What gear to run

[davez]

So it's time to go through the rear in my 4 speed car. Currently has a locker with 3.89's. At 65 mph the tach shows 3400 rpm. Higher than I prefer. Going to be used for driving until a restoration years away. Was going to build a center section with a limited slip trac lok and what ratio? Was either going with 3.25 or 3.50's. Any suggestions or experiences with what you are happiest with? Is 3.00 too far the other way?
Thanks dz

[deathsled]

I have a 3.50 in big red.  It revs enough.  Would not want it higher.  If you're doing a lot of highway driving go to a taller gear.

[JD]

I had put a 3.50:1 limited-slip in the car I had for the same reason, to keep-up with highway traffic and not run the engine so high especially on 1-2 hour trips.  If I did it again I'd do the 3.25:1.  (One opinion, also kept the original 3.89:1 open and that went with the car)

[J_Speegle]

Guess we should ask, rather than assume, what transmission your running just to establish a base line to work from

[davez]

Close ratio t10
Thanks

[GT350Lad]

I have actually stuck with the 3.89. Had the diff rebuilt in 2014 and thought about changing the ratio but didn't. Car hasn't moved much since, did rev damm hard on highway but good fun between stop lights 

[SFM6S087]

What size tires are you running?
What rpm would you prefer at 65mph?

Steve

[davez]

205 65 15 tires
I used a tire size calculator that came up with 25.5 inches height but when I measured them they are actually 24.5 inches tall.
Motor seems happiest at 2800 to 3000 rpm.

[gt350hr]

  The 3.50 will drop about 400 rpm at cruise. "Some" change in letting the clutch out in 1st , but not bad.
        Randy

[mygt350]

I had the original 3:89 locker installed in the car and I now find at my age, keeping up with traffic spins the engine more than I wish. So I located a date correct 3:50 posi from Jim Cowles at Shelby Parts and Restoration. It still makes engine spin up bit more than I would like, but it is definitely better ratio for me at this point in life. Have the locker with its original markings bagged and tagged. T10M with Sebring gears, again, from Jim years ago.
Had I to do again, I could have my arm twisted to go to 3:25. But its a real rush to run thru the gears and let it stretch its legs. Get nasty looks from Prius drivers though.

[Side-Oilers]

Not for the concours guys, but a Gear Vendors overdrive (0.80:1 ratio) drops the revs significantly when you want it to, yet gives you all the feel & acceleration of the original setup when you switch it off.

Have had it in my KR (3.50 rear, Top Loader) for about two years so far, and really like it.   

[Bigfoot]

3.89 Detroit is a dream

[GT350Lad]

3.89 Detroit is a dream

Sweet sweet dream

[SFM6S087]

205 65 15 tires
I used a tire size calculator that came up with 25.5 inches height but when I measured them they are actually 24.5 inches tall.
Motor seems happiest at 2800 to 3000 rpm.

If your tires are 24.5 inches tall that should make their circumference around 76.9 inches. Starting with that, then a rear gear of 3.25 should net you around 2900rpm at 65mph. A rear gear of 3.50 should give around 3125 rpm at 65mph.

If your tires are really 25.5 inches tall that would make each of those tach readings a little over 100rpm lower.

Well, that's what the math says. Someone with real world experience might chime in with something different.

Steve

[shelbydoug]

It all depends on what you want out of driving the car.

Theoretically you could run a 2.50:1 9" rear but as far as acceleration, forget about it.

The best combination of having your cake and eating it too would be to run something like a Richmond 5 speed with a 3.26 first gear and use a 3.25 rear ratio.

That would give you a reasonable cruising rpm, a first gear acceleration ratio of what you would put in a GM with a 2.20 first and a 4.88 rear for acceleration with the close spaced gearing of a T10.

If you want lower rpm cruising then that, then you need the 6 speed where 6th is in overdrive by about .7:1


These considerations were never possible back in the day and along with electronic fuel injection are the largest changes to the original packaging of a "current real high-performance vehicle" for the street.