Ldubfounded

Hey guys, new to the form, but I'm a not an idiot just that signed up just looking for answers, but I've ran into a problem that I could use some opinions on. I appreciate in advance any one that could throw some advice my way

I just bought a 98 GT with a few minor problems, I noticed that my speedometer is lagging and my odometer was not working at all.
So I did the obvious and replaced the gears in the cluster, old gears were of course shot. Well that did not fix the odometer, so my next step is to replace the motor that drives the gears.

Now with the speedo it lags 5-15 mph, and I'm wondering if anyone has had these similar combined problems and what was done to solve them. Do I need to replace the speed sensor or the speedo cluster.

I really appreciate any advice I can receive.