Using shorter drag radials cause any problems?? - Page 5 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-14-2005, 04:42 PM   #41
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Originally Posted by rjolly87
okay, lets blow this out of proportions, and put little 6 inch tires on <no they wont fit, but im making a point> and set him loose on the track. he takes off, and when the vss reads 50 mph he shifts correct? it is after all at the top of the gear according to the PCM calculations. but what you are saying, is that it will shift lower, at say 3000 rpm, instead of 5800, because that is when it is doing 50mph. if that were the case, and you could magically change the shift points to be at the top of the gear, you could roll 100 mph in 1st gear<according to the vss> which doesnt make sense to me. am i correct in saying that first gear is made taller with smaller tires?

***please note, this is pretty much what i understand you saying, just with differant numbers***
well, due to the "artificially" higher speedometer reading (caused by the short tires), the car will "see" 50mph much sooner. BUT, the car is NOT at 50mph in reality...and that'* where the problem would lie. The VSS tells the speedo it'* at 50mph, and the PCM tells the transmission it'* at 50mph and it'* time to shift...If rogue is correct, and the car'* upshifting is controlled by BOTH the mph and rpm, I'm going to have a problem...the pcm will be confused and who knows where or when it will shift into 2nd gear, or even if it will upshift... It may only be at 4000 rpm'* or whatever and that'* solely because of the shorty tires faking the car out into thinking it has a steeper gear ratio, but not having a different speedo gear to compensate...obviously since the VSS picks up it'* signal from a reluctor wheel, there'* no way (mechanically) to compensate for that.

I'm going to try and get some rims somehwere tomorrow and try this out...
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Old 09-14-2005, 04:44 PM   #42
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Originally Posted by Rogue
The VSS calculates speed based on how many times a magnet reads a tooth in the 32 tooth reluctor wheel around the differential. The PCM Calculates speed based on how many times a hit (signal) is made on the reluctor.

When gearing is changed in the car (example 2.93 to 3.29) we used to put a reluctor spoof on, change the amount of teeth to 34. The signal hit rate would be the same time per rotation now with 34 teeth instead of 32.

Without telling the PCM anything has changed it will contintue to calculate speed based on the hit rate of the relecutor. Because the tires are smaller it will spin faster give a false reading which is why your speedometer has an inflated number (doing 75 instead of 72 for example). Because the PCM calculates shift tables based on rpm and vehicle speed an artificially inflated number will cause that calculation to be off.

In rjollys example above the VSS will see 50 mph almost instantly with 6 inch tires due to the rotation of the reluctor spinning so fast and so it will shift way way WAY early.
okay

also in my example, i mentioned the shift being @ 50mph @3000 rpm. it this what you are saying also <principle only please>
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Old 09-14-2005, 04:46 PM   #43
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I just wish someone KNEW if I could simply manually shift the car and avoid all this possible problems???? If I have the shifter in LOW 1or whatever it says on the console, there'* no way it will upshift itself electronically, is there?
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Old 09-14-2005, 04:50 PM   #44
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Originally Posted by rjolly87

okay

also in my example, i mentioned the shift being @ 50mph @3000 rpm. it this what you are saying also <principle only please>
yes it is...I totally understand Rogue'* theory now.

I guess the problem is that which one "rules" the PCM...rpm or mph??? If they conflict with one another, that'* where the issues will be...
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Old 09-14-2005, 04:50 PM   #45
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I just wish someone KNEW if I could simply manually shift the car and avoid all this possible problems???? If I have the shifter in LOW 1or whatever it says on the console, there'* no way it will upshift itself electronically, is there?
on the new stuff it does shift on its own, even though its in 1, but, i dont know about yours though.
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Old 09-14-2005, 04:52 PM   #46
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Originally Posted by rel3rd
I just wish someone KNEW if I could simply manually shift the car and avoid all this possible problems???? If I have the shifter in LOW 1or whatever it says on the console, there'* no way it will upshift itself electronically, is there?
You cannot hold 1st gear, you can hold 2nd and 3rd manually to the limiter.
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Old 09-14-2005, 04:52 PM   #47
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Originally Posted by rjolly87

on the new stuff it does shift on its own, even though its in 1, but, i dont know about yours though.
We'll know soon enough...

Going to check the local pick n pull junkyards tomorrow for two rims...wish me luck...
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Old 09-14-2005, 04:53 PM   #48
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Quote:
Originally Posted by rel3rd
I just wish someone KNEW if I could simply manually shift the car and avoid all this possible problems???? If I have the shifter in LOW 1or whatever it says on the console, there'* no way it will upshift itself electronically, is there?
You cannot hold 1st gear, you can hold 2nd and 3rd manually to the limiter.
well....that sucks...
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Old 09-14-2005, 04:54 PM   #49
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Originally Posted by rjolly87

okay

also in my example, i mentioned the shift being @ 50mph @3000 rpm. it this what you are saying also <principle only please>
yes it is...I totally understand Rogue'* theory now.

I guess the problem is that which one "rules" the PCM...rpm or mph??? If they conflict with one another, that'* where the issues will be...
but i believe that the rpm-mph relationship would remain the same <please just forget about actual vehical speed, we all know its going to be lower than what the vss reads> according to the VSS. but, if i understand correctly, rogue is saying that they change, implying that the gears change too. that is what i am trying to understand.
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Old 09-14-2005, 05:00 PM   #50
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Originally Posted by rjolly87

but i believe that the rpm-mph relationship would remain the same <please just forget about actual vehical speed, we all know its going to be lower than what the vss reads> according to the VSS. but, if i understand correctly, rogue is saying that they change, implying that the gears change too. that is what i am trying to understand.
The gears themselves don't change. the shorter tires simply make the car "THINK" they have been changed...like he said above, when he did an actual gear swap, he had to modify the reluctor to make the VSS read accurately. With my temporary tire swap, I cannot compensate, so the rpm-mph relationship will have no choice but to be "different" than it would be with the stock tires on it...

Basically, the shorter tires are a poor man'* gear swap...
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