The way I found out what belt I needed for my Mustang (UDP, and only running Alternator, no A/C no Smog pump, no Power Steering) Was I took a string and rapped it around my pullies and then measured it in MM and round up to the nearest multiple of 5. The belt size numbers on most belts in your local parts store are the size in MM and the number of teeth. (i.e. 400K5 is the Drive-Rite part number for the alternator - a/c belt for my car. 5050400 is the Dayco part number. It is 400mm and K5 or 5050 means 5 teeth. 355K5 is without a/c, and 275K4 or 5040275 is for P/S because it is 275mm and has 4 teeth) But being how most parts store employees are COMPLETE MORONS!!! (I know, I worked with them) you will probably have to tie a knot in the string and bring them the loop. My store had a cool slide ruller belt measuring thingy, but I was the only person in the store who actually knew how it worked. If they are half way smart they could use that device, but being the they are also LAZY BASTARDS they might tell you they either need a part number, or they can´t find it. So then you tell them to look up the stock belt, and plug in the measurement for your new belt in the part number where the measurement for the stock belt is then tell them to get that one. And if they give you any grief call them out as the morons they are and never go to that place again.