After looking at the thread mentioned by Travis, then the wikipedia article, IMO most people don't have or won't build a motor (small block with mild RV type cam, or big block with a mild cam) that will be happy pulling a .50 or .62 overdrive on the freeway at a reasonable speed say 70mph
-especially with a carburetor-.
Just for fun, I put these numbers into a calculator using a 3.91 rear gear ratio: (28 inch tire would be like a 275/60R15)
the .50 overdrive (6th) gear ratio is for the viper T56
Input Parameters Are the Following:
Differential Gear = 3.91 to 1
Transmission Gear = 0.50 to 1
Tire Diameter (Inches) = 28.00
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 1643
Input Parameters Are the Following:
Differential Gear = 3.91 to 1
Transmission Gear = 0.62 to 1
Tire Diameter (Inches) = 28.00
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 2037
This last one has a common overdrive (4 speed auto or 5 speed manual) ratio of .76
Input Parameters Are the Following:
Differential Gear = 3.91 to 1
Transmission Gear = 0.76 to 1
Tire Diameter (Inches) = 28.00
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 2497
I agree with with a poster in the other thread, the 6 speed is more of a bragging thing than useful unless you build the powertrain around being able to pull these gears.
Just for comparison, for those with low profile 17/18/19" tires, many come out around 24.5" tall (thinking 275/40R17). The T56 might work out better:
Input Parameters Are the Following:
Differential Gear = 3.91 to 1
Transmission Gear = 0.62 to 1
Tire Diameter (Inches) = 24.50
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 2328
Once you start going in the direction of more common rear end ratios (say 3.23 or 3.55), the rpms start to get too low at cruising speed again.
Input Parameters Are the Following:
Differential Gear = 3.55 to 1
Transmission Gear = 0.62 to 1
Tire Diameter (Inches) = 24.50
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 2114
Input Parameters Are the Following:
Differential Gear = 3.23 to 1
Transmission Gear = 0.62 to 1
Tire Diameter (Inches) = 24.50
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 1923
Input Parameters Are the Following:
Differential Gear = 3.55 to 1
Transmission Gear = 0.50 to 1
Tire Diameter (Inches) = 24.50
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 1705
Input Parameters Are the Following:
Differential Gear = 3.23 to 1
Transmission Gear = 0.50 to 1
Tire Diameter (Inches) = 24.50
Vechicle MPH = 70
Computation Results:
Computed Engine RPM is 1551
I would stick with the A-833 with an overdrive gearset or a 5 speed.