Wow... I've been off for a while and missed one of my fav topics -- wheels & tires!!
Well it seems they got you on the right track, but just FYI:
The first number is the width of the tire in millimeters. So all 275 tires are about the same width...
The next is called the "aspect ratio" It really just means percentage. So a 35 has a sidewall that equals 35% of the width. (.35*275 = 96mm = 3.8inches).
And last is the diamater - which you knew.
So 8" rims get a 215-225 tires, 8.5" get 235-245, 9" get 255-265, 9.5" get 275-285, 10" get 295-305, 10.5" get 315s, basically.
FLASH = 98 V6, chrome yellow, black & blue accents, 146k daily driver POWER
: divorced exhaust with glasspacks; BBK CAI, TB, UDPs; (shift kit in the garage) STYLE
: black headlights, black taillights, black grille, black hood pins, black window tint, blue hood & side scoops, blue billet fuel door, interior billet, 12000k HIDs RIMS
: XXR 006 black spoke with blue lip; 18x8.5 front, 18x9.5 rear TIRES
: BFG KDW/2 245/40R18 front, 275/35R18 rear