You can get/build a 17" Excel rim for the Tenere, you don't have to go for an 18" if you don't want/need to.

The 18" option gives you more choice of decent off-road rubber, but less choice of on-road/dual-purpose rubber. It will also raise the rear of the bike, which will affect handling unless you do something about the ride height/sag at the front.

Going to an 18" rear raises the gearing by a tad less than 6%, which is roughly equivalent to loosing 2/3 teeth off the rear sprocket (on a 17" stock wheel). To counteract this, if you fit an 18" rear, you'd need to go to a 14T front, or gain 3 teeth on the rear to retain stock(ish) gearing. Obviously there are many other permutations for gearing ratios depending on what you want to achieve.

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