I can imagine the original owner taking delivery of his car and immediately taking it to the local drag strip and pounding the crap out of it. You see the 1964 Pontiac GTO is regarded as the first true muscle car and It is also probably the most famous muscle car in high-performance automobile history. You basically take a light mid-size car and drop a big block V8 motor into it. The results were fantastic!
Our 1964 GTO came from the factory with a Muncie M21 close ratio 4 speed transmission and 3.90 gear ratio in the rear differential. What do those numbers and names mean? It means the car was ordered to get from A to B as quickly as possible, but not made for long trips on the expressway. When I first started driving our GTO I was pulling around 4000 RPM at 70 MPH. It seemed to handle the RPMs OK, but I could visibly watch the fuel gauge move towards empty. If we were going to enjoy taking the GTO on any sort of road trip we would have to do something about it. We had several options which seemed like good ideas. One was changing the gear ratio to 3.21 or so to help drop the RPMs. This was probably the cheapest and easiest solution, but we would loose the seat of the pants take off feel you get when mashing on the skinny pedal that I had become accustom to.
The solution we chose was to replace the M21 4 speed with a Tremec TKO-600 5 speed. The swap is as close to bolt in as you can be. That being said there is a little body work that needs to be done do the the Tremec being larger than the original transmission. You can compare them in the picture above. In the picture below you can see the stock transmission in the stock location.
Here is the M21 in it's stock location
Front view comparison
Hole cut to clear the Tremec
As you can see there is definitely a size difference and some body work is needed. I used a template and plasma cutter to make sure the hole was as precise as can be. I have heard you can modify the rear transmission mount to lower the transmission so that it will clear without cutting the floor, but it also changes the angle of your drive-line and could cause it to bind at the wrong moment and cause serious failure.
Getting the Tremec in place
The other item I had to address was the fact that our GTO came with a center console. Which meant we would need to offset the shifter. Tremec sells several different top plates to help with this issue. Now that we had the transmission in place and the proper top plate it was time to cover up the gaping hole we cut. A rolled piece of sheet metal cut to fit will work best. Some companies offer this with their transmission kits. The piece needed to be massaged and tacked into place.
Patch panel being fitted
Patch panel from the passenger side
The patch panel needed some work to get it to fit decently. This was probably the most time consuming piece of the project. It could be that I’m a caveman when it comes to metal shaping (getting better) and need to learn finesse. The only other issue we ran into was that now the floor was taller the carpet needed to be “stretched” out. Other than that it fit very well.
Added carpet over the patch panel
This is by far the best modification we have done to our 1964 GTO. We still have the gears to get offline quickly yet can cruise at 70 MPH all day long circa 2100 RPM. We immediately drove it 700 miles on a road trip without any issues. We increased or fuel economy to boot. I would recommend this upgrade to anyone who was thinking about doing it.
Gear Ratio Comparison
M21
1st 2.20 | 2nd 1.64 | 3rd 1.28 | 4th 1.00 | 5th X.XX | R 2.27
TKO 600
1st 2.87 | 2nd 1.89 | 3rd 1.28 | 4th 1.00 | 5th 0.64 | R 2.56
Dog dish hubcaps!
