1974 PontiacTrans Am Gears - Pontiac Tech

Transmission Speedo Gear
Changing the rear-gear ratio requires a recalibration of the speedometer. This can involve changing the speedometer drive gear, driven gear, or both.

In this case, we determined that our speedometer was turning 5-mph faster than it should be because 50 mph on our speedometer showed an actual speed of 55 when tested. If we divide 55 by 50, we get 1.1 for our correction factor. That means the driven gear in the transmission is moving 0.1 faster than it should for our gear set. This can be corrected by adding teeth to the driven gear or by subtracting teeth from the drive gear.

There are two types of carriers for the driven gear. The first will hold driven gears that have 34-39 teeth, while the other can run driven gears with 40-45 teeth. The carrier in this Turbo 400 was for 34-39 teeth.

Driven gears are color-coded to help determine the number of teeth they have. These colors can fade and be hard to identify. We confirmed our tooth count by marking a tooth with an indelible marker and counting the teeth. We did this more than once to be certain of the count. The driven gear that was matched to the original 3.08 ring-and-pinion gear set had 36 teeth, according to our factory service manual.

Our previously calculated correction factor of 1.1 can be used to determine how many teeth are needed to correct the speedometer. The existing driven-gear tooth count can be multiplied by the correction factor (36 times 1.1 equals 39.6).

We have two choices based on the results-either round up or down. Since our existing driven-gear carrier is capable of holding the 39-tooth gear but not the 40-tooth gear, we elected to go that route to avoid disassembly of the transmission to change the driven gear. If our speedometer is off, it will be minimal. The proper 39-tooth driven gear was purchased at a local GM dealer, and the speedometer correction was complete. TCI has a great chart on driven gears, colors, applications, and more on its Web site, www.tciauto.com.

Drag Testing
As with any track testing, it's virtually impossible to duplicate every factor. The biggest variable can often be the weather. For this reason, we attempted to return to Centerville Dragway with similar weather conditions.

The '74 Trans Am test car features a 462 engine, unported SD heads milled for 9.2:1 compression, Crower 247/252-degrees duration at 0.50 with 0.505/0.517-inch lift., a port-matched Performer RPM intake, a Speed Demon 850 carb, Hooker Super Comp 2-inch primary headers with 3.5-inch collectors, a 3-inch Torque Tech exhaust system, a Dr. Gas X-crossover, and Dynomax Super Turbo mufflers. The tires are M/T 275/60-15 drag radials pumped up to 17 psi on widened 15x8 Rally IIs.

Our initial runs were made on a crisp spring morning with temperatures in the upper 60s to lower 70s. The humidity was virtually nonexistent. In short, it was one of those perfect days at the track. The result was new personal bests for the T/A with the old 3.08 gears. Our follow-up testing was on a day that was an average of 10 degrees cooler but with similar humidity levels.

The first number to notice when comparing our runs from before and after the gear swap is the improved 60-foot time, which dropped an average of 0.14 seconds. This tells us the deeper gear ratio is getting the heavy car off the line quicker. That improvement was amplified at the 330-foot mark with an average drop of 0.19 in elapsed time. The averages kept getting better as the e.t.'s dropped by 0.23 seconds at the end of the eighth-mile track with a mile-per-hour increase of 1.49.