Rear Diff Input Needed

[Raven]

Hey guys, looking for some driveline advice. My ‘78 has a mild 455 (bored .40 over but otherwise stock), a S-T10 4-speed, and a 4:10 posi rear diff. This is what it had when I bought it and while it’s fun for the strip, it really isn’t street friendly as you’ll hit 4,500 RPM or so at 60/65 MPH. While I hope to one day be able to swap in a 6-speed that isn’t possible with the current budget. That being said, I’m looking for input on what ratio I should swap the rear to as that is within the budget. This isn’t a track build, all I’m after is a fun car to drive. I have no personal experience or expertise in this area so your input is greatly appreciated.

[b_hill_86]

Not sure what your timeline for an OD trans is but when I 4 speed swapped my 77 I had a 2.41 and went to a 3.08 due to fairly regular highway driving. I run a 27” tire and get around 2700 rpm’s at 70 mph. I kind of wish I’d gone with a more factory ratio like 3.23 only because I think you can sneak an OD trans in at that ratio whereas I think my 3.08 is a bit too low to go with an OD trans without regearing again. I don’t think there would’ve been much noticeable difference going 3.23 vs the 3.08.

[81Blackbird]

I bet the 4.10 is a kick in the pants when the light turns green.  Here is a quote. "Multiplying the 2.48 First gear by the 4.10 rear axle results in a final drive ratio of 10.16:1 (2.48 x 4.10 = 10.16).
For most street performance applications, a 10:1 final First gear ratio is usually considered optimal."  You're running at 13.41. Assuming the ST-10 has a 3.27 first gear.  With the options of either a
2.89 or 3.06 with the Richmond 5-speed, you can get that ratio down to 12.55 in first gear.  Still high but with the final drive of 0.77 with the Richmond would bring the final drive ratio down to 3.16.
That is much better but some like a rear end ratio of 3.23 or 3.08 even better. Then the question comes in.  How much highway driving do you do?  Myself, I'm running the original DNE street five speed. 
First gear is at 3.27 and the final drive is 1 to 1 with a 342 rearend. So at 3000RMP it's running at 65 MPH.  That is the original setup. But over the years a 6th gear with a drop to  0.76 for the overdrive
would be great.  Unfortunately a DNE 6-speed in no longer made. With the price of a Richmond 5-speed around $4000.00, a rearend swap would be a lot easier on the wallet.  Just remember that when you do
opt for a new transmmission, the optimal 1st gear street ratio of 10:1.  BTW, The Doug Nash cost me $1388.47 plus $250.00 for the Long shifter back in 1986.
   
   
   
   
   

[Raven]

Thanks 81BlackBird, that’s just the kind of info I needed. You’re quite correct, with all the low-end grunt it’s like hitting warp speed!  At this point I’m just trying to get this car back on the road, and as this is the third engine rebuild in as many years (long, sad story), I’m not really focused on future upgrades very much. The way I see it, if I’m in a position for a tranny swap, changing the rear (if needed) will be a minor expense. The use case is primarily highway cruising, no multi-state road trips or drag strip runs.

[70_71_78]

I have been looking to repair a worn out rear axle on my shop spare SUV. Not cheap. Just the parts add up to about 1k, partially because my carrier is damaged, but still, why not make sure everything is sound and correct while it is apart?

[79GoldnTan]

I run 3.08 in mine. I think it's perfect for what use it for. Rarely on the highway but cruising the local area here it's great.

[N PRGRES]

Not sure what your timeline for an OD trans is but when I 4 speed swapped my 77 I had a 2.41 and went to a 3.08 due to fairly regular highway driving. I run a 27” tire and get around 2700 rpm’s at 70 mph. I kind of wish I’d gone with a more factory ratio like 3.23 only because I think you can sneak an OD trans in at that ratio whereas I think my 3.08 is a bit too low to go with an OD trans without regearing again. I don’t think there would’ve been much noticeable difference going 3.23 vs the 3.08.

I also run a 3.08 and I want to OD it, the 2700 RPM at 70 makes me cringe and I want to do moer highway driving with it.

[Raven]

Thanks for the input everyone, I really appreciate it!

[firebirdparts]

A lot of the 1980 and 81 cars had 308 gears, so that is a good place to look for one already assembled.  If you're looking on the cheap and all that.