I am getting some new wheels to put on my wifes bike, the ones she likes are 18" and she has 17" on there now. The back is going to be a 18x5.5 and the front is a 18x3.5, so what size tire do I need to put on there to keep it at the same height? We had to have it lowered for her so she would be more comfortable on it, so I need to keep as close as possible.