Hmmm... by the factory tach, I get my best runs shifting at 5800. Stock 87GT engine, BBK hiflow, 2.5" exhaust, K&N.
I'll lose 2mph shifting over or under.
Now, you say you've been shifting all over the place. If you were shifting really high in the rpm's, which is what the taller 3.08's would have liked, when you switched to the 3.55's, you'll lose ET. Simply because you won't drop as many rpm's, and you're outside your powerband.
You should be shifting around 600rpms lower than what you were, at least.
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