Did you buy it new ???
I did a fair bit of work on the Tenere gearing last year, have a look at
http://www.xt660.com/showthread.php?t=8876
There's a lot of guff about fitting a speedo healer but the content is fairly explanatory.
The standard is a 15 and comes with a speedo error of about 8-10%.
A 16 will bring it more into line but it will throw the odometer out. The odometer is fairly accurate with a 15t.
On top of that you may find that it goes faster in 4th than 5th especially loaded or with a head wind or slight incline. (with the 16t)
As Kev said, it also makes a lot of difference as to how you want to use the bike. I currently have a 14t on the front and it make off road and traffic driving a lot better, but I fitted the speedo healer to bring it back to the original error of 8-9%.
OGR