The speakers are probably 6" X 8".
The speakers in the doors of a 1991 Ford Ranger are 6-inch speakers.
i have a 2001 ranger xlt and my door speakers are 6 X 8
the doors are 5 inches and the back ones are 4 inches
i believe the doors are 6" rounds and the back speakers are 6x9s, if not 5x7s.
You can use 5x7 or 6x9
On all doors front and rear 6x8.
Depending on what year model 1990-1999 is either 6x8 (5x7 is the same thing) or 6.5". The 1996 Ford Ranger XLT model the front and rear speakers are 6x8(5x7).
You have a 1999 Ford Ranger 3.0.
Rear speakers: simply unscrew the 4 screws that hold on the cover then you have access to the speakers. Front speakers: you will have to take the interior door panel off this should give you access to the front doors.
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Crutchfield shows the DOOR speakers to be 5 1/4" and the REAR speakers to be 6" X 8."
Correct STOCK size is 5" x 7" for front and rear speakers. You have to cut the doors metal to make 6x9 speakers fit. REFERENCE : http://www.modifiedlife.com/1999-ford-explorer-car-radio-stereo-wiring-diagram/