my gs500 revs about 6500rpm at 70 mph, is that normal,  can I change my sprocket to get a better top end?  I counted all the gears so I know it is in 6th

That's right where it should be.

Yep thats normal, how high do you want it to rev? To rev higher get a front sprocket with less teeth or to rev lower get one with more. But if you get one with more it will affect acceleration also (slower acceleration less revs in top gear) and the opposite if you get one with less (faster acceleration more revs in top gear)

Where in OH are you located?

If thats normal, then why with a 15tooth front and a 39tooth rear and a 140/70/17 on back, 110/70/17 on front im reading 5500 at 65mph? my speedo reads 3 mph over at 60mph as clocked by 3 other vehicle

Quotemy gs500 revs about 6500rpm at 70 mph

QuoteYep thats normal, how high do you want it to rev

All vehicle speedos are off. Cars usually high by 4 to 5%, bikes by about 10%. Different makes of the same size tire are very different circumferences.

If you check your speedo against a better reference (GPS), you'd find it is more like 6 mph difference.

There is a good spread sheet for calculating the speeds with different tires and gearing on the GS at:

http://www.evilpumpkinracing.com/eprlinks.htm

Downshift 2 or three times and you'll get plenty of revs That's what I do when I want to pass someone, and until my new sprockets come in (15 front 40 rear, 1 less front, 1 more rear) I'll run in 5th if there's a strong head wind because it requires less throttle.