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My '68 fastback currently has the factory 3.25 gears with an open differential. I'd like to add a limited slip and I'm contemplating just getting another center section to use instead of taking apart my original.

This brings me to gear selection :).

The 428 has a stupid amount of torque and it's an absolute blast even with the 3.25 gears. I've got 3.70 gears behind a 306/4 speed in my '69 and it's stupid fun around town, but the RPM is definitely up there on the freeway.

I genuinely enjoy being able to cruise comfortably with the fastback on the freeway, but I also love mashing the skinny pedal (a lot) so I'm debating between building a limited slip center section with 3.40 gears. The other option is to simple add a limited slip to my existing 3.25 center section. I did the math, we're looking at 138 RPM difference at 70 MPH between the two ratios. Transmission is a close ratio 4 speed (also super fun).

I also happen to have a new in box 3.50 ring and pinion that was supposed to go in the '69, but I kinda like the 3.70's in that car :pirate:.

What do you all recommend? If a totally different option, why?
 

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I have 3:70s in my 68 Torino with a 427 S/O and a 4 speed. Yes, tons of torque and super fun but the highway sucks. I’d use your new set of 3:50s and have an all round great package. That 428 will still roast the tires with 3:50 gears. As for adding a new carrier I’d go with either an Eaton Tru-Trac or a Detroit Locker. The new lockers aren’t as harsh as they use to be but are still bullet proof. I have a Detroit Locker and love it.
 

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Well I feel you should gear it for how you drive and what trans you have. It should work well with each gear in the trans and not biased to some of the gears. Back in the late 60's and early 70's a lot of these cars with 400+ cubes would run the quarter mile pretty much the same time with 2.80 gears or 4.30 gears.

With my mild GT40P, wide ratio Toploader and 2.80 gears I'm pretty happy. Great gear on the highway. Low enough I'm not downshifting on any inclines or hills and tall enough for comfortable crusing at 70. The whole point of a big engine is massive torque. If it were my car I might even think of 2.50 or 2.75 gears. The car is light so it's not a big issue. I believe @rpm is running 2.50 gears with his big block
 

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3.25 is what I planned to run with my Toploader. Still deciding but I already have the 3.25s so... I would consider going lower, 3.55s or 3.80, but not higher. There's always Gear Vendors.

Here's a little spreadsheet I made so I can easily see the difference between the rear ratios, also did it with Gear Vendors Overdrive ratio out of curiosity. Tires are 25.6".
 

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Discussion Starter #5
3.25 is what I planned to run with my Toploader. Still deciding but I already have the 3.25s so... I would consider going lower, 3.55s or 3.80, but not higher. There's always Gear Vendors.

Here's a little spreadsheet I made so I can easily see the difference between the rear ratios, also did it with Gear Vendors Overdrive ratio out of curiosity. Tires are 25.6".
Jdub, that’s a great spreadsheet! I’m conveniently running the exact same height tire. I think I’m pretty set on the 3.40 ratio now, the big block really doesn’t need much gear.

A high winding small block however, loves a numerical higher gear. I might go for broke and swap in 3.89’s into the ‘69 ;).
 

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I like the new all billet case Traction locks, I have one for the ( original) 3.50 open diff for my 68.5 CJ . I built an extra ( keeping the original C/R in tact) Wide ratio big /out toploader for better street manners. 3,000 rpm on the freeway isn't bad. I would switch to a 3.00 "if" I was planning a long drive. Right now it has a 4.30 for "spirited" short drives and I stay off of the freeway with it.
Randy
 

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i had a 460 / c6 / 3.55 combo in my 85 tbird. 255-60-15's on the back. plenty of gear with all the torque, and very comfortable driving anywhere. man I miss that car...…

 

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Jdub, that’s a great spreadsheet! I’m conveniently running the exact same height tire. I think I’m pretty set on the 3.40 ratio now, the big block really doesn’t need much gear.

A high winding small block however, loves a numerical higher gear. I might go for broke and swap in 3.89’s into the ‘69 ;).
I think you’ve got it, no need to go lower with that engine unless you are going drag racing.
 

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My 70 Ranchero GT was a factory top loader 429(uh slightly modified) car/truck with 3.73's and hidden headlights(1 of 268 configured like it in 1970) and it ran fine out on the highway. I was incredibly stupid for letting that car get away from me after spending 2 years restoring it.
 

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You know most people see the Gear Vendors Overdrive as having another gear in the transmission (or 3-4 more gears if you like pushing buttons). I see it more as having two rear gear ratios, one for taking off, one for cruising. You can have a 3.89 and a 3.03 or a 3.40 and a 2.65. Or even a 4.11 and a 3.20!

I've done a lot of mods and purchased a lot of parts for my car (though many are still on the shelf)- touched just about every aspect of the drivetrain, suspension and brakes. I'm getting into esoteric modifications territory like overdrives, stereo systems, A/C and fuel injection if I want to keep going. The overdrive is really making me go "hmmm..."
 

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It would be nice if they weren't so expensive but then they are built for race cars. They also have no competition. I need one and it is still my least expensive option for an overdrive gear. Borg Warner apparently used to build one. I have seen it and they sold many of them so you can probably still(maybe) find one but if it isn't working you are kind of SOL because there is no source of parts to fix it with.
 

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There is a lot of great discussion here about what axle ratios work best.
When Modern Driveline works with customers, we always ask for the axle ratio and rear tire size when determining the best transmission for your use. The transmission, axle ratio and rear tires together complete the power transfer. If one of the three components don’t match, you will either burn up a clutch trying to get the car going or zip through the gears and not have a usable overdrive.
Our Approach is simple for STREET use.
First gear 3000 RPM at 19-21 MPH +/- a bit. This provide a nice smooth acceleration and will get you through the intersection before having to shift gears.
Overdrive (5 or 6 speed) Low 2000 RPM range at 65 MPH.
When we achieve the above, all the other gears will be fine and not an issue.

How to achieve this and use the information provided. Go to the Tremec web page and find the “Tool Box”. If you don’t know your rear tire diameter, use the tire size calculator to get the diameter. Then use the speed calculator and enter the transmission of your choice, the axle ratio and rear tire diameter. Try different combinations until you achieve the above.
It’s really that simple.
 
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