With 205/50/16's, you'll have a diameter that is 9.5mm less than with 195/55/16's on the same wheel. In other words, you'll go from 620.9mm overall wheel/tyre height to 611.4mm, a variance of only -2%. This is probably less than the error in the speedo so shouldn't change it's accuracy to any measurable extent. But to answer your question, yes - to travel the same actual road distance the wheels will have to rotate through 2% further than the narrower tyres. edit---- PS. If they'd fit the 6inch rim, a set of 225/45/16's are also only -2% shorter in overall height than stock, same as the 205's... PPS. On 17 inch rims, the best match are 235/40's - almost 0% variance from stock (actually 1.1 mm shorter).