3.23's to 3.73 08 4.3L

[piecesparts]

I am running a set of 265/50/20 tires with a 3.42 geared rear end in my 2011 GMC. This truck also has the 5.3 engine and a 6 speed auto tranny. My highway RPMs are simple 75 MPH = 2000 RPM. This combination works well with the engine.

The change in tires from a 245 to a 285 tire may not make a lot of difference, you did not state what the ratio of the two tires are 55 series, 50 series, 60 series etc.... That number will give a change in the amount of sidewall that you have on the tires and affect your RPMs greatly. Not to mention the acceleration.

For instance a 245/55/17 tire is 27.6" tall and a 285/55/17 tire is 29.3" tall. Are you keeping the ratio the same. A 285/50/17 tire would be 28.2" tall.