Tire size question

Changing from 215/60R15 to 225/60 would make the speedometer off by 0.4mph at 20mph, 1.1mph at 60mph, and 1.8 at 100mph, very minute speed variations really. But according to discount tire, 94's came with 205/65R-15, and if a 225 tire would fit on the rim (I think the rim is 7" wide, 225 is 9" wide, usually a tire is under 1.5" wider than the rim), then 225/60R-15 would put you under 1mph speed difference even at 100mph, something you'll never notice. But on 01+ stangs, they have a 7.5" rim, that's why 225's fit, it's wider. My 2000 is a 15x7 and 205/65 is what is on them.