Speedo will be fine, depending on tire. I've spec'd out a new set of tires for mine, the stock 205/55/16 EL400's are 25" dia, 834 revs/mile, and it so happens that 255/35/18 Michelin PSS are 25" dia, 832 revs/mile, so close it's practically negligible. 225/40/18 is a teensy bit larger than 255/35/18, but only barely, so I wouldn't worry about it.
As far as computer tuning goes... don't know anybody who has gotten past the Skyactiv's two-fuel-pulses-per-firing quirk. I don't even know if you can go standalone with this engine, it kind of depends how good the fuel injector control is, and even then, nobody has declared that they know how to tune the Skyactiv DI system.