I see that JamesC has a thread showing the difference between the 3.42 and 3,73 rpms at 70 mph.

3.42 (RPO GU6) / Approx. RPM’s at 70 mph--2255
3.73 (RPO GT4) / Approx. RPM’s at 70 mph--2463

If I replace my 3.42 rear end with a 3.73, would the rpm still only go up a couple hundred rpm. It used to be a V6 car, but now its a 350/350. At 55mph, It does 2500 rpm. Id like to use the 3.73 rear end for the posi unit plus the disc brake set up. I just didnt want to get into replacing the gears right away.
Thanks for any advise.

I believe you are in the ballpark with your estimate. It all depends on the tire size too. If you are running stock wheels and tires, you should only see a few hundred rpm increase.

Step up to a 6 speed and you won't have to worry about the crusing RPM :-)

you wont even really notice the difference between the two.

Are you swapping the entire rearend or are you just wanting the posi unit?

I'm gonna do the whole thing. I found a 84 T/A w/ the G80 carrier and GT4 gears. A bonus is the disc brakes. I can get it for $200.00. It does have the B/W 5 sp and I could get the pedals, but I got alot of $$$ into my TH350.

I just had it out and at 70 mph, the rpms are right about 3100. So a couple hundred more won't really hurt.

my car runs at about 3300 rpms at 70 mph with my 3.45 9 bolt in 5th gear.

my codes say i have a 3.42:1 rear, but based on those rpm readings, i am beginning to think that either my speedometer or rear end are lying to me.
at 75 mph indicated on my speedo, with a 74 confirmed on gps, running 255-50-16 tires, i am just a hair over 2100 rpms.
hmmm.
do you maybe not have lockup working properly?

i have a t5 ,there is no lockup

that was actually to shelad sorry.
but yours is pretty high too.
i couldnt stand running 3000+ rpms cruising at 70.
unless you are running a 1.8L engine with 10 lbft of torque, you are simply pouring gas out your tailpipe.

sorry to get off topic.

I'm running a straight TH350 w/2500 stall. No lock up. No wiring. All of the harnesses and the computer have been removed. Also, 215/60-15 tires. I picked up the rear end today. Weather permitting, I will be installing it tomorrow. It was 115 degrees today.

also it'll be quicker shifts going 3.42-3.73

The rear end is in. I also changed the proportioning valve. I need to bleed the brakes still, then I can test it. As far as faster shifts, It already shifts too fast. My next project will be to figure how to adjust the shifting.

Test drove today. Rear end works great. Still having other issues, but I believe its engine, so I,m moving to tech/general engine. Thanks all, Larry