I wish I could find it again, but I thought our Getrag 238 6 speeds had their 1 to1 ratio in 5th gear. If I remember correctly the autos had a better ratio in overdive than the 6 speed manuals. The RPMs would be fairly high with the 3.92s at highway speed, even more so with the 1st granny gear with 3.92s you would probably shift into 2nd at 5 mph.

 

Something about being afraid of the warranty claims when people blow their transmissions because of the additional power the gearing gives was the line I got. It's crap, and I realize that, but not a whole lot we can do, either. They're probaly covering themselves from all the idiots out there who can't drive.

I do have the V-8, though, and what gearing you can get might be different with the V-6.

 

So many guesses and rumors surrounding gearing ...its just multiplication! Let me drop a few numbers (I'm going to assume the truck in question has 245/70 R16 tires):

-245/70 R16 Tires measure 29.5" diameter nominal. C=pi*D, so every revolution of the tires = 7.72 ft. 60 MPH then translates to 684 RPM
-The overdrive on the Getrag 238 is 0.79:1, so with a 3.21 rear end the engine is turning (684*3.21*0.79) = 1735 RPM at 60 MPH
witha 3.92 read end the engine is turning (684*3.92*0.79) = 2118 RPM at 60 MPH

Because of the higher mechanical advantage of the 3.92 gearing, theengine does not require as forceful an explosion (i.e. as much fuel) in each cylinder to counter friction and aerodynamic drag. Therefore, your throttle will likely be more closed at highwayspeed. However, the higher RPMS will increase viscous drag in the engine, tranny and rearend,and pumping losses in the engine will be higher, so mileage might still take a small hit. That's still certianly amanagable highway RPM.

MyDak's manual suggests shifting out of 1st at 15 MPH with 3.21's, or 171 axle RPM. (171*3.21*4.23)=2322 engine RPM. The 3.92 gears would have you shifting out of 1st around 12 MPH at that RPM. As for a 0-60 run, the 3.92's would get you to 49 MPH at 5500 RPM in 2nd, and require another shift to 3rd, while3.21's would have you at60 MPH in 2nd at a hair over 5500 RPM. Buuuut...the 3.92's give you 22% more rear wheel torque, and therefore 22% more force accelerating the truck, so each gear would run out 22% sooner. Granted we don't have short throw shifters by any means, but even with the additional2-3 shift,the 0-60 time has got to improve significantly.

Using good oldF=ma, the engine curves,and finite element method, you can use Microsoft Excel to estimate your 0-60 times, 1/4 mile times, shift points, etc. I'm working on getting an accurate model put together;mine currently predicts about 8 seconds to 60 with my 3.7/6spd/3.21's...a bit optimistic, but I'm working on it.Approximating the aerodynamic and frictional lossesis the real sticking point.Once you get such a sheet dialed in, it would be very easy to see, for example, how adding bigger wheels, or different gears, would affect the performance of the vehicle.

As for the gearing change being "too much" for the truck.....no. The RPM hike isn't significant; you'll be shifting a bit sooner, maybe burning a bit more gas on the highway as the engine fights to pump a greater volume of water, oil and air. You won't overload any components or run the risk of twisting anything in two ; a 4.7 HO with a 545RFE,and 3.55's, hammer-down in 1st,is putting (theoretically) 4355 FT-LB through the differential carrier and axle shafts ; a 3.7 6 speed with 3.92's checks in at 3897 FT-LB.

Find a good shop and swap those suckas. If I had a few hundo to spare I'd be taking a good look at that mod.

 

My head hurts...

But thanks for all that...

*dazes around* I think.

 

that was extremely confusing.... but at least someone knows what they're talking about.