Tach Install Question??

[Zilverado]

So I just finished installing my tach (first time). Seemed simple enough, but I have a few questions.

At idle, the tach jumps around between 1000 and 2000 rpm. As soon as the throttle is pressed and the truck starts moving, the needle drops to approximately 1000 and begins to slowly rise with the acceleration. All seems well until I take my foot off the gas and the engine again is not under load. The needle again starts to jump. What could this be? I thought my wiring was solid and the ground was good. Any thoughts?

Next, I just want to verify my engine speed. At approximately 60mph, the needle shows around 1500 rpm. This is with a:
305
700R4 tranny (in OD)
3.07 rearend
LT235/75 R15 rubber

Does 1500 rpm sound right for that set-up. It would explain my good gas mileage .

BTW the tach has a 4-6-8 switch. I have it set on 8 (obviously).

Any thoughts would be greatly appreciated. Thanks.