Tire Size Question....please help.

My wife has an altima and the factory tire size is 215-55/17 with an overall diamater of 26.31". She is wanting to put a 215-50/17 with an overall diamater of 25.46" on her new wheels. My question is with the tire being smaller how bad will it affect her mileage? When shes doing 60 will it read 63? Or when she has driven 5 miles will her odometer show she has driven 6 miles? How will this difference affect things? Thanks

215/50/17 rotates 815 times per mile.
215/55/17 totates 800 times per mile.

So depending on your speed you will be off about 2% 70MPH error of roughly 1.4MPH

45MPH Error of roughly 0.9MPH

if its an automatic they can sync the tranny to the new tires size.

Why not use the original size?

Here's a tire calculator for your speed questions:
http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp