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Old 03-21-2004, 10:01 AM   #1
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Default something i learned today??

i just found out that when you put on bigger rims or tires your speedo doesnt change because they speed of your car comes off of the tranny no the wheels i never new that or maybe i did but just didnt thin kthat hard??
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Old 03-21-2004, 11:30 AM   #2
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Part of that is true.

When you upgrade to larger rims its critical that you maintain overall tire diameter. Right now you have 225/60R16'* so if you want to upgrade to 20" rims, you need to find a tire that is very near (within 5-10%) of the stock tire diameter.

Your transmission is geared and operates on a specific rolling distance or total circumference of the tire. If you greatly increase the total circumference of the tire then your tranny will be doing extra work in order to operate properly. The speedo is read off the computer and off the tranny yes. So if your tranny working off a different total rolling distance the speedo will read wrong.

GM corrected this issue with installing new software capability to the 2000+ cars, their computers are Tech 2 and can be programmed to suit new rolling diameters.

I didnt make this up, this came from a GM service manager when I asked about going +1 rims on my car.
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Old 03-21-2004, 11:55 AM   #3
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That is good to know, thanks for the clarification
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Old 03-21-2004, 12:01 PM   #4
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Great writeup Jim!

In a nutshell, your vss sensor in the trans is only telling you what the car WANTS to do for speed, or rather what it'* trying to. Because the PCM can't see the tires, it doesn't know what'* going on there. You can still screw it up, and be going faster or slower than the vss/pcm think.

It'* best to match when upgrading.
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Old 03-21-2004, 12:21 PM   #5
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Thanks!

Joe..fyi 225/60/R16= 26.6" total diameter if you want 20" get 255/35R20 = 27" total diameter

.4" is not that bad...but, I'm leary of dubs on a car simply because of the diameter issue. Maybe 19'* are better?

Its up to you bud
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Old 03-21-2004, 12:26 PM   #6
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You'll get a very harsh ride. Maybe some new struts would be a good idea too...
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Old 03-22-2004, 08:47 AM   #7
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so total diameter is 27? how much difference could that give me in relation to the speedo? say the speedo says im going 35mph how much could i really be going?
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