Backspace confirmation please

[usa-69z]

Guys , i just want to ask , im runnining 17 x 9.5 with 5.5 backspace now. I will be purchacing new rims asap. I want to go with 18 x 10 , Question is ...if i keep same backspace do i gain that half inch 'outwards' and inside stays the same? I seem to have plenty of room toward the outer edge.