There should be a tag on the differential cover that is held on by one or
two of the differential cover bolts. It'll probably be 3.73 or 4.10. If
there is no tag. you could lift the rear wheels off the ground, put the
transmission in neutral and count how many times it take to turn both wheels
in order to get the drive shaft to make one complete turn, or a dealer could
probably tell you just be running the VIN.
Or look at the sticker in the glove box
AXLE REAR, 2.14 RATIO
AXLE REAR, 3.62 RATIO
AXLE REAR, 3.36 RATIO
AXLE REAR, 3.15 RATIO
AXLE REAR, 3.08 RATIO
AXLE REAR, 3.08 RATIO, PERFORMANCE
AXLE REAR, 3.27 RATIO
AXLE REAR, 3.31 RATIO
AXLE REAR RATIO, ECONOMY
AXLE REAR, 3.55 RATIO
AXLE REAR, 2.37 RATIO
GT4 AXLE REAR, 3.73 RATIO
Look in the glove box for the RPO options tag. It will tell you what
gears it has along with most of the options on the truck as well.
Then look at
or http://www.c2e.info/rpocodes /
to find out what they mean.
Me thinks this is your problem, there is no overdrive button. OD is another
notch on the shift lever, the button is two/haul mode. If your running
around in tow/haul mode that could account for the low mpg.
My Chevy is a diesel so I can't compare it to your GMC, but comparing it to
my Dodge, which runs just below 2000 RPM at 70 MPH, it may be that the
overdrive isn't engaging, which could account for the difference. Please
note however, the overdrive on my Dodge is pretty tall (IIRC .67:1 gear
2006 1500 Z71 5.3L 3.42 gears Reg Cab around 5k on the odometer
Most of my driving is just puttering around town and I get 15.5 MPG.
Drove it 1100 miles with about 1000 lbs in the back and got 19 on the road.
I buy cheap gas, Wal*Mart, Kroger's, etc.
Since you can't tell what gear you are, why don't you redneck it.
The stick on the column, put it in 1st, go, then 2nd, go, than 3rd, go, than
D, than go some more.
You should see something happen on your tach.
That will tell you if your tranny still had OD.
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