Maybe your worrying about nothing--maybe not, discount tire has a calculator for tire size and tells you how much you'll be off...for instance a 235/45 compared to a 255/40 is roughly 3/4 of a mile an hour@ 65mph..not worth fretting over..

 

how hard would it be to install a manual speedometer to make up for any changes in tires/wheels? I can't hardly see my stock speedo when driving anyway... I'd be all for ripping apart the panel and using all aftermarket gauges, except for that pesky odometer!