Hehe welcome to the BEST PLACE IN THE WORLD to get help and friendly advice about the XT range. I too have an XT660X (see list of mods below) and I get a consistent 20 km per litre and even more if on A roads and out of town, so you must sort out the plug, the CO setting, the snorkel and the air filters, and by all means a good exhaust set will help too.
Have fun!!!!!

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(Now sold on, sob) 2011 Blue XT660X with gold wheels, was 26,500 km.
Engine mods: K&N Stage 1 filter, DNA Stage 2 filter, snorkel removed, Kev fuel mod fitted.
Plus: smoked Puig screen, Yam aluminium sump guard, Yamaha / Acerbis handguards, Givi Trekker aluminium side cases, Leo Vince X3 cans plus Kev front fork mod and Fender Xtender. Just fitted Michelin Road Pilot 3 tyres.

SOLD the XTX for a Super Ten 1200. And just bought a Raptor 700 so staying on here for some tips....