What affects RPMs?

I've been noticing that my RPMs can differ as much as 300RPMs at the same speed... why is that?
And it doesn't seem like T-tops on or off is a factor...
Went on a trip yesterday... 340 miles one way with the t-tops off and the windows down.. made 28.6 MPG... came back the same trip windows up and t-tops on made 27.8 MPG... weird
I noticed going up at 70, RPMs were 2200... but on the way back at 70 it was at 2500+. I thought that was weird. Any thoughts?
-Geno 1985 Blue Camaro 2.8L auto w/T-tops (aka "Tazmaro") 1988 Blue Firebird Formula 5.0L auto w/T-tops 1985 Brown Firebird 5.7L (360) 5-speed NO T-tops (still looking for an '82 Black T/A w/T-tops)
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It wasn't uphill coming back was it ?
--
Mitch - 1995 Z28 LT1 M6
terminal snipped-for-privacy@sand-hill.freeserve.co.uk
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Lol.. no.. I was noticing it the whole way... and it was the exact same trip... mostly flat roads...
-Geno 1985 Blue Camaro 2.8L auto w/T-tops (aka "Tazmaro") 1988 Blue Firebird Formula 5.0L auto w/T-tops 1985 Brown Firebird 5.7L (360) 5-speed NO T-tops (still looking for an '82 Black T/A w/T-tops)
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Torque converter clutch not locking up on a 700R4 auto trans is what you're describing. To see if it is locking up, (driving in 4th gear of course, not coasting), slightly press the brake pedal enough to make the brake lights come on. The RPM should increase about 250 rpm on level ground. Many things come into play to lock it up. Brake light switch, manifold air temp sensor, coolant sensor, 4th gear switch inside trans, throttle position sensor, vehicle speed sensor input, and the ECM. Usually the ECM will set an error code when any of these is amiss, but not always. For example the 4th gear switch not closing.

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LT-1 with a 6-speed - 2500 rpm in 6th will get you about 100. 2000 is 6th is worth a shade less than 80.
Tom 97 Formula Convertible, Red LT-1 w/6 speed
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snipped-for-privacy@uwplatt.REMOVE.edu says...

Sounds like the torque converter isn't always locking up.
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Dennis Smith
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A tail-wind versus a head-wind?
I know thirdgen f-bodies have decent aerodynamics even by today's standards but that's got to make a difference.
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KITTvsKARR wrote:

I'd say your lock up torque converter isn't always doing it's job.
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