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1998 Chevrolet Silverado 2500 4x4/6.5 Turbo Diesel
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Discussion Starter · #1 ·
Hey y'all I have a 98' ck2500 4x4 with the 6.5 I believe it's a 4l80e trans my grandfather gave me the truck it has 194k miles the truck maybe sat for close.to two years and it would start but then shut off. I picked it up took it to my buddy's diesel shop and they found the tank and lines were clogged up with sludge they got it all cleaned out and got it back up and running. It runs really well and shifts fine but on the hwy running about 20miles home it burned just under a 1/4 tank at 65-70mph. Rpms were around 2500 it just seems like its not hitting overdrive. Also it got up to around 230degress. It's going back to the shop tomorrow to get a new t-stat and coolant flush. I looked at rear end to see if there was a tag but I'm guessing it's possibly the 4:10 does this sound normal for that speed. I pushed it past 70 and it never shifted past where it was it just doesn't seem right. It really feels like I'm not getting into overdrive any and all help with this is appreciated
 

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Hey y'all I have a 98' ck2500 4x4 with the 6.5 I believe it's a 4l80e trans my grandfather gave me the truck it has 194k miles the truck maybe sat for close.to two years and it would start but then shut off. I picked it up took it to my buddy's diesel shop and they found the tank and lines were clogged up with sludge they got it all cleaned out and got it back up and running. It runs really well and shifts fine but on the hwy running about 20miles home it burned just under a 1/4 tank at 65-70mph. Rpms were around 2500 it just seems like its not hitting overdrive. Also it got up to around 230degress. It's going back to the shop tomorrow to get a new t-stat and coolant flush. I looked at rear end to see if there was a tag but I'm guessing it's possibly the 4:10 does this sound normal for that speed. I pushed it past 70 and it never shifted past where it was it just doesn't seem right. It really feels like I'm not getting into overdrive any and all help with this is appreciated
From the Trans section of the forum: Info: - The most common problem facing the 4L80-E NO...
This may help (y)
 

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Look in the glove box. There should be a build sheet stuck on the bottom with a bunch of 3 digit codes. They will tell you what gear ratio you have from the factory.
Gear ratio starts with G.
GT4 = 3.73
GT5= 4.10
GU6= 3.42
If you also see G80. Means you have a Gov lock auto locker.
2500 RPMs with 4.10 gears, in OD, @ 70 MPH seems about right to me. Assuming you are on stock height tires.
If you are not going into OD or the torque convertor is not locking you should get a transmission trouble code. (Trans codes do not light the CEL)
 

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1998 Chevrolet Silverado 2500 4x4/6.5 Turbo Diesel
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Discussion Starter · #4 ·
Ok sounds good I'll take a look to see what it has I'm willing to bet it's 4:10. It just runs like it's just running out of gear at 70 like it needs to shift once more and just driving about 25 miles it ate almost a 1/4 tank. If it's turns out to be 4:10 and not a trans issue I'm swapping to 3:73. It's going back into the shop tomorrow morning once I get to work load it up on my flatbed wrecker and get it dropped off. It also got up to 230 if the gauge is reading right I wasn't running it hard at all. I pushed it to 80 once just to see if it would shift or not and it didn't. Just feels like its running really hard just to do 70mph
 

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1998 Chevrolet Silverado 2500 4x4/6.5 Turbo Diesel
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Discussion Starter · #5 ·
I have brand new (265/75/R15) Cooper Discoverer AT3's on it
 

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My 95 runs around 2500 at 70 with those sized tires. 75 is pushing it i thinking I'm turning almost 3000rpm Never have tried pushing to 80. I have 4.10s. But I do not use anywhere that much fuel in 25 miles. If I'm out on the highway I can get roughly 100 miles per quarter tank. I would look to make sure you don't have something else wrong causing the poor mileage.
 

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Yes unless your gauge is wrong. Your fuel consumption is very high.
 

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1998 Chevrolet Silverado 2500 4x4/6.5 Turbo Diesel
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Discussion Starter · #8 ·
Yea it very well could be the sending reading wrong.I drove it earlier and I noticed the gauge moving back and forth it also didn't run as hot as it was this morning when I was on the hwy. I guess I'm just use to driving new vehicles with better gearing it just seems the motor is running harder than it needs to at 70
 
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