Speedometer issue Help Please

I have a 1997 chevy venture i just updated my speedo to a montana speedo. Every thing works but my gas gauge the person that sold it to me sed that thats what happend when you update to a tach something alse dont work. But i think it bull because the montana and venture are the same vans pretty much. So i dont really think that GM is going to change up the wiring harness for the venture and the montana. But anyways if anyone knows any info on that please help me out before i replace my fuel tank float.

if the fuel gauge worked before you changed the speedo the the float ain't the problem. did you replace the cluster[speedo] out of the same year montana or go newer? because g.m. did change the harness in a newer year from a '97. get you a haynes or chilton manual and check out the wiring schematics for a '97 and what ever year the speedo came out of.