 - 15 to 25 degree temps - Yet another trip, same roads, same trailer, but with a 1000lb. snowplow blade on the deck for about 250 miles of the trip: 418.6 miles/24.85 gal. = 16.85 mpg :crybaby:
 
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