I got a 96 dakota in feb and it had 29-10.5 snow tires on it and when i took it down the highway i noticed that my speedo was off a bit so i gps'ed it and at 70 i was going 61 or so. so i got 30-9.5 bfg-a/ts on it now and its still off by about 3 to 5 mph i was wondering what the final gear in the diff would be for that and in fith gear the rpms say 2.2k @ 70 speedo speed i was wondering if it might be off or odd or anything

 

I don't know what to tell you, it sounds to me like either someone changed out rear gears in your truck or you have a different speed sensor than factory. You're going to have to do some investigating on your own.

What I can tell you is look in your door jamb and see if the sticker is there for the factory size tire. Its probably a 235/75/15, which is 28.88 inches in diameter (width doesn't matter).

So basically, if a bigger diameter tire is correcting your speedometer, you either have lower rear gears or a speed sensor calibrated for lower gears.

And 2,200 rmp at 70 sounds about right for 3.92s and 30 inch tires.