Question about my Tach

[xtreme80]

I just put a new tach in the truck, and I'm not completely sure if it's accurate or not...

I did a WOT run in the truck, and according to the tach, it shifted first to second at 4000 RPM. Shifted from second to third at 4200 RPM.

Here's the engine I have...
http://www.jegs.com/webapp/wcs/store...55609_-1_10763

Now either the tach is inaccurate, or it's shifting too early, because the specs on that engine show 290hp at 5200 RPM, and I'm about 1000 below that according to the tach. I've got it hooked to a main power source in the fuse box, and I've got the actual RPM wire hooked to the "tach" terminal on the dist. cap. It also shows it's idling at about 500rpm in gear, which I think is a little lower than it actually is. I could be wrong though..