Using shorter drag radials cause any problems?? - Page 3 - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 09-14-2005, 03:32 PM   #21
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so....is my solution to simply unplug the ABS fuse and manually shift the car at 5800 like it seems to like now, if need be??
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Old 09-14-2005, 03:38 PM   #22
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Ok, where does the VSS get its signal? How does tire size change what the VSS reads? I though the VSS determined speed by tranny output RPM and stock tire size. So the VSS signal would be related to engine speed, in whatever ratio corresponds to the gear it'* in, not actual car speed.

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With smaller tires your speed will reach 50mph (according to the VSS) before 5800 (probably 5600 or so dependng on amount of tire change)
Explain...
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Old 09-14-2005, 03:49 PM   #23
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Quote:
Originally Posted by jwikoff99
Ok, where does the VSS get its signal? How does tire size change what the VSS reads? I though the VSS determined speed by tranny output RPM and stock tire size. So the VSS signal would be related to engine speed, in whatever ratio corresponds to the gear it'* in, not actual car speed.

Quote:
With smaller tires your speed will reach 50mph (according to the VSS) before 5800 (probably 5600 or so dependng on amount of tire change)
Explain...
I can explain that one...maybe. In fact, this is the reason I asked about having an ABS light problem...because of the artificially higher MPH on the front tires (and speedo), due to their smaller height. At any given vehicle speed, the rear tires will be rotating slower than the fronts because of the fronts being a smaller diameter. Smaller diameter = more revolutions (of the tire) per mile, Larger diameter = less revolution per mile...As soon as I find some cheap 16" wheels to mount the darg radials on, I will be immediately trying them out to see exactly what happens...There seems to be some debate, so hopefully I can find out some definitive info for everyone...just need 2 wheels...

I would have thought someone had tried smaller tires, other than me, before now solely for the benefit of getting the car artificially geared higher by doing so...As long as the car doesn't run out of rpm'* before it runs out of track, it has to be quicker than it would be with stock height tires...The DR'* are 1.9" shorter in height, so that "should" be like swapping in a 355-373 gear ratio in the car...More gear = quicker acceleration, quicker acceleration = lower ET'*...as long as the car stays off of the rev limiter for 1320 feet, and I really don't think that will be an issue, unless the PCM won't allow me to manually shift at 5800...that'* what I need to know.
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Old 09-14-2005, 03:50 PM   #24
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Originally Posted by Rogue
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.

.
but wouldnt the rpm-speed relationship <according to the VSS> remain the same? i would think the only thing that changes is the actual speed of the car <which the only thing that would notice the difference would be the abs>. i would think the car would run through the gears just the same.
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Old 09-14-2005, 03:56 PM   #25
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Quote:
Originally Posted by rjolly87

but wouldnt the rpm-speed relationship <according to the VSS> remain the same? i would think the only thing that changes is the actual speed of the car <which the only thing that would notice the difference would be the abs>. i would think the car would run through the gears just the same.
It can't possibly remain the same since the car will "think" it has a steeper gear ratio than it really does. The speedo will be off, probably 10mph at highway speeds, if not more simply because the tires are rotating faster than they would be if they were stock height. I definitely hope you're right about it shifting at the right time, automatically, because I intend to bracket race it that day and bring home the bacon...lol...

Being a veteran Mustang guy (sorry fellas), one of the first mods we do is swap in a steeper gear, usually 373'*. Then, the speedo reads a good bit higher than you're actually going, so you also have to swap in the correct speedo gear...maybe that'll help the comparison?
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Old 09-14-2005, 03:57 PM   #26
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I completely understand that two tires of different diameter at the same rpm will be moving at different speeds. I'm with rjolly on this one.

The rotational speed of the tires stays the same, regardless of tire size, with respect to engine speed. Outside of the ABS and mismatched tires, how the hell would tire size change anything with regards to the VSS or PCM other than making the speedometer incorrect?
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Old 09-14-2005, 04:00 PM   #27
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The VSS gets it signal from the rotation of the reluctor ring on the differential. With smaller or larger tires the rotational speed of that ring is off thereby effectiveily changing the final drive ratio.
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Old 09-14-2005, 04:09 PM   #28
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Originally Posted by Rogue
The VSS gets it signal from the rotation of the reluctor ring on the differential. With smaller or larger tires the rotational speed of that ring is off thereby effectiveily changing the final drive ratio.
adding acceleration. but the only clue the car would have is increased acceleration, which it doesnt care about
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Old 09-14-2005, 04:11 PM   #29
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Quote:
Originally Posted by jwikoff99
I completely understand that two tires of different diameter at the same rpm will be moving at different speeds. I'm with rjolly on this one.

The rotational speed of the tires stays the same, regardless of tire size, with respect to engine speed. Outside of the ABS and mismatched tires, how the H*** would tire size change anything with regards to the VSS or PCM other than making the speedometer incorrect?
If you understand the different speeds compared to one another, the rest is easy...
Rotational speeds vary GREATLY when the tires diameters are different.

Another good example is you guys swapping supercharger pullies. If you want to slow the blower down and lower boost, you put a bigger pulley on. If you want the blower to spin faster (at the same engine rpm), you go with a smaller pulley...In that case, the "VSS" would be the "BOOST" gauge... It'* simple math really...
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Old 09-14-2005, 04:15 PM   #30
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Quote:
Originally Posted by rel3rd
Quote:
Originally Posted by jwikoff99
I completely understand that two tires of different diameter at the same rpm will be moving at different speeds. I'm with rjolly on this one.

The rotational speed of the tires stays the same, regardless of tire size, with respect to engine speed. Outside of the ABS and mismatched tires, how the H*** would tire size change anything with regards to the VSS or PCM other than making the speedometer incorrect?
If you understand the different speeds compared to one another, the rest is easy...
Rotational speeds vary GREATLY when the tires diameters are different.

Another good example is you guys swapping supercharger pullies. If you want to slow the blower down and lower boost, you put a bigger pulley on. If you want the blower to spin faster (at the same engine rpm), you go with a smaller pulley... It'* simple math really...
but the only differance is actual vehicle speed, compared to what the speedo reads. we all agree that the smaller tires in effect change the final drive ratio, and increase acceleration capabilities, throwing off the speedometer and everything. what rogue is saying, is that the car will shift early <in the rpm band> because of this, which is what we are trying to understand.
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