[cadgear]

Find a used tach-equipped cluster on ebay, ensuring that you find a column/console shift as applicable. Its a straight drop in, you might want to adjust the new cluster's ODO. As far as rough approximation, my 3100 runs about 2200 at 70mph. Approximate. If you want exact, you'll probably need to be electronically inclined and radioshack-friendly or get a scan tool. Or even a cheap scan tool ($120 from auterraweb.com) will display some sensor info.

A rough shift point diagram (from the Unit Repair on service.gm.com):
1-2
TPS %: 12/25/50
Speed: 12/18/29

2-3
TPS %: 12/25/50
Speed: 24/32/51

3-4
TPS %: 12/25/50
Speed: 42/59/101

Downshift:
4-3: TPS 0%, Speed 31mph | TPS 50%, Speed 49mph
3-2: 19mph
2-1: 10mph

TCC Apply:
3rd:
TPS%: 12/25
Speed: 25/38

4th:
TPS%: 12/25
Speed: 39/51