Morning!
I have recently replaced the shock on the K75 with a Hagon which has reduced the height in a positive way. I am wondering:I have to replace my rear tyre, so could I use a 130/80/17 tyre instead of the 130/90/17 currently in use, to further reduce the height?
Thanks.

couldn't hurt

might change your actual speed reading to the calibrated setup ex factory  (figuring that it will show a few steps higher speed than what you are actually doing  )

if you want to recalibrate at some time ....search the forum for the utility karumba ...found here   https://www.k100-forum.com/t9583-karamba-speedo-software   edit .... oops sorry try this link   https://www.k100-forum.com/t2038-karamba-speedometer-calibration-program-tutorial

just be aware

good luck

Thanks. I'll go with the 130/80/17. There was one on the bike when I first got it 5 years ago. I'll keep an eye on the speedo!

You're changing the geometry of the motorcycle by installing a shorter shock and contemplating a smaller tire.  You might have to compensate for the rear end changes by raising the stanchions in the triple clamp an appropriate amount.  The changes you made will affect front trail and consequently steering, as well it could affect spline wear, rear swingarm travel, sag and other characteristics that will affect handling and safety.  Tread carefully.

Another consideration on lowering the bike is how far you have to lift it to get onto the centerstand.  I just raised my recently purchased K75S to the stock height because it was such a bitch to get on the centerstand.  Previous owner had dropped the front end about 13mm and the rear about 20mm by using a shock with a shorter shaft.

Lifting about 220kg an additional 17mm can be pretty hard.  Just be aware that there is a downside to lowering the bike.