97 Chevy 1500 speedometer / fuel guage issues

[Tpotter1100]

Hello,
I am purchasing a 97 Chevy 1500 and during a test drive i noticed that the speedometer goes from zero to 100 extremely fast. in traffic in a 45 mile zone i was pegged at 100. as you slow down it comes down and will rest on zero at complete stop. (at least from what i remember) I will confirm once i purchase on Thursday) This is my first Chevy and i'm curious at how these instrument clusters operate. is it a sensor, or cable, digital/mechanical? The fuel gauge bounces all over the place as well. could that be a separate issue?

I'm curious as to what to test and what to look for.

Thanks in advance for the help and advice.