wheel size questions

my 00 sport has the ugly snowflake-lookin wheels on it and i want to move to 18-20" 5 spokes. the rim is small (15"?) and the tire isnt. if i get a rim/tire that is the same height overall as the old i shouldnt have a problem with anything should i? im looking at afs 03 cobra's 18x9/18x10.5 (245/35zr18 275/40zr18 i think) or any deep dish 5 spoke with widths like that. ive heard losing gas mpg and speedo being wrong from wheel size etc. i also might want to lower 2"-4" later on, will i have any problems?

also pls explain backspacing to me.

 

[crazynavajo]

I bookmarked this a while back on backspacing:

Backspacing is the distance from the inside rim surface to the backside of the wheel mounting surface. This will have an effect on how far the wheel/tire sticks in or out of the vehicles wheel wells. The smaller the backspacing, the farther out the wheels will be. Having the correct backspacing is a consideration on preventing interference with suspension, braking and steering components. Oftentimes, a lift manufacturer will require a certain offset rim to work properly with their kit.

hope this helps

 

[TheFox88]

Not to be an @$$, but all the info in this thread has been covered countless times before on this site. There is a search feature that you should try first. You will be able to find out all of the info. Good luck.