I found some more information...
It was a good idea in 1984; you could order your new Corvette with a four-speed manual transmission that had an overdrive on the top three gears, and suddenly had a seven-speed transmission in your Corvette. This 4+3 transmission was really nothing more than two separate transmissions mated together to form one unit. By combining these two transmissions, Chevrolet was able to give us seven forward gears. We have the EPA to thank for this marvelous combination, but more on that later.
This poor 4+3 transmission was put down from day one. Everyone complained about the way the transmission shifted, and the way the overdrive unit worked. Most of this wasn't justified. A lot of the people who complained about the 4+3 had never driven one that was properly adjusted. The fact that the average Chevrolet dealership didn't know, or didn't care, to properly adjust the transmission, didn't help its reputation one single bit. This unit also suffered because the German six-speed that replaced it in 1988 was really much better. Nonetheless, if you have a 4+3 in your Corvette don't get all worked up. This is really a good transmission.
The 4+3 manual transmission was the old Borg-Warner T-10 that had been around for decades. Doug Nash, who is no longer in business, bought the rights to the transmission from Warner Gear in June of 1982, and built the transmission to Chevrolet specifications. The case was strengthened, and several flanges were added to the rear for the mounting of the overdrive unit. This basic four-speed unit has proven to be bullet proof, and very few people have ever had a problem with the manual section of the transmission.
One of the critical engineering tests Chevrolet was using at the time was called a "spike test." The transmission was run at a specific rpm and the clutch was engaged for both low and reverse gear. The test was done three times at 2,000 rpm, three times at 3,000 rpm, and so forth until 5,000 rpm was reached. This was one major reason for the strengthening of the case.Another reason for the different case design was the due to the fact the fourth generation Corvette was designed so that the transmission and rear axle operate as a single rigid unit. The channel beam that you see under your Corvette is put there to tie the transmission and rear differential case together as a single unit. In earlier Corvettes this load was taken up with the frame crossmembers. The rigidity of this assembly is a real benefit to the Corvette, and the same design was retained when the six-speed was introduced in 1989.
The overdrive unit was hung off the back of the four-speed transmission. In a great many ways the overdive unit is very similar to the old two-speed Powerglide. This particular unit was originally designed for the Jeep CJ-7 and CJ-5. Chevrolet was working on such a tight deadline for the fourth generation Corvette that there was no time to develop a new unit specifically for the Corvette. The early thoughts were that the unit from the 1934 Chrysler Airflow could be used. The engineering team also looked at the GKN/Laycock de Normanville units that were used in a lot of European cars.
The problem with these units was that they couldn't handle the torque of the small block Chevrolet engine. In the end Chevrolet went back to the unit that Doug Nash had setting on his shelf. Delco Electronics solved the computer problems and made sure that the manual section and the OD were truly integrated units. All of this was really done to meet the EPA fuel mileage regulations, but at the same time the overdrive fourth gear provided a high-speed cruising capability that no previous Corvette ever had. The top speed of the C4 Corvette is tremendous. At the same time you get the great acceleration in the lower gears with the overdrive disengaged.
The EPA was the major reason that there was never a thought of putting a five-speed in the Corvette. There was no way that a five-speed could have had a 0.68:1 top gear. This was the secret to the 4+3; a tremendous ratio for the top end. Keep in mind that there were no six-speeds available at the time. The irony of this whole situation was that a wide-ratio three-speed transmission would have easily met the EPA mileage figures.
As Dave McLellan explained in 1983:
"the best possible manual transmission is a wide-ratio three-speed. I'm not saying it's much good for a Corvette driving down the road, but it's best for EPA numbers because of the way the EPA test is structured. EPA rules say you can't shift into second below 15 mph, into third gear below 25, or fourth below 40. And if you've got a fifth gear you can't get into fifth until you reach 45 mph. lf you run a five-speed transmission with a top gear for cruising fuel economy, you'll benefit the customer all right, but your advantage in the EPA highway cycle is only about a half a mile per gallon. If on the other hand, you've got only three speeds you can get into top gear a lot sooner, and that'll save mileage considerably, more than half a mile per gallon. Anyway, in a Corvette, what you need for acceleration are four normal ratios, lower than that will simply light up the tires."
_ken
It's currently... "history." 
