RPM at freeway speed?

[Guss]

I have a new crate motor, 6.2L LS3, 4L65E trans. Current rear gear is 3.73. GM recommends 3.42 for this engine/trans combo. With the 3.42 and my tire size, RPM's calculate to 2201 at 70 MPH. My question is this, is 2200 rpm going to be in the right range for optimal fuel mileage. I would assume so from a GM engineering standpoint but was looking for real world scenarios. Thanks

[solidaxel]

Sounds like a perfect combination to me!
I would not think you would notice that much difference in MPG , are you thinking that 2000 RPM's is the perfect number?
I am thinking out loud, the difference between 20 and 21 MPG difference.
You could always reduce the tire height by 1/2 inch or so.............. after you burn the first set off!!

[GMC-YA68]

I don't know about fuel mileage because I don't care, but I'd keep those 3.73s all day long...

[clinebarger]

3.42 compared to 3.73 is not much of a ratio change to warrant changing gear sets.

[Guss]

Originally Posted by clinebarger

3.42 compared to 3.73 is not much of a ratio change to warrant changing gear sets.

I get what you are saying. The 3.73 gear has to go anyway. it isn't a posi rearend and I like to burn the tread off my tires evenly!

[BR3W CITY]

Originally Posted by Guss

I have a new crate motor, 6.2L LS3, 4L65E trans. Current rear gear is 3.73. GM recommends 3.42 for this engine/trans combo. With the 3.42 and my tire size, RPM's calculate to 2201 at 70 MPH. My question is this, is 2200 rpm going to be in the right range for optimal fuel mileage. I would assume so from a GM engineering standpoint but was looking for real world scenarios. Thanks

Which rear end are you running? If your using something like a 10 bolt, 12 bolt, or aftermarket GM case, your not going to find 3:42 gearsets anywhere nearly as often as finding a 3:73. Usually then next "common" drop down would be to the 3:23 (again from an availability standpoint) if needing less gear a-la those needing to build load on a boosted motor.

2200 is fine on the highway, and I'd say its right around where most of us sit. Your stall converter and lockup settings will also affect the seen vs calculated rpm. My 4l80e/ 373 12bolt sits about 150-200rpm higher, but the converter has a slipping problem when hot in 3rd.

[mooseknuckles]

I'm running 3:42 gears with my 6.0l/4l80, and it's right around 2200rpm @70mph....and 2200 is right about when this thing starts to pull... I wouldn't be able to keep tires on it at all with 3:73 gears! if you were doin a lot of freeway driving you could run 3:23 gears easily with that ls3...

[solidaxel]

Originally Posted by Guss

I have a new crate motor, 6.2L LS3, 4L65E trans. Current rear gear is 3.73. GM recommends 3.42 for this engine/trans combo. With the 3.42 and my tire size, RPM's calculate to 2201 at 70 MPH. My question is this, is 2200 rpm going to be in the right range for optimal fuel mileage. I would assume so from a GM engineering standpoint but was looking for real world scenarios. Thanks

Guss, is this truck on the road?

[Andy4639]

6.0 4L80E with 4:10 gears and I'm cruisin at 2400RPMs @70MPH, tires are 29" tall. Truck gets about 16-17 MPG.