I am thinking about treating my Ten to some Excel rims for Xmas. So, I have a noob question. The rear is 18 inch (if I'm not mistaken); what are the advantages /disadvantages over the standard 17 inch rear? Any changes to gearing; and most importantly (to me), what effect does it have on mpg ? Better, worse, negligible?
Cheers.

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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Exactly the info I was after. Thanks Pleiades.

If you are going to get a bigger rear sprocket with your 18" wheel, don't forget that you will also need a longer chain, as the wheel will be placed further to the rear. But that will increase hight speed-stability very much.

As a result of that, i still have a chain with only 300 kms on it, that i could not use anymore...

Also don't forget that every change in gearing, including wheel size, has an effect on the speedo reading.