Please do not choose a (wrong) part number. Belts stretch a LOT; old ones are 5% to 15% longer than new! Provide COMPLETE information so we can find appropriate replacements for you.
We need to know:
1--Type of equipment: Cassette Tape Recorder; VCR; CD or DVD; Reel to Reel Tape; 8-Track Player; Phonograph; Telephone Answering Machine; Record Player; Movie Projector, Key Machine; or ?????
2--Brand: Make, Model Number, approximate Year made. so we can use cross reference books if it is listed.
3--Style of Belt: Is it Square, Flat, or Round? V-groove pulleys use square belts! Many customers mistakenly ask for round belts when they need SQUARE belts!
4--What function is the belt for: Drives a tape capstan; Moves the load tray: Drives the take-up reel; Drives a position counter; Rotates the turntable?
5--What is the Condition of the old belt: Broke ten minutes ago; Broken six months ago; Stretched a lot and falling off; Moderately Stretched and Almost Working; Stretched very little and still working great?
6--What are the measurements of the old belt: Width, Thickness, and Circumference (length from END to END when broken or cut?) Please tell us HOW you measured the belt or belt path along with the measurement results you obtained.
Measuring the CIRCUMFERENCE (Length) of Old Belts: Broken belts--lay them next to a ruler!
Unbroken belts: mark with a ball point pen; then roll along a ruler until you reach the mark again!
Lost belts: wrap some 20 to 24ga hookup wire tightly around the pulleys; make a half-twist to kink the wire, then measure the distance between the kinks with a ruler.
Compare the thickness of square belts to the edge of a Coin; a Dime is about .047 inch thick; a Penny is about .055 inch thick; a Quarter is about .065 inch thick; a Nickel is about .077 inch thick. On a ruler: 1MM is .039 inch thick; 1/16 inch is .062 inch thick; 2MM is .078 inch thick.
Width of a flat belt: 1/8 inch is .125" wide; 3/16 inch is .187" wide; 7/32 inch is .219" wide; 1/4 inch is .250" wide; 9/32 inch is .281" wide; 5/16 inch is .312" wide.

Free Technical Repair Tip--Rubber Belt Life Expectancy: Flexible plastic compounds (synthetic rubber) are affected by temperature, humidity and air contaminants. "Air Purifiers" that generate OZONE drastically shorten the life of "rubber" drive parts used in electronic equipment!

Free Technical Repair Tip--CD Players & DVD players: Many Computer CD Drives and DVD & CD players use a square rubber "load belt" to open and close the drawer. If the drawer doesn't open or close--you probably need to replace the "load" belt. Please measure the circumference (=length) and thickness (.047" is the thickness of a dime); (.055" is the thickness of a penny); (.065" is the thickness of a quarter) (.077" is the thickness of a nickel) of your old belt.
Many CD "load belts" are between 2.5" and 4.5" circumference
Please tell us the actual measurements of the old belt?
and--tell us the condition of the old belt (is your old belt broken?)
or (is it stretched a little and still working?)
or (is it stretched a lot and slipping badly?)
so we can select an appropriate replacement for you.

Many Chrysler & Honda Autosound Cassette Players use 16" 1407-459 rubber belt.
Many Ford Autosound Cassette Players use 15" 1425-91 rubber belt.
'87 Lincoln Ford Autosound uses SCQ6.0 & SCX9.5 rubber belts
GM/Delco Autosound Cassette Players 1980 thru mid-1990's come in 4 styles.
(1) Non-auto reverse models use SBX7.4
(2) Auto reverse Fijitsu 3 (Mechanical) Button Mechanisms use SBS8.6
(3) Auto reverse Blaupunkt 4-button Mechanisms use 2 belts: AV-SBS4.9 & AV-SBS9.2
(4)Pioneer (88 up) and Alpine Mechanisms (91 up) have electronic switch buttons use AV-SBS9.2