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severe drop in fuel milage

J

jrsavoie

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severe drop in fuel milage

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I had a few problems with the 95 Tahoe.
We installed a new transmission and a junk yard rear end - it looked good inside.
We are now getting about 8 mpg less than we were. We have 265/75's on the Tahoe which should make the speedometer read slower than the speed you are traveling - as it does on our 2 Suburbans of the same set up. The Tahoe shows on the speedometer about 5 mph faster than traveling speed as determined by the same GPS used in all 3 vehicles over close to 1000 miles per vehicle.
We turned the drive shafts slowly so as not to slip the differntials, front & rear 3 3/4 turns per one revolution of the wheel.
Is it possible that our method of checking the gear ratios is flawed?
My first suspicion was that the junk yard rear end was a lower gear. But I kind of blew my own theory.
Is there a more accurate way to check the ratios? Is it possible that we did not end up with accurate results?
Should the Tahoe have 3.42 gears?
The front gear is original equipment as far as I know
The last 8 digits of the Vin SG113169
Is there anything else that would affect the fuel milage and the speedometer? I can imagine there could be something with the transmission or other electrical problem but I don't have any ideas.
I'm still not trusting my results and think the rear dif should have showed to be a lower gear. But it didn't.
Thanks
 
How about popping the cover off the rear and counting the number of teeth on the ring gear and number of teeth on the pinion. Divide the number of teeth on the ring gear by the number of teeth on the pinion. ie- 41 ring gear / 10 pinion = 4.10 gear ratio.
Kind of a pain in keester way to do it, but with 4wd you don't want it to off.

Did you look up the code on the glove box?

Also is trans going into lockup? Like BudTX asked what are rpms at cruising speed.
 
I'll have to check the RPM's. The tach shows 19000rpms at 60 mph on the speedometer. I didn't have the GPS along but I'm guessing that is probably closer to 56 mph on the GPS.
The RPM's for the Tahoe are more than the RPM's for the suburbans at the same speed.
The speedometer is about 5 mph off at 70 mph. With the cruise control set at 75 mph the GPS says the Tahoe is going about 70 mph.
Would the trans going into lockup cause the speedometer to be off?
We thought about pulling the cover but it is pointless unless you do the front and the back. & since the front doesn't have a cover it's even more of a pain.
I incorrectly stated that the front differential is OEM factory. I assume it is. But since I am the 2nd owner I can not be positive. The Tahoe had 130,000 miles when I bought it. This should have been checked against the old rear end prior to install. If I had been available at the time I had a whole differant not stock swap of both axels plan. The old rear is long gone.
 
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I have noticed my speedomet SOMETIMES doesn't match my GPS. Especially at highway I can see this, thing is, I don't see my RPM's drop, I can feel/hear engine work a little harder to maintain speed, Speedo stays constant, GPS Speedo drops up to 3-4mph, Not sure if the computerized way of calculation speed via VSS has lag?

I'm doing 60 at around 2200 rpm, or damn close to it. I have 4.10's stock tires and GPS most of the time matches speedo exact.

It kills me to read TD is doing 75 at 2200 rpm.... Must be nice
 
If you changed the whole rear end and not just the 3rd member you might want to check your brakes...they could be draggin'...
 
There is the slip factor when turning the wheels with the drive shaft. It isn't to hard or unusual to have slipage while turning the driveshaft. I was wondering if there was a more definitive way to check the ratio. other than counting teeth.
We only turned the front drive shaft once. The back we did 4 times. Just ran out of time.

The speedometer was reading properly before the switching. Are there other problems that could make the speedometer show a faster speed than what you are traveling?

AS far as stamps and numbers where do I check and what numbers do I check to verify the OEM gear ratio?

Is it possible for the transmission to have a lower gear ratio?
 
well im shocked no one has said this yet, but are you positive you got a diesel tranny and diesel torque convertor in there? if so deffinately check the rear brakes and check the gear mesh in the differential it could be set too tight causing excessive drag.
 
I'm not sure how the brakes would affect the speedometer.

So what's the differance between a diesel tranny and one for a gasser?
The tranny part number was gotten by way of the vin number but it wouldn't be anywhere close to the first time that I got the wrong part from the dealer especially if it was a certain counter guy
 
the brakes wouldnt throw off the speedometer but it could explain the loss in fuel milage if they are causing drag. i would also climb under the truck and make sure all sensors are plugged in on the tranny and transfer case.
 
jrsavoie said:
I'm not sure how the brakes would affect the speedometer.

So what's the differance between a diesel tranny and one for a gasser?
The tranny part number was gotten by way of the vin number but it wouldn't be anywhere close to the first time that I got the wrong part from the dealer especially if it was a certain counter guy
Click to expand...


One difference is the torque converter. The diesel torque converter has a much lower stall speed than a comparable gas motor because the diesel can make more torque at lower rpm and the gasser needs a higher stall speed to engage at a higher rpm.

I am not sure if this is your problem but it may help you determine the cause of your fuel mileage problem.
 
Slim Shady said:
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One difference is the torque converter. The diesel torque converter has a much lower stall speed than a comparable gas motor because the diesel can make more torque at lower rpm and the gasser needs a higher stall speed to engage at a higher rpm.

I am not sure if this is your problem but it may help you determine the cause of your fuel mileage problem.
Click to expand...

Would this also throw my speedometer off? I'm pretty sure if I get the speedomteter working right my fuel consumption problem will be taken care of also. But there is no gaurantee that 2 differant things could not be messed up at the same time.
 
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.

Is the Tahoe 4X4? Check in your glove box and look for the Alphanumberic code slip.

3.42's should be GU6 code.
 
jrsavoie said:
What would the symptoms be of the torque converter not locking up?
Would this throw the speedometer off?
Click to expand...


The engine rpm would be higher but the speedometer would read correctly. That would kill your fuel mileage, it would in effect be the dame as running a lower gear ratio (4.10).
 
What code was there? Any at all? Or is the tag gone?
Is it 4X4?
I believe codes for these trucks (most common ones) are
GT5-4.10
GT4-3.73
GU6-3.42
GU5-3.23
 
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