LQ4 Final drive ?

[89_Wagon]

Soon, I will get going on the meat of my GM V8 swap. I have the vast majority of parts and a rebuilt engine. I will also need to rebuild the diff as it's getting very noisy and was wondering if I should take the opportunity to change to a more optimal gear. Right now, I run 3.31. I was thinking of running 2.72s (or whatever the "normal" gear was) for better mileage and lower RPM at highway speed. I use this vehicle more as family hauler with some occasional hauling of a few hundred pounds of camping gear. Below are more some more specs

6.0 LQ4 with 862/706 heads (10.5:1 compression)
4L80E
Stock suspension and tires

What do you guys think? Difference is 300 rpm at highway speeds.

[Yeller]

I have an LQ4 with a 4l80e and 33's with 4:10's and am very happy. Doing the math back to a stock tire with the same ratio would be a 3.59:1 gear ratio. I think you will be quite happy with the 3.31 gear. That works out to 2022rpm at 70mph. remember this is a brick in the wind and not a needle, it needs a bit of gear to be efficient. 2.72:1 would be 1661 at 70, in my opinion too low, I would not even consider doing less than a 3.08 gear. Also remember if you change the gear ratio from 3.31 to something else you have to do the same for the front axle as well.

For reference, my truck is at 2192 rpm at 70mph.

[rocklaurence]

I know that newer vehicles with VVT lug down to 1700-1900 rpm at 70mph but a none VVT needs to be in its power range [2000-2300 rpm].

[Stuka]

With an over drive transmission, you want lower gears, not higher. Over drive will be unusable with 2.73’s, and likely unusable with 3.31’s. Those gears were used because over drive transmission didn’t exist yet.

Most newer 1/2 ton trucks have 3.42-4.10 gears with 30-32” tires. So you can see how the 3.31’s work out. The most important thing is to keep the engine in its torque range for cruising. If you lug it, it will either down shift or unlock the converter, both are bad for cruising.

[89_Wagon]

Thanks for the input! Looks like 2.72 is too low. Appreciate the guidance!

[will e]

This web site, written and supported by one of our members, is hands down the best gear ratio calculator for rigs with transfer cases

http://www.grimmjeeper.com/gears.html

[Stuka]

Thanks for the input! Looks like 2.72 is too low. Appreciate the guidance!

Its actually too high, as its a ratio. 2.72:1 is a higher ratio than 3.31:1. Its numerically lower, but is a higher ratio.

[89_Wagon]

Got it. Thanks for clearing that up.

[dodgerammit]

I vote 3.73 ratio for stock size tires. If running 31s or 32s, then 4.10s to utilize OD. Of course, those may require a new carrier depending on where the 'break' is. If that's the case, then a street friendly traction aid is a nice, minor cost upgrade at that point.

Do note, you don't have to do all of this at one time. You can do the swap and rock the 3.31s for a bit. Then upgrade the axles when budget allows.

Just know you will:
a)simply want to leave OD off or not be in OD gear so you don't lug the motor.
b)need to possibly have the pcm recalibrated when you change ratios.

[Yeller]

3.73:1 with 30-32” (depending on truck) tire was stock for an LQ4/4l80E in millions of 3/4 and 1 ton Chevy trucks, and a few had 3.42:1 so 3.31’s with a 29” tire is well within range. ECM is well capable of managing the shifting a 4l80E. The ECM will not lug the motor beyond factory specifications unless someone has been messing with that part of the programing, if it needs to down shift it will just like it would in the Chevy truck it came out of.

[89_Wagon]

Thank you all for the info! I like the idea of matching the OE setup.