This morning as I was going to work, I was traveling at 65 mph with a strong tailwind, I looked down at the tachometer and it was reading about 1400 rpm. I couldn't believe my eyes but yes it was reading 1400 rpm at 65 mph. I had always thought the rpms will be the same at a given speed depending on the gear. Is there a gear that I don't know about in the Allison? Does anyone know what this is about? With all of the torque that this engine produces if I could figure out how to run at that speed and that gear it would make for really impressive fuel mileage. Any ideas? The Sierra has the Duramax with the Allison and 3.73 gears.

having just driven in east blowing wind (was heading east) at 66 MPH i noticed no difference in RPMs. i got 20 MPG, i have 4X4 and there was some city driving about 10 miles between fill ups.

Your tach ispossibly messed up.

heading to kc few weeks ago i had good tailwind and was getting 22mpg(dic calc)rpms stayed the same as always.

Yeah your gauge cluster is most likely bad. Usually its the speedo that quits working or reads eracticly, but it can be any gauge. Had one that the oil pressure gauge quit working, oil pressure stayed at 60 psi and it would flicker around every so often. The gauge did its sweep check like normal and was 0 with the engine off, start the engine and went right to 60 psi.

x2 gauges

Gm sent out a warranty bulletin which gives us 8 years or 80k miles and they will replace the gauge cluster for those reasons. Actually having mine done today. My Engine temp gauge is off by 60 deg. lights are out and when you rev the engine in park the speedo goes up!!

X2, 3, 4, or 5... it's the gauges. At 65 mph on stock wheels/tires, you should be right around 1900-1950 rpms. Analog gauges are close but not precise; with my Predator showing real-time data, 2000 rpm's shows IIRC 67 mph. No way it's 1400 rpm.

The speedometer went out on our long trip to Az but 6 days later it started to work and still is. This isn't the usually way from what you guys are telling about your trucks.
chevman:beerchug:

Where can you find that GM bulletin on the gauges, my speedo is acting up but everything else is fine.

The dash repair was only for 04-vehicles and up to 70k miles. However some dealers were doing 05 vehicles as "Goodwill". You will have to talk to the dealer. :rolleyes: