Gearing is much less than an exact science. I definitely
don't have any concrete answers for you here. I can only give my thoughts and
assumptions, and the directions I'd consider. What you end up doing
is up to you only, and
either way you go, it should end up fast ('cuz it's an Olds, and that's
inherent in the design, right?). The gearing and RPM range of the motor will
just dictate when that fastness is most noticed.
What do you want? Good street performance or 1/8 or1/4 mile performance?
If you want good street performance gear it so you keep the rpm's in the
optimum power range of your motor at your typical cruise mph. If you
want straight line performance gear it so that the motor is at the top
of the power band at the end of the track. You also want to make sure
that your tires can handle the extra torque that a steeper gear will
create at the tires.
Running 3.23's, I think it's too steep for a BB and the
highway. For racing, though, I'd go a bit steeper. 3.50's might be fine, or
3.73's might be where to shoot for. It will probably all boil down to your
engine's redline. If your engine can stay at 8,000 RPM for a while without the
mechanical rev limiter (aka rod) taking over, then the steeper gears will be
great. The 455's usually don't get that high, though, (without BIG$$$)
which should leave you topped before 6000, I'd assume. With 4.11's and that
range, I'd guess you'd likely be topped out before the end of the quarter,
I'm not sure. My GS with the 3.42's goes through at just over six, usually,
but its RPM characteristics are markedly different than the Olds was for me.
OTOH, with the really steep gears, you'll top out faster, but you'll have
alot more speed keeping you going. You'll only go 100 MPH, but you'll be
there alot sooner than the other guy, and he'll still have trouble keeping
up, even if he goes through the traps at 120 MPH.
I guess in my eyes, it will depend on what RPM range your motor
produces it's torque. If you'll have monster low end torque, but
limited high end, then a lower (numerical) gear would probably be better.
If you have a lot of power at lower RPM's, you may be happy with a 3.23
to 3.55 gear, and still maintain highway drivability.
If you have nothing but high end power, than the motor will need all the
help it can get to get the sled moving, so the 3.73's and higher
(numerically) would be a better choice. You need a 3.73 - 3.90 gear
in order to get the RPM's up high fast.
With 3.90's and 245R60/15's you are turning 3500 RPM at 60 MPH.
If you plan on streeting it, you'd
want to modify those numbers a bit, too. I'd have a hard time recommending
anything steeper than 3.73's for the street, unless you get an overdrive in
there. It'll drive ya nuts on the highway. It looks like with the proper
selection of tires, you'll have no problem locking them onto the pavement.
The stall and gears will like that.
The rear end gears you pick depend on what you want the car to do. The
Dart's 3.55's and high RPM HP isn't too practical for around town (35mph-45mph
speed limits), because to enjoy it, it has to be wound out too much, which
results in going too fast when really enjoying it. 3.73's or 3.90's
would be better suited for this car for power, but it will kill all highway
drivability.
I recommend 3.23's highly. I like the good
combination of torque at lower rpms, and gas mileage that is acceptable.
If you're running a tall tire (30"+), then you need to take that into
account in your gear selection. That 3.50 gear and a 27" tire might be the
equivalent of a 3.00 gear with a much taller tire. Figure what RPMs you
can live with at highway speed, and then figure out what gears will put you
there with your tire and tranny selection (you might have to go with 4.56's
to get you there with a tall tire).
Some examples:
1970 Buick GS Stage 1 with a 3:23 posi. Used for 3 years as a family
car. The RPMs were not objectionable at all; my wife never complained.
The car was always quiet & smooth. I believe that I was
turning a little under 3000 RPM (27-2800?) at 70 MPH. Gas mileage was
14 MPG on the highway.
1973 Vista Cruiser w/455 & a 3.23 posi was always very quiet at 70 MPH
& (after modifications) got 14 MPG on the highway. The Vista Cruiser was
owned for 13 years with 130K miles on it. It was used often for trailering long
distances.
1962 Jetfire with a 3.31 diff is just fine as far as highway engine
noise. The 1968 442 has a 3.91 posi which is far too noisy at highway
speeds. I am trying a 3.42 posi diff in my current 1970 GS, hoping that
I will like it. It may be borderline though.
1964 Olds (330/4V) is a 3.36 diff and 215R70/14's: the 3.36's are a good mix
of matching the 330's mid range torque and high RPM HP with a drivable
car on the freeway. It turns 3,000 at 60 MPH; good passing, 40-70 MPH (kick
down 2nd gear) acceleration is a lot of fun.
1969 Dart (built 318/4V) with a 3.55 diff and 225R70/14 tires.
The engine is built
for high RPM HP, it has nothing below 3000 RPM. The torque comes in at 3,000 RPM, and the the HP come in at 4,000. It turns 3,200 RPM at 60 MPH.
1974 Plymouth Satelitte (440/4V) with a 3.23 diff and 255R60/15 tires:
tons of
low end torque match the 3.23's at 35 MPH to 90 MPH (with 2 gear changes)
quite nicely. The 3.23's allows a kick down to second at 50 MPH. This thing
is a monster between 45 MPH and 70 MPH. (ex-cop car).
1970 4-4-2 with the standard 455 (mildly modified) and 3.23 diff.
At 60 MPH, I could punch it and instantly pass anything I ever met.
Yet the RPM's were comfortably in the 2500-3000 range.
Cutlass with 3.42's and 25 in. tall 235/60/14 tires. It turns about
2700 RPM @ 60 MPH. I like the 3.42's for all around performance
and driving ease.
Additional Alternatives
Why compromise? Go with the overdrive transmission and keep the
3.73s! Alternatively, get a Ford 9" and two center sections, one
with 3.91's and one with 3.08's. Switch gears for your upcoming driving
needs.
I have 3:91's in my H/O and, while the engine tachs out on the
freeway, I don't mind the noise (I'm on the far end of that
tolerance scale).
Actually, have you really considered the cheapest way out? Get a
set of tall rear tires for highway cruising. This will
effectively give you a lower numerical rear end for improved
highway mileage and reduced rpms. Just swap the shorter tires
for strip terrorizing. The only problem is that your speedo will
be wrong with at least one of these configurations!
BFG makes some T/As for street trucks which are pretty tall (up to 35"
diameter, which obviously won't fit in your wheel well). These
are not the off-road tires which most stores stock, so you may
have to order them.
[ Thanks to Doug Ahern, Charley Buehner, Bill Culp, Joe Padavano,
Dave Wyatt, Bob Barry, Daren for this information ]