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What is the top speed of the typical course for you? Do you try and stay in second the whole time or shift? I am trying to help a friend gear his car accordingly. Thoughts? I don't have a lot of experience with auto cross. He has a 1.98 second ratio and a 24.6 inch tire. A 3.55 would yield 70 mph at 6700 rpm. He would have to shift at that point. Worth time going to a 3.33? 500 HP 408. Thanks.
 

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My set-up used 3.36 differential, 1.94 second gear and 23.5 inch tires when I was autocrossing. I almost always started off in second gear and rarely had to shift. 6700 rpm for me is 72 mph.

It mostly depends on the courses in your area as to whether it would be worthwhile going to a 3.33 rear gear. Some of the national event courses can reach 80+ mph but most local and regional courses rarely exceed 70 mph.
 

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Frankly, I don't look at the speedo while AXing. My s550 has 3.73s, 285/40/19s and I stay in second - rarely hit the limiter (65mph) on the local courses. But, a number of guys with other makes/models than Mustang hit the limiters all the time. My '86 Mustang has 3.23s, a T5 and 255/40/17s. Left in 2nd, the 23s seem to be a great gear for a stock revving 5 liter. But I'm moving to a 3.55 and 305/30/18s.
 
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