[TrailLeadr]

If you get bigger tires, you're going to need to adjust the speedo. There's no way around it. Unless you change the rear diff to compensate for the larger tires.
It would just be easier to have the speedo re-calibrated.

Were you asking how big you could go on the '00 Sierra without a lift?

 

[TrailLeadr]

Thats right , I don't have a lift, and I don't want my speedo off at all, but I was wondering if there was any way to have the computer reprogramed for the larger tires at a shop or something like that

You should be able to have that done at a shop, but I wouldn't know what the approx charge would be for it.
Maybe someone else would know.