Need help on tire and rim decision

[mattman_13]

I have a 2003 f250 4wd with a ????? lift on it. The rims currently on it are the micky thompson ones that look like soft 8s. From what ive found research wise has a 5" or 4 7/16 backspacing..i could be wrong. On that rim im running a 315 75 16 kumho which ends up being 34.6 tall and 12.30 wide section.

Im wanting to switch to 18" rims for ride quality and better choices of tires. Im looking for a tad big taller tire but skinnier. wanting to get some rolling resistance off and gain some speed on the highway as my trucks mostly used to travel for work. im looking at the toyo oc at in 285 75 18.

I'm questioning the backspacing needed for a skinnier tire. Most the results ive found from my search have been "will a 285 75 18 fit on stock rims" and etc. i want to keep the tire sticking out the same or in a little.

What rim would you all recommend for that situation.
Thanks!