There is other technical thread
http://www.xt660.com/showthread.php?t=4816&page=6

I have installed XT-X rim's to XT-R and I didn't have any problem get certificate for both tyres in Finland. I don't know about Slovenia law.

I just printed XT-R and XT-X technical specifications from Web-page and drove bike to local registration office.

Officer checked those specs and my bike with XT-X tyres. Because XT-X and XT-R are "parallel models" there wasn't any issue.

I got official registration certificate and there are written allowed tyres are
Front 90/90/R21 or 120/70/R17
Rear 130/80/R17 or 160/60R17

http://www.yamaha-motor.eu/eu/produc...view=techspecs

http://www.yamaha-motor.eu/eu/produc...view=techspecs

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MODs - XTX/XTR wheels - Supermoto front fender - Handguards - Engine crash bars - OTR Bash Plate - Renthal 971 Handle bar - Bi-Xenon lights - Auxiliary Driving Light LED - LED tail light mod - Speedo healer - GiPro gear indicator - Thermometer - Heated Grips - Navigator - Tachometer - 12V Output plug - Front Spacers 30mm + 15W Fork oil - Lowered seat 25mm - Adjustable side stand - Kev Fuel Mod - DNA Stage 1&2