I'm going to disagree on this one.
As long as you're not running some crazy radical cam, I'm sure you'd be fine with 3.23 and overdrive. Especially when you factor in typical highway speeds these days. The 383 should have enough low end torque to cruise along at a pretty low rpm. You would not want to over-carb in this situation. 600 cfm max on a 383 to keep the air velocity sufficient for good atomization.
The standard set up in many of the 1994-96 GM full size cars is a 2.56 rear end ratio paired with the 4L60E transmission, which has a .7:1 overdrive, for a final ratio of 1.79:1.
Jeff
There is ONE difference between the GM vehicles referenced and the proposed 4smOD and 3.23 gears. Actually TWO differences. The main one is electronic fuel injection, whether throttle body or port. The other one is the fact that the GM vehicles mentioned have an automatic transmission with electronic computer controls.
The EFI is signiticant as it meters fuel regardless of the air flow going through the motor. Rather than the carb relying upon intake manifold air flow to pull the fuel out of the venturis. This is one of the reasons for the customers raving about how much better their 5.7L TBI motors ran than their old carb'd motors, but by the time for their first free oil change, they were complaining about fuel economy. Why? Because with each throttle movement the computer adds fuel to the mix, rather than the air flow having to pull the fiuel from the venturis AFTER the throttle was openned.
The computer-controlled automatics are significant in that they will not let the engine lug, but will first unlock the lock-up torque converter and then downshift as needed under loads. This happens seemlessly rather than the driver having to be sensitive enough to do it, as would be the case with a manual transmission. Respectfully so.
When it was popular to install the THM700 OD automatic in the place of a THM350 automatic, almost all of them had a 3.08 or higher-geared rear axle ratio. What those people ended up having to do was install something in the range of a 3.55 (or lower) rear axle ratio (on their carb'd 5.7L V-8s). After they discovered that they couldn't use OD until over 70mph.
Almost all of the recent GM piclup trucks have taller tires, direct-injection, and the approx 3.23 axle ratio with the 6-speed automatics. Cruise rpm at 70mph is right at 1700rpm. More throttle and it kicks out of 6th and goes down about 2 gears until the need for power is gone, when it goes back into its normal 6th gear configuration. Even a 6.0L box truck ran 1700rpm at 70mph, but didn't get better fuel economy than about 14mpg average on the road.
Remember, too, that the old 3-speed OD manual trans cars usually had a 3.70-area rear axle ratio and 7.50-14 tires. Which was back when most highways were two-lane and 60mph speed limits.
Now, there was one model year when GM was seeking better CAFE from their 1/2ton pickups and came out with their "Special Economy Truck" option package. NO factory a/c, OD manual trans, 5.0L V-8, detonation limiter ignition, P205/75-15 tires, and something like a 2.56 rear axle ratio. We had aftermarket a/c put in them. In other words, not a really nice Silverado, but more like "Work Truck" with some mpg tweaks. The ones I drove did run pretty good, but OD could not be used until about 70mph or so. IF a hill happened, it was downshift time (as an automatic would do by itself). These trucks worked well enough on the chassis dyno to get the extra 2 mpg GM needed (compared to a normal 5.0L THM700 combination), but they also bought GM some time to get things sorted out better for the next model year's pickups. Understand, too, that they had spreadbord QJet 4bbls, too, which combined with the lower gears in the lower trans gears, worked pretty well with the shorter tires for off-line performance. But when the detonation sensor wires shorted to ground on the rh exhaust manifold, they did good to pull themselves up a moderate hill in 3rd gear at 55mph, by observation.
NOT to say that a 383 and 4smOD could not make a good combination, BUT with the correct axle ratio (to not lug the engine at lower highway speeds), carb size (500-600cfm), intake manifold, and CAM (something like "idle to 4000rpm" for very good low-to-mid range torque, for better throttle response on the road in OD). Then basically build a 4000rpm motor that will make more rpm with a better dual exhaust system . . . "a torque motor that rpms". AND it might well be that with the old 252 degree cam, a modern Edelbrock Performer, with a smaller AVS2, with HP manifolds and a 2.25" dual exhaust might well be closer to this than not, even with the normal 2.76 rear axle and a normal 4-speed trans (non-OD). Fuel economy gains would come from the manual trans being more efficient than a TF and the modern carb and intake.
One other observation . . . a friend bought a '69 Buick Wildct 4dr hardtop from the orig owner in AZ. It had the 430 V-8, power steering, a/c, and a 3-speed manual trans (with 3.08 rear axle). The first owner ordered it that way. Obviously for lots of steady-state highway driving in AZ? BUT my friend soon discovered that the automatic trans was "a pain" in DFW region traffic (and all of the construction or accident-related stop and go traffic on the freeways). AND he likes to find manual trans Buicks, too. Or anything else with a clutch pedal in it. But he does not like to drive them in metro traffic. Too much "clutch action", I suspect.
I kinow that some perceive that they desire a manual trans for the performance/sporty aspect of things. Plus the exclusivity of it all. Nothing wrong with that! Just that sometimes such a project can end up not meeting expectations in all respects, without making other changes to make things work better. By observation, some of my friends seem to be "at one" with the 4-speed cars they have had in the past, which is fine, but they came with the trans already in them from the factory. Me? I can change atf pretty well and that can be less expensive than a clutch set-up. Which was my final decision to order my then-new '77 Camaro LT with a THM350 rather than a 4-speed manual trans. To me, much neater to just "floor it" for a two-lane blacktop pass than to get staged in a lower gear to make my move to pass a slower vehicle (when possible). But cruising down the Interstate at 75+mph, no big deal, either way.
Just some thoughts and observations,
CBODY67
