Cheating non-Slovak speakers/nationals seems to be normal for some taxi drivers at the train/bus station: I have paid 250 SKK for a 5KM trip while the normal road is much shorter. When later I have realized that and went with a friend speaking Slovak to complain (with the receipt stating clearly the starting point and the address of my "pension"), we were told by this driver that's kind of normal and that his boss told him to do so with non-locals.
Of course, DON'T EXPECT LOCAL AUTHORITIES DO TO ANYTHING, even when it's written black on white that you were cheated. The tourist information center does basically nothing for you to fix it in friendly way ("address your complain to this office in Zilina). That's rather sad for a place that lives firstly and mainly from tourism IMHO....
Make sure you get your "VIGNETTE" in Austria and Slovakia if you drive on the Motorways (Autoroutes). These are available at the service stations on route as you enter the country. They are valid for 1-7 days min , 1 month, 3 months and 1 year.
A vignette is the motorway tax pass and is important. It is not a rip off when compared to the government proposal for the UK.
THESE ARE VERY CHEEP SO IT IS NOT WORTH TAKING A RISK OF GETTING CAUGHT AND FINED.