Hello!

I have a 1993 Cadillac Fleetwood Brougham. L05 engine, 4L60E trans, w/3.73 rear end.

I do a lot of highway driving in this car. I don't do a lot of highway driving in my Chevy K3500 for obvious reasons

My gas mileage has peaked out at 22mpg at the best of times but usually can be found around 19mpg. I'd like to do a little better. I think I'd like to shoot for the 20's consistently for my highway mileage.

I have considered swapping the engine for a 6.2 diesel but scrapped that idea because it was too costly, time consuming and everyone here would probably be upset. I may swap out my 7.4 in my 3500 for a 6.5 and then consider putting the 7.4 in the caddy. Consider is the key phrase.

The above paragraph is digression from why I'm posting. I was considering changing the rear end gear in my Caddy (GT4) 3.73 to something in the neighborhood of a 3.42 or even a 3.08. In my guestimate, the L05 has plenty of torque to push my 4600 pound vehicle down the road at 60mph with a 3.08 but it's better to ask someone who has already tried this. I wish I could just find a rear end from a 94 Fleetwood (GU6?) which I'm guessing was a 3.42. Theoretically, if it's the same bolt/spline count and other considerations are similar, it MIGHT work.

I'm looking for someone who has firsthand knowledge of the after effects of the swap. I know it will be more sluggish. That's OK because everyone passes me on the road now There are other things that I have not put in here that I do know about the swap, but I'd like to get some other perspectives so I won't post them now.

Oh yeah, I don't tow with this vehicle. I tow with the K3500.

Thanks, in advance!

 

Changing the gear ratio in the rear you have would be the best way to go, rather than swapping in a whole diff from another car.
You shouldn't have any speed sensor problems with this swap, but your fuel mileage calculator might be a bit off...I don't know for sure. Going to a 3.08 will definitely help with the hi-way mileage, but any in-town driving is gonna kill it.
I haven't done a swap like this, so I'm just going on theory.

 

AA, Thanks for your reply! I assume that "in town" driving will result in worse mpg with a higher ratio.

I have to do more research on this. I have a service manual and need to pour through it more regarding the VSS and other info.

It seems like all the forums I visit talk about lowering the gear ratio for performance so there should be some 3.08's out there in good condition (I hope). If anyone else has a site for me to check for info/parts, I'd greatly appreciate it.

 

I wouldn't go for used gears in the diff...9 times out of ten they will make noise. New gears would be better.
Check with Randy's Ring and Pinion
I use their stuff all the time.