Car in question/theroy, 2017 Mazda 3 GT 18inch wheels(215/45/R18). Theoretically, say I wanted to run 205/50R16's next summer. Tire gap? bah who cares, not me. The speedo will be reading like 10km/h lower than actual speed. With modern technology, could I not take it to Mazda and have them adjust the speed sensors? Are they programmable like that yet? In the ideal world you plug into the ECU and just change the tire size on the programming and presto chango, magic, the speed reading is automatically adjusted to the tire size. Should take all of 2 minutes tops to do said change. Does that exist? Yes Id be paying twice yearly for the speedo to be adjusted and having tires mounted twice a year(until I have money for 2nd set of wheels) but honestly for 2 minutes of theoretical work for the speedo adjustment, should be like $60 TOPS for the adjustment.