5200rpms at 70mph, too high!! kz1000m2

Hello everyone,
I've been really struggling to figure out why my rpms are so high from 5200rpm at 70mph on my fix-it-up kz1000 that I stole for $300. I should be getting closer to 4300rpm.

First thing to check was sprockets to make sure someone didn't change them; nope they're stock (15f/39r). Went to check them and the rear sprocket has 39 stamped on iy and I had to count the front but it has 15 teeth. I got really confused but then decided to check the wheels and tires; they're also stock. They are pretty worn but i dont think it would add a 1000rpm to my speed. I found this problem in looking to convert to a 530 chain and measuring rpm differences in different sprockets. I've put a couple hundred bucks into the bike to get it to run good by doing valve clearances, rebuilding carbs, and making sure everything works right. Another thing to note is that I AM getting into 5th gear.

Long winded but thanks!

Should be about 4550 rpm.  Did someone install a 16" rear wheel, by chance?

You indicate you are getting into 5th gear when this happens, so what happens when you are in 5th and keeping it at 70 do the rpms drop or stay the same. 

It rpm doesn't change when I cruise. I figured out the tachometer was the problem by using my tach from my 440. Showed a solid 4300-4400rpm. Gonna tear at it in a couple days. 

The rear wheel is spokes and the csr has spokes wheels so I'd assume they're stock. aren't stock wheels 16 inch so it wouldn't matter? Anyways I swapped my 440 tachometer and it showed around the rpm you indicated so I'm to believe that the tachometer is the culprit. Is this the right line of thinking? If it is then I might disassemble it. I've had problems with this tachometer returning slowly so I wouldn't be surprised if there's more wrong with it.