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severe drop in fuel milage

I have 3.73 in mine, and I'm at 1900 RPM exactly at 60 MPH,
with the torque converter clutch locked.
I think there is a chance you are in 4WD, that'll kill mileage fast.
Jack up the rear, get in and put it in drive, see what happens.
 
You probably got a tranny for a gasser since you only gave them the last 8 of the VIN. Did you specify it was a 6.5L diesel at the time? Even if you did the VIN letter of importance on that is not in the last 8. Its the 8th digit which you didn't provide.

That would have told them it is an L56 or L65 6.5L diesel.

The parts guy probably doesnt know the difference, you need the Heavy Duty version if thats how you can differentiate it. There are different internals, and some may affect input and output speed sensors that feed the speedo and PCM.
 
For your brakes to cause an 8mpg loss of fuel mileage....you wouldn't need to check em - they'd be smoking.....and you would see higher psi from the turbo - just like when towing.

I think you can rule out the brakes dragging.
 
buddy said:
You probably got a tranny for a gasser since you only gave them the last 8 of the VIN. Did you specify it was a 6.5L diesel at the time? Even if you did the VIN letter of importance on that is not in the last 8. Its the 8th digit which you didn't provide.

That would have told them it is an L56 or L65 6.5L diesel.

The parts guy probably doesnt know the difference, you need the Heavy Duty version if thats how you can differentiate it. There are different internals, and some may affect input and output speed sensors that feed the speedo and PCM.
Click to expand...

I'll be checking on the tranny tomorrow.

GT-4 & MT-1 codes

I didn't make the call but the counter does know the vehicle and knows it is a diesel. But he may not have checked and just got a 4L80E Without knowing the differance.
 
We have come to the conclusion that the Tahoe probably had a 3.42 rear end that did not match the front end before the swap. After all the parts checking it's the only answer that makes sense. I always thought that it was a 3.42 but I don't remember if it's because I was told that when I bought it or if I had previously checked it before it went out. A 100,000 miles & a lot of other vehicles worked on since then. It's going to grate on me that the Tahoe isn't going to get the mpg it got before. Since all I ever tow ith it is the 16' boat or small trailers I've never had to worry about high gears.

I did learn the following as well as a couple of other things along the way.

If you want to check your gear ratio, pop the diff cover off and rotate the pinion until you come across a number combination. 11/41 will be a 3.73, etc. You take the bigger number and divide it by the smaller number to find out the ratio. The bigger number is your ring spline count and the smaller number is your pinion spline count.

Thanks guys.
The help as always is much appreciated.
 
buddy said:
You probably got a tranny for a gasser since you only gave them the last 8 of the VIN. Did you specify it was a 6.5L diesel at the time? Even if you did the VIN letter of importance on that is not in the last 8. Its the 8th digit which you didn't provide.

That would have told them it is an L56 or L65 6.5L diesel.

The parts guy probably doesnt know the difference, you need the Heavy Duty version if thats how you can differentiate it. There are different internals, and some may affect input and output speed sensors that feed the speedo and PCM.
Click to expand...

Could anybody get me the GM part number for the transmission that should have been installed?
The part number isn't on the receipt they are trying to look up what I bought.

I'm starting to do some stuff but I'm almost a year behind.
 
Now I'm confused!

jrsavoie said:
Could anybody get me the GM part number for the transmission that should have been installed?
The part number isn't on the receipt they are trying to look up what I bought.

I'm starting to do some stuff but I'm almost a year behind.
Click to expand...

Had to call a GM Parts dealer:
4L80E part# 1784206
Torque converter for the 6.5 part#8689960, "same used for big block gassers (I'm quoting the parts guy)."

Neither of those parts hit on the GM Parts direct type web sites so I couldn't confirm the numbers or get a ball park price.

Hope this helps

By the by...In doing the searching on line, and it seems tht goodwrench.com doesn't agree as to what transmission you have in there.

MT1 = 4L80E :thumbsup:
But according to www.goodwrench.com that transmission wasn't available until 99 in a 4WD Tahoe, those had the 4L60E instead. :confused:
http://www.goodwrench.com/_res/pdf/TransmissionPdf/Transmission Application Chart.pdf :mad2:

Local GM parts guy said there was a non turbo 6.5 listed with the 4L60E on his screen. Learn new things every day.
 
Last edited:
I just got off the phone myself with a local dealership & http://www.newgmparts.com/.
They both gave me Part # 17804206 I don't know if the first 0 after the 8 is significant or if you missed it.
I'm pretty slow on the go. It's been almost a year since I have done anything to speak of. I should have made these calls myself before I posted.
I apologize for putting you out on something so simple.
Thank you for the info. Especially the extra info that I didn't find out.
 
jrsavoie said:
I just got off the phone myself with a local dealership & http://www.newgmparts.com/.
They both gave me Part # 17804206 I don't know if the first 0 after the 8 is significant or if you missed it.
Click to expand...

Not sure if I missed it or not...both numbers were 7 digits. Now I'm really confused. :confused: But, I'm betting the number is significant as in the difference between a transmission and a windshield washer pump. However, 17804206 on the gmparts direct web page does result in an automatic transmission but no specific type. Just the "contact us" with an estimated price of over $2,000.

No problem running things to ground, I had the time.

Paul
 
Does anybody know of anything else that would have thrown the speedometer off?

I didn't drive the vehicle between the trans change and the rear end change. My daughter drove it during that time span. The first time I drove it I knew things weren't right. Had I driven it after the trans change at least one possibility would be ruled out.
 
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j k auto said:
Did you figure out the problem yet?
Click to expand...

No, I've only been doing stuff that even resembles work for about the last 2 weeks. I've got a years worth of struff to catch up on.

With the RPM's matching the speedometer I'd almost have to guess that the speed on the trans is set for a differant rear end gear ratio.

The drop in milage could be somewhat do to the miles on the engine.
 
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