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<blockquote data-quote="GaryM" data-source="post: 31223" data-attributes="member: 9373"><p>Exactly. The paddle may show that 8th is selected but unless all the criteria is met it will stay in 7th gear, which is 2200 RPM. You can achieve 8th gear w/o the paddle shift by adhering to this criteria, &lt; 62 mph, no throttle. It will shift every time. However, if you give it a bit too much throttle to get up to road speed it will down shift to 7th where it will stay. If it's shifting right, at 70 mph engine RPM will be 1800. The criteria is quite complex but in short it contains speed, throttle position, acceleration rate, ATF temp, loading % (combination of acceleration and engine load factor). The TCM has programming that uses all that plus fill time in the shift cylinders to determine if it's &quot;safe&quot; to move to the next gear. That's where the ATF viscosity comes into play. The factory fill is GM Dexron VI. Good stuff but in cold weather it's thick, which slows down the fill rate in the shift cylinders. That could cause it not to shift to the next gear, or in their terms. reject the shift. In some cases it may allow the shift but because the shift cylinder fills slightly slower you get a &quot;shudder&quot; in that gear. I had it in second gear until I changed the ATF to Mobil 1 Devron VI which has a better cold weather pour point. Shudder is gone.</p></blockquote><p></p>

[QUOTE="GaryM, post: 31223, member: 9373"] Exactly. The paddle may show that 8th is selected but unless all the criteria is met it will stay in 7th gear, which is 2200 RPM. You can achieve 8th gear w/o the paddle shift by adhering to this criteria, < 62 mph, no throttle. It will shift every time. However, if you give it a bit too much throttle to get up to road speed it will down shift to 7th where it will stay. If it's shifting right, at 70 mph engine RPM will be 1800. The criteria is quite complex but in short it contains speed, throttle position, acceleration rate, ATF temp, loading % (combination of acceleration and engine load factor). The TCM has programming that uses all that plus fill time in the shift cylinders to determine if it's "safe" to move to the next gear. That's where the ATF viscosity comes into play. The factory fill is GM Dexron VI. Good stuff but in cold weather it's thick, which slows down the fill rate in the shift cylinders. That could cause it not to shift to the next gear, or in their terms. reject the shift. In some cases it may allow the shift but because the shift cylinder fills slightly slower you get a "shudder" in that gear. I had it in second gear until I changed the ATF to Mobil 1 Devron VI which has a better cold weather pour point. Shudder is gone. [/QUOTE]

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