I did a lot of research into this over the last few months looking to top 190mph at the Texas Mile.
Last March I did 161mph at the half and 186mph at the full mile. (We did have a bit of a tailwind that day, so these are a bit inflated.) I would like to bust 190mph with the GenIV engine and I still need to port and polish the intake. Currently it is around 600rwhp. So as you can see from the numbers above, the car is a dog in 5th gear in the second half of the mile. I looked into different gear for 5th and 6th, and then had my mind made up that was what I was going to do. I then took a 3,000 mile DustBall Rally trip in it and I was DAMN GLAD I did not change out 6th. I am going to go to 3.55 rear end (which I did in my Gen III and like it) and see how it does in the Mile. If I still think gearing is holding me back after that, I will change the 5th gear out.
Here is a great gearing calculator if you are looking to see how the match works out on changes. You will need to put your own data in for rear gears tires etc. Stock ratios are below.
http://xse.com/leres/ss/calculator.html
0809: 2.66, 1.82, 1.30, 1.00, 0.741, 0.50
2010: 2.66, 1.82, 1.30, 1.00, 0.796, 0.50
2013: 2.66, 1.58, 1.19, 1.00, 0.770, 0.63
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