I have created my own spreadsheet and used every one of the calculators referenced in http://www.chevelles.com/forum/Forum1/HTML/003846.html . Everyway I figure it, my car should be going ~80 mph at 3,000 rpm, but it's not. At 3,000 rpm, I am going 72 mph (verified with a police radar unit in my car). I have measured the rolling distance of the rear tires AND counted the teeth on the ring and pinion gears. I can only think of two things that could cause my calculations to be that far off. Either the factory tachometer is incorrect (which for purposes of setting the idle speed vs a hand-held tach/dwell meter was correct at 800 rpm) OR there is a problem that I cannot imagine with the transmission (T400) causing the engine to spin higher rpm than it should. The transmission has been recently rebuilt and has a stock converter. I am inclined to trust the tachometer, although I'll probably try to verify that next. Does anyone have any other ideas?