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Discussion Starter #1
Trying to max out the performance of 4.3 Silverado (not diesel sorry)

Thinking about a higher ratio going from a 3.42 to a 3.73 or 4.11.

Looking for more pulling power and off line acceleration... Now the question.
How does the trans see shift points in a change out? Is it going to be actual road speed or trans rpm. Is there a way to maximize shift points without reprogramming. How does tire over size change things?
 

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Trying to max out the performance of 4.3 Silverado (not diesel sorry)

Thinking about a higher ratio going from a 3.42 to a 3.73 or 4.11.

Looking for more pulling power and off line acceleration... Now the question.
How does the trans see shift points in a change out? Is it going to be actual road speed or trans rpm. Is there a way to maximize shift points without reprogramming. How does tire over size change things?

Is this is a 1/2 ton? 3/4 ton? 4L80E? 4L60E?
This more of a question for a PCM programmer. I do a little performance work. I will do my best.

The basic's are the same for either unit, the programming is a little different between them.

They both use a MAF, MAP, TPS and VSS to make all shift speed calculations The MAF sensor is the main input for calculating load and internal pressure rise. The MAP is sort of the back up unless the vehicle is speed density calculated, then it is the main load measurement device.
The TPS tells the computer of the user requested throttle opening.
VSS tells the computer road speed.
The PCM makes a calculation based on tables programmed in.
When you change rear end gears. Most computers have a window for the difference. 3.42, 3.73 & 4.11's are all factory gear ratios and are probably in the pre-programmed parameters. if you look to buy a PCM, they only want the # for the computer. They don't ask gear ratio or tire size. The Beauty of the the newer PCM's.
If going to lower gears say 4.56's the PCM will need to be given a buffered signal or will require programming. Same with huge tire changes, because they will take a 3.73 gear down to 3.42.

The tire size will affect the overall load to the motor, larger tires have less RPM's per mile. They will effect shown MPH and actual MPH.
Tiresize.com has a huge data base for tire comparison. There is also a page for calculating overall gear ratio based on the new tires.
I hope this helps...
 

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Discussion Starter #3
Helps a bit. So if I wanted to retain factory shift points with a axle swap I would need to get the computer that came out of a truck with the ratio in that axle? Where would I find the maximum shift points for a given truck / transmission? Or is that info even obtainable?
 

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Discussion Starter #5
As always you pull a rabbit out of your hat. Any chance you can tell me what Trans Cal means on the left hand column? A B C Thanks a Bunch!
 

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As always you pull a rabbit out of your hat. Any chance you can tell me what Trans Cal means on the left hand column? A B C Thanks a Bunch!
I do not know what that means, might be what Tranzman57 was talking about.
The gasser PCM's work differently than the diesels. Our diesels use the VSSB to send an "adjusted" speed signal to the PCM, speedo, ABS, even the radio. The gasser has that function built into the PCM and AFAIK it has to be programmed to match conditions or any changes to tire size and/or gear ratio. I was guessing on the vehicle for that chart, I should have the chart for your gasser if you give me the make, model, year, and tranny.
 

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As always you pull a rabbit out of your hat. Any chance you can tell me what Trans Cal means on the left hand column? A B C Thanks a Bunch!
Sorry i did not know you had asked this question. Trans cal is the slight spring changes made for the different gear ratio's/car/truck. If you look at the chart the differences are usually less than 50 Rpm's different. Most of this is so insignificant that it goes unnoticed after a swap. Most of us hotrodding these transmission's just live with the slight change in MPH/RPM on the shift points
 
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