tach off by 500 rpm

[grego]

hi all
well after installing the tach about 7 months ago its been driving me crazy that its 500 rpm higher then everybody elses in the world. instead of 1000 rpm per 10 miles an hour its 1500. so at 65 mph it reads 7000 rpm. is 7000 rpm to high for riding down the freeway or am i worried for no reason ?
the questions in my mind are
1. is there a bike mod that would do this ?
2. is the clutch slipping ?
3. is the tach off by 500 rpm ?

was wondering if this tach would work on the gz to answer question #3. could install it to see if the tach is off or if its something else.

http://www.ebay.com/itm/Hour-Meter-Tach ... 561wt_1150