Al G. -- The fifth gear issue on a TKO is a perfect example of why you can not take a blanket statement on any aspect of your Cobra build and rely on it as the gospel truth. You have to research the issue until you are at the point where you can tell yourself
"I understand this issue, I understand how the variables that are involved will comparatively impact the outcome, and I can now figure out the answer that best suits me all by myself." To help you along (even though we all know that one-time posters 99.999% of the time will never get a Cobra), here is the decision tree for deciding between a .64 and .82 TKO.
If you are going to be doing any serious racing at speeds above the redline speed of your fourth gear then you must choose the .82 transmission. For most of the folks around here, that's roughly 125-135 mph. Otherwise, you must ask your engine builder
"What is the lowest RPM that my cam will comfortably drive on the highway without hiccupping?" Take that RPM value, along with your rear end ratio, and your tire size, and using an RPM calculator available on the web, calculate the resulting speed with the .64 ratio fifth gear. If that value is equal to or less than the customary highway cruising speed that you expect to be driving, then the .64 gear is a viable option for you. Otherwise, you must go with the .82 gear.
As an example, I have 15" wheels, a 3.54 rear, and my cam is comfortable on the highway at 2000RPM. Using the .64 fifth gear, that calculates out to 68MPH. In my neck of the woods, the far right lane on the major highways moves along at 70MPH and state troopers will be passing you on the left. Thus, for me and my build, the .64 gear was the right choice (and my first four gears will still take me to 140MPH). The .82 fifth gear would be the wrong choice for me.
Al, I'm not going to rag on you, but you really have to strap on your thinking cap when your spec'ing your Cobra. Don't rely on what we tell you other than to use it to understand the issue itself -- then you have to figure out the answer. After all, we're not going to be driving your car.
(But Mr. Mustang will buy it from you for a song after you spec it wrong and put it up for sale on Cobra Country.)
