1993 Fleetwood Brougham
 

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.