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Old 10-31-2007, 10:04 AM   #8
Z15
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Join Date: Jul 2007
Location: Lake Superior, Michigan
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Re: 1994 Gmc Sierra MPG ISSUE

You say is has 265 tires but what does the tire pressure lable on the door say the tire size is? If it does not say the same, thats part of your problem. OS tires fool the sensors that tell what rpm the trans to shift at, when the torque converter is to lock up and give you inaccuate odometer and speedometer reading. Meaning, you are going faster and driving more miles than are recording. I think you need to determine if 265 are the correct tire size before going any farther.

No O2 sensors maybe part of your problem. They are plumbed into the exhuast pipe before the converter. I am not sure how many would be on your truck. I do recall my 92 305 engine had only one.



From a dealer servive bulletin
Quote:
Before any diagnosis is performed for these types of concerns, inspect the tires/wheels for the proper tire and wheel combinations.

If a truck is found to have other than the above listed tire sizes, no further diagnosis should be undertaken until the original size tires and wheels are reinstalled on the truck.

The Engine Control Unit (ECM) and Transmission Control Module (TCM) use (RPM) and (VSS) inputs as operational parameters for engine torque, transmission shifting, cruise control operation and speedometer indicated vehicle speed.

Calibrations for different tire/wheel combinations are not available.

Therefore changing of tire and/or wheel size should not be undertaken by dealerships or truck owners.

Warranty Information
Reinstalling the proper wheels and tires on the vehicle, prior to performing any type of diagnosis of the truck, would be considered truck owner responsibility and not a warranty repair.
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Mike
2010 GMC Sierra SLT E. Cab Z71 All Terrain
2009 Cadillac CTS4 3.6L AWD Performance Sedan
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