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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?
 

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I vote for the torque convertor...how did you account for it's effiecency in your calculation? Everything else is pretty much a direct drive, without any loses.
 

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I think its the converter or exact tire measurements...


at a given speed i can be doing 3000 rpm on deceleration, or at the same speed, 3400 or so at full throttle.. so theres quite a variation there..

Mike
 

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I'll second that, unless you run a lock up convertor or a manual trans the faster you go the bigger the deviation from the calculated figures. There are other factors like tires growing at high speeds and wind resistance that will affect the #'s also.

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Philip Valentine
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I think that the converter is the cause of part of your puzzlement, but certainly not all. Your converter may "slip" about 2%, but it cannot be responsible for the 10% disparity that you have seen. Can you install a second Tach and do the measurements again? It is probably 2% converter and 8% tach error. If your converter were slipping 10% , the oil coming out of it would likely overload the trans cooler.
 

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I would never put too much faith in the calibration of any gauge, especially a 30-year old one. Everything must be suspect until proven otherwise. --- Carl
 

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Discussion Starter #7
Thanks for the replys guys. I also tend to think it is the transmission. I have ordered a supplemental tach from Jeg's in an attempt to eliminate that as the cause.

Torque converter slippage might explain why the car's temperature seems to run hotter the longer you drive. Maybe it's exceeding the trans. cooler's ability to do its work.

On the other hand, the torque converter is very tight - feels like it's locked up. When you let off of the gas, it will really slow you down like a manual transmission.

I'll keep you posted.
 
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