Using shorter drag radials cause any problems?? - Page 2 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-14-2005, 08:28 AM   #11
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Shift points (at least on the 2k+) are based on both rpm AND mph. Smaller tires mean you shift sooner, taller tires mean you shift later (and possible to late, hitting the limiters). For those at bonniefest 05 they saw this on BADSSEi.
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Old 09-14-2005, 08:34 AM   #12
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It'* possible to program for different tires correct?

Only how do you program between street and track? Two pcms?
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Old 09-14-2005, 10:11 AM   #13
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But the stock PCM in this 95 SLE will not know he is going a different speed than the VSS indicates, and shift like normal, even though you are actually going slower.
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Old 09-14-2005, 10:14 AM   #14
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Quote:
Originally Posted by jwikoff99
But the stock PCM in this 95 SLE will not know he is going a different speed than the VSS indicates, and shift like normal, even though you are actually going slower.
That is why I'm asking if it can be done. Might not be the current answer, but a future solution.
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Old 09-14-2005, 10:24 AM   #15
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Quote:
Originally Posted by jwikoff99
But the stock PCM in this 95 SLE will not know he is going a different speed than the VSS indicates, and shift like normal, even though you are actually going slower.
It wont shift like normal, it will shift based on its normal programming. The shift will come early (in relation to stock tires) or later depending on the size difference. If this size difference is large the car may shift WAY early or not at all if it bounces off the limiter (depending on which way the tire is different.).
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Old 09-14-2005, 10:43 AM   #16
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Quote:
Originally Posted by Rogue
It wont shift like normal, it will shift based on its normal programming.
Yes, it will shift based on normal programming, that'* what I meant by normal.

Do you mean late/early in relation to RPM or actual MPH? If the car doesn't know the tire size has changed, why would it shift at a different RPM?
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Old 09-14-2005, 11:16 AM   #17
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Quote:
Originally Posted by jwikoff99
Quote:
Originally Posted by Rogue
It wont shift like normal, it will shift based on its normal programming.
Yes, it will shift based on normal programming, that'* what I meant by normal.

Do you mean late/early in relation to RPM or actual MPH? If the car doesn't know the tire size has changed, why would it shift at a different RPM?
Yes, In relation to RPM. There are tables in the PCM that are based on RPM, MPH, and Throttle Position. If any of the 3 are wrong (in this case MPH) your shifts will be wrong. The severity of which is based on the amount of size difference in the tires.
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Old 09-14-2005, 11:30 AM   #18
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So, if it normally shifts at 5500 rpm, WOT, at 45 mph (just an example), then you change the tire size, wouldn't it still think you are going 45 mph, WOT, at 5500 rpm and shift at the same time?

What don't I understand?
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Old 09-14-2005, 02:16 PM   #19
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Quote:
Originally Posted by jwikoff99
So, if it normally shifts at 5500 rpm, WOT, at 45 mph (just an example), then you change the tire size, wouldn't it still think you are going 45 mph, WOT, at 5500 rpm and shift at the same time?

What don't I understand?
Thats what I thought the only thing that would be running faster is the actual speed you are moving and the cars Guage would be off afew MPH
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Old 09-14-2005, 03:21 PM   #20
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Quote:
Originally Posted by jwikoff99
What don't I understand?
The PCM does not know what speed your car is going. What it does know is the tire size that should be on the car, the differential final drive ratio, throttle position angle, and the input from the VSS (Vehicle speed sensor). Based on this information it calculates the proper shift point in the table. If any of these are off such as you change the tires size, or put different gears in the car, then your shift points will change sometimes drastically.

For example if your moving at 20 miles in first gear and floor it the PCM calculates that to shift at 5800 your speed should be 50 mph with your stock tire size. With smaller tires your speed will reach 50mph (according to the VSS) before 5800 (probably 5600 or so dependng on amount of tire change) and the shift will occur early.

In the same scenario if your moving at 20 mph and you floor it the PCM calculates that to shift at 5800 you should be 50 mph with your stock tire size. With larger tires your speed will reach 50mph (according to the VSS) at say 6000 instead of 5800. If your limiter is set to 6000 your going to bump it.
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