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The 74-78 Mustang II's and Cobra II's with automatic transmissions all used the Ford's C4 transmission. The V8 models used a one of a kind bellhousing, torque converter, and 141 tooth flywheel that was smaller than all the other C4s to help with clearance problems. The Stock C4 transmission is a very good, reliable and strong transmission with 3 speeds (gears) that can take alot of horsepower and abuse. The 3rd gear had a 1-1 ratio and would show about 3000 RPM at 70 MPH with stock wheel and tires. In later years Ford came out with a 4 speed transmission they called the AOD. The AOD was used on Ford cars and trucks from early 80's until about 1992. AOD stood for Automatic Overdrive which had a 0.66 -1 overdrive or 4th gear. This means that at 70 MPH the engine would only turn approx. 2100 RPM. That's 30% less than the C4 transmission at highway speeds meaning 30% better gas mileage. If you are getting 15 MPG with a C4 tranny you could achieve about 20 MPG or better on the highway. The AOD has a much wider bellhousing and transmission pan (about a 5-6 inch difference) but it will fit and work in a all older Mustang's and Cobra II's . There is some clearance problems with the exhaust and the shift lever on the transmission that will need addressed. you will have to use the stock exhaust manifolds or have custom headers made as there are no manufactures making headers for this application. There are one or two companies that make a set of headers which require cutting the inner fender walls that may work but none that will work without cutting as far as I have found. The overall swap is not too complicated and there isn't alot of fabrication but you will need to do some welding on the new tranny crossmember brackets and the shift lever if you want to keep the stock M II shifter. You will need to spend about $200-300 in extra parts besides the tranny cost. Finding a good AOD used tranny will be most of the battle unless you are going to start with a rebuilt one. One last word about whether or not to make the swap. The Stock C4 is a great tranny for this car. it is cheaper to repair and upgrade and takes all the abuse you want to throw at it and it was made to fit. If you are just going to drive this car as stock and want to keep it original or don't have a good mechanical ability then the AOD swap may not be for you. If you are considering the AOD swap and have some mechanical ability, like working and modifying your car then you may want to consider this swap. Besides the reason of increasing gas mileage another advantage of this swap is gear ratio. The other reason for swapping to the AOD is if you want to swap your gear ratio in the rear end and not lose any gas mileage. You can actually swap out to about a 4.00 rear end gear and still have the same final drive ratio. Example: you have a 3.00 rear end gear and swap to a 4.00 rear end gear. With a C4 tranny you will have to turn 30% faster rpm to make the same amount of speed. With the AOD's 0.66-1 (30%) overdrive gear you would turn the same RPM's as you did with the 3.00 rear end gear ratio. If you Swapped to a 3.73 rear end gear or less you could actually get better performance and a slight improvement in gas mileage. I want to close this with a word of recognition to a great friend and ford man " FERG" He was a huge help in making this possible.
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