Gauges are incorrect.

After cranking up on a flat level surface, the estimated mileage will drop suddenly. It drops about 15 miles. It drops about a mile a second. Another problem I noticed happened om the interstate. While zi was driving 70+mph, it was showing 40. Later it dropped down to 0mph. I know I don't drive that slow. It's not keeping me from driving, it just makes me worry. I really have enough to worry about right now. I drive a 2005 Avalanche 1500 2wd. Other than the tailgate lock being mounted on a flimsy piece of plastic that can be popped of easily and quietly, I still like my SUT.:smokin:

After cranking up on a flat level surface, the estimated mileage will drop suddenly. It drops about 15 miles. It drops about a mile a second. Another problem I noticed happened om the interstate. While zi was driving 70+mph, it was showing 40. Later it dropped down to 0mph. I know I don't drive that slow. It's not keeping me from driving, it just makes me worry. I really have enough to worry about right now. I drive a 2005 Avalanche 1500 2wd. Other than the tailgate lock being mounted on a flimsy piece of plastic that can be popped of easily and quietly, I still like my SUT.:smokin:

Get it on a good capable scanner and check for codes and the speed sensor reading to computer when the speedometer is droping off.
Ans also watch odometer for reading miles right.
Speed sensor reading and speedometer should match.
Speed sensor has to be reading right to work speedometer right.
If any codes post back code no.
Spedometer not right may affect your MPG readings.
Also ckeck for recalls on speedometer head.

GM used some defective servo motors in the gauge clusters of many of it's vehicles between 2002-5. You can: 1) purchase updated servos and install them yourself, 2) Purchase the service online and send the cluster out to have them replaced, or 3) have the dealer replace the whole cluster.

I've had it happen to my Envoy. I opted for #2 a few years ago and haven't had problems since.

Good luck with however you proceed.