Does anyone know how much mpg I will gain if I changed gears from 3:73 to 3:08 on a 05 CC Duaramax diesel with allison trans. What kind of problems will this cause.

you will kill all your low end power. Even going to a 285 tire, I notice a pretty big difference in the low end take off power. I really dont think it would be worth it, unless you are driving 90 mph all the time.

Is that ratio available? I would like to know what all is available for the 11.5" rear.

buy a gear venor if you want to switch. That way when your on the road you can flip into a higher gear. I hate to tell ya.....MPG will stink with a 3:08.

How Much Does A Gear Vendor Cost For That Truck?

I doubt you'd ever pay for the cost of the Gear Vendor with fuel savings. That goes for a Gear swap as well if you have 4wd.

FWIW most engines make their best economy when riding along the natural torque peak.

3.08 would be too tall to pull, and mileage would suffer. 3.55 would put you at the cusp. bear in mind with the taller gear it will require more throttle to maintain a given speed in windy or hilly conditions.

IIRC gear vendors works well on a gas motor but I do have a friend that has killed 3 of them on his BOMBed CTD.

I favor the US gear overdrive. It mounts between the trans and transfercase and is very durable. Cost runs around 2k + driveline mods.:eek: