SteveG75

All,

Looking for some advice regarding rear end gear ratios.

My '75 was delivered with 3.36 gears and Super T-10 with 2.64 first. This gave me an 8.87 first gear multiplication and 3.36 final drive in 4th.

Back in 2001, I swapped in a Tremec 3550 with 3.27 1st and .68 OD. Ended up with 10.99 first gear multiplication and 2.28 in OD.

Right now, I am about to get a 12 bolt conversion rear with 3.73 gears built. Plan right now is to swap to a T-56 Magnum with 2.66 first and 0.63 OD. This will give me 9.92/2.35 overall ratios. I am concerned that I would be happier with a 4.11 rear which would mean 10.93/2.59 overall ratios.

Other option is to get a TKO-600 with the 3.73 (2.87/0.64) which with a 3.73 rear results in a 10.71/2.39 overall ratios.

Current engine is a warmed over 350 but plans are for a big block swap (debating 454 or 496 right now).

Just looking for some thoughts/options.

bashcraft

Without knowing how the car will be driven, nobody can say what is best, let alone what you'll be happy with.

I have a 502 with an M-21 and 3.08 gears. That gives me a 7.75 first gear multiplication and I'm perfectly happy with that. I had 3.55's and 1st gear was useless.

SteveG75

Bashcraft,

Thanks for the info. Never owned a big block so used to RPM's vice torque (other toy is a '77 308, 3 liter V-8 that comes alive at 4000 rpm).

Starting to feel that the project is getting away from me. Gary Ramadei is building the rear and we already have the 3.73's on order. Just second guessing my decisions.

Car is a semi-daily driver with enough power to back up the looks. Not looking to go drag racing.

0Willcox Corvette

My personal choice between the two would be the 3:70....

bashcraft

With which transmission?

DRC

So many variables to consider or not yet determined. But, you're talking about probably 500 + Lbs. Torque from a big Big Block. Just in general I would say 3.70 would be overkill.

If your build includes a cam that comes on in the 2000 to 2500 RPM range, low end torque, even a 3.36 rear would be like launching a Saturn rocket.

SteveG75

Well, the choices for a 12 bolt are 3.73, 4.11 or 4.33. Gary looked into doing a 3.55 but it was cost prohibitive.

DRC

Who made these choices? GM 12 bolts came stock from the factory with 4 speeds for example with 3.08, 3.36, 3.55 etc. up to 4.88.

bashcraft

Why are you using a 12 bolt in the first place? It doesn't sound like you're going to have enough motor to warrant it.

DRC

The OP is suggesting a 454 or 496 BB. That would need a 12 bolt.

bashcraft

Really? My rear is doing just fine behind my 502. It's been 4 years now.

Is the OP using sticky tires?

DRC

Well yes, I know your point. I assumed he is doing some performance work, so to be safe a 12 bolt is really the best way.

Your 10 bolt is fine, but would not hold up too well I think if you had drag radials and were able to hook up and hammered it.

63mako

3.73 with TKO600. That said I have the 4.11 with T56. With 2 OD's it is fine but the T56 is more of a pain to install and more $ but Im running N20, total hp 700. Needed stronger trans

SteveG75

One reason that I am doing the 12 bolt is that I want this car to rock solid for any future upgrades. I have owned the car for 20 years and plan on owning it another 20+.

The other reason for the upgrade is that the supply of 30 spline spiders has basically dried up. No one is making any more and everything is turned into powdered metal crap made offshore. Richmond Gear has also shut its doors. So, do the upgrade now or maybe never be able to do it. So, I am spring for the full Tom's 12 bolt setup with 30 spline inner and outers and 1350 halfshafts.

Plus, the last used setup that was for sale went for $6k so really just investing the $$$. LOL.

Right now, I am leaning towards 3.73 (original plan). 4.11's would be better for small block that revs but I do so want a big block. LOL.

larrywalk

Multiply your first gear ratio by the rear end ratio. A total ratio of 10:1 is ideal - any greater and you won't hook up; any less and you'll wear out clutches from slipping them to start.

MotorHead

Rear is gears are bandaids for no torque. Anyone with 500ft/lbs or more a 3.08 is fine. The best setup would be for you currently is 3.36 and TKO 600 with 2.87 first gear and when you get the torque monster you don't have to change anything. We always worry about the dragstrip then never even go. Spend 5 grand and never go and still no guarantee it won't blow up

How many post do we get where someone has blown their rear end. I have the Dana 3.08 in mine with tons of torque and HP and yet it works perfect.

Big difference comes when your toting around time and smashing gears at the drag strip becomes 50/50, if you go to strip a couple of times a year a sub 2 second granny launch and then get on it will still get you a good timeslip

corvettedave383

Ive got a 12 bolt rearend package coming for my 78 aswell, thats going to be mated to a T-56 Magnum, close ratio, 2.66 for 1st gear, Ive decided to go with the 4.11 rear end, my vette will be getting turbo charged soon, so need a bullet proof rear end and tranny, Mike (tracdogg2) has been busy building it all for me, including doing me a reinforced crossmember to hold the t -56 in place.

good luck

dave

Vette5.5

Since you're pretty well versed in the overall package gearing deal, here's what I've observed. If running a tranny with 1st in the 3's and say 0.70 od top gear, then a mid 3's axle is a hard combination to beat on the street. You pretty much get it all. Good digs on launch, and decent highway cruising.

OzzyTom

I have a TKO-600 (0.82 OD) fitted with a 383 sb. which lacks torque below 2000 rpm.
I had 3.36 diff initially, but motor didn't feel comfortable.
The overall gearing had me either lugging in 3rd gear, or screaming in 2nd gear in the hills twisties I enjoy driving.... speeds in range 40~70mph.


I changed diff gears to 3.73 and it transformed my entire driving experience.
The gearing now works perfectly for my driving style.

The 3.73 will be good for your 350ci sb.
But as many have already stated, a big block can pull much taller gearing....

You could fit taller tyres when you get the big block to offset the 3.73 gearing if you have issues.
Or better yet, when you build that big block, aim for a spec which pushes peak torque higher in the rpm, thereby keeping torque at lower rpm more manageable, whilst also providing a genuine adrenalin rush when you do get into it.

TheSkunkWorks

A lot depends on the torque curve of the prospective engine (and where it falls RPM wise), but FWIW...

TKO 600 w/3.73:
1st - 2.87 = 10.71
2nd - 1.89 = 7.05 (34%)
3rd - 1.28 = 4.77 (32%)
4th - 1.00 = 3.73 (22%)
5th - 0.64 = 2.39 (36%)

TKO 600 RR w/3.73:
1st - 2.87 = 10.71
2nd - 1.89 = 7.05 (34%)
3rd - 1.28 = 4.77 (32%)
4th - 1.00 = 3.73 (22%)
5th - 0.82 = 3.06 (18%)

Richmond SS w/3.73:
1st - 2.89 = 10.78 (optional ratio)
2nd - 1.85 = 6.90 (36%)
3rd - 1.31 = 4.89 (29%)
4th - 1.00 = 3.73 (24%)
5th - 0.77 = 2.87 (23%)

From a strictly performance standpoint, I'd caution going too tall (in 5th) without huge top end HP, and I'd also note how the steps will work out for your intended uses. The 600's 5th gear is fairly tall and a big step, so it's pretty much just for highway cruising.

And, be careful not to build a dump truck motor that's done before the fun really begins.

Oh, and given enough torque and sufficient bite, IMOE one doesn't have to be on slicks or dumping the clutch at insane RPM's to break a 10-bolt. Thus, no regrets for having gone with a 12-bolt IRS conversion (3.55:1 w/Eaton) long ago.

My $.02