Stock size, and thus speedo calibration, is based on a 215/65-16 tire size. By running a 205/55-16, your rolling diameter is smaller by about 8%. That is, at an actual speed of 60 mph your speedo is going to read about 65mph (dependent upon whatever error is included in the calibration and there is some error put in at the factory). The odo is also off by 8% indicating that you have traveled farther than you actually have. Now with all this said and as far as your instrumentation is concerned, the car will still think it is spinning the motor at x rpm at y vehicle speed over z distance and therefore burning a certain volume of fuel over that distance regardless of what that actual distance is. In other words, if you are driving at an indicated 60mph on either set of tires, the engine will still be turning the same revs and, in theory, still burning the same volume of fuel, if you follow me on my thinking. None of this, however, takes into account any differences in tire construction and tread compounding, which IMHO is probably the biggest factor in mileage differences at a given speed, or aero effects (if you are driving an indicated 60mph your true air speed and aero resistance is less) having a small effect on mileage.

So why are you running a 205/55-16 on an XC? Typically if you don't want to run the stock size, most folks run a 225/60 or 235/60 so that their speedo is closer in calibration with a wider tire. You've gone to a narrower and shorter tire which has no advantage over a stock size no matter how you cut it plus it probably looks a bit strange. Unless you got a slamming deal, I also suspect that any difference in price would have been small.

Cheers,

Bill