My camaro has factory size tires that are bald which are P245/50R16. And my younger bro just bought some like-new tires for his eclipse which are 215/60/16....but theyre too big! Will they fit on my rims? If so, will they affect the speedometer calibration?

Yes they will fit, and they may throw off your speedometer calibration. lets look at P245/50R16. The 245 is how wide your tires are at the tread, not the rim. The middle number is a ration representing the height of the sidewall (for example low-profile tires will have a smaller number here) and the last number is the diameter of your rims. When in doubt, look up the stock tire fitment for perfect results. I just put new tires on my 89RS this a.m. I took the P255/60R15s and put on the factory recommended P215/65R15s.

You speedometer will read 1.8% to fast.

A 215 is too narrow to fit a 3rd gen 8". If you can get it on there, it will be real distorted & wear out the tread pretty badly. I wouldn't run such a narrow tire on the 8" wheel.

The height isn't as much of a problem as is the width.

i was afraid of something like that...it would probably look a little funny, too.