I have a '99 Silverado, 3/4 ton, 4x4, extended cab. It has a 6.0 liter gas engine. I put taller tires on it and jumped from 13 MPG to 13.5.

Worst mileage I ever got was pulling a 6000 trailer thru the mountains at 70 mph. I dropped to 10. something. When I got to 8000' altitude and no trailer it did jump to 19mpg.

It has the trailer towing package, so I'm sure it has a low final drive. Does anyone know how I can check with Chevy and find out what it is?

I use Mobil 1 oil. I've noticed it runs much better and smoother. I change it every 5000 miles. It starts much easier in the winter using this oil.