The 1973 AMC Javelin AMX that I purchased has a 401 engine, 4 speed on the floor BW T10 transmission,  and a AMC 20 rear end. The car has power steering, power brakes, and the GO PAK option. The 401 is equipped with Hedman Hedders, Crane cam (click for specs), custom head work, and an Edelbrock Performer Series Square-Bore 600 cfm Electric choke (#1406) carburetor.

The VIN for the car is A3M798Z1xxxxx and decodes to:
A -> Manufacturer is AMC
3 -> Model year is 1973.
M -> 4 speed on the floor
7 -> Javelin
9 -> 2 door hardtop
8 -> AMX option
Z -> 401 engine
1 -> Built in the Kenosha WI plant

The code from the tag on  the engines valve cover is 509Z28 which means a 401 built on 9/28/1972.

The Borg Warner T10 most likely has the close ratio of 2.23, 1.77, 1.35, and 1:1. 

Trim code is 341S which is black vinyl.

Paint code is D7 which is Trans AM Red, this is a sort of orange red. The car is currently painted 1995 Ford truck Electric Currant Red.

I took a quick look at the rear end for the gear ratio code but could not find it. I like math so I decided to try to figure out the ratio. Knowing the tire size, rpm, and speed should be enough to make an approximation of the ratio. The tire size is P225 60VR15, 225mm/25.4 gives a tire width of 8.858 inches. The tire has an aspect of 60, 60% of 8.858 inches gives a height of 5.315. Rim size of 15 inches plus tire height of 5.315 plus height again gives a diameter of 25.63 times PI gives a tire circumference of   80.478 inches. To determine the number of times a tire needs to rotate to travel 1 mile divide 5280 feet (or 63,360 in.) by the circumference of the tire, 63,360 / 80.478 is 787.296 rotations/mile. Next I drove the car at 50 MPH because it was easy to maintain in 4th and get a good look at the tach. At 50 MPH the tach read 2,600 RPM. For the tire to travel 50 miles it will rotate 787.296 X 50 which is 39364.795 times. An engine running at 2600 RPM will turn 2600 X 60 (minutes in an hour) times in an hour 156,000. Engine rotations divided by tire rotations 156,000/39,364.795 says the gear ratio is 3.96. Using more accurate values gives 3.94. Looking through the '73 sales brochure shows two axle ratios available on the 401 4sp either 3.54 or optionally 3.91, so I will guess the car has a 3.91 ratio. Of course this whole thing depends on how accurate the tach and speedometer (which is affected by tire size but a 225 60R15 is darn close to the original E60X15) are.