In exchange for your time, here is some data:
The short cables are probably for those cars with cruise control. Pull yours out and measure length. Remove it from the back of the speedometer by pushing the friction ring towards the speedo, then pull back on the cable.
Here are some speedo cable part numbers from RockAuto.com a 1985 Monte Carlo
ATP Part # Y814 {Click Info Link for Alternate/OEM Part Numbers} One of our most popular parts
W/o gear adapter; Cable Length 61.00 inches
Part Image
$9.09
PIONEER Part # CA3008
w/ADAPTER LOWER - 20.00" CABLE - 3.8L - 4.3L - 5.0L; Base Model
$9.83
ATP Part # Y815 {Click Info Link for Alternate/OEM Part Numbers} One of our most popular parts
Upper; W/gear adapter; w/intercon.; Cable Length 80.00 inches
Trans. code THM200-4R; W/o gear adapter; Cable Length 80.00 inches
Part Image
$10.43
ATP Part # Y816 {Click Info Link for Alternate/OEM Part Numbers}
Trans. code THM700-R4/4L60/MD8; W/o gear adapter; Cable with 1 female end; Cable Length 100.00 inches
Part Image
$12.30
ATP Part # Y808 {Click Info Link for Alternate/OEM Part Numbers}
Lower; W/gear adapter; w/intercon.; Cable Length 20.00 inches
Part Image
$12.53
ACDELCO Part # 88959478
CABLE,SPEEDO Upper w/4-SPD AUTO TRANS(MW9); GEAR ADAPTER, FLEX W/CASING, 77.80 IN/1976.12 MM LONG; CC1014
Part Image
$32.79
ACDELCO Part # 88959479
CABLE,SPEEDO Upper w/3-SPD AUTO TRANS(MV9); EXC GEAR ADAPTER; FLEX W/CASING, 83.66 IN/2125 MM LONG; CC1076
CABLE,SPEEDO Upper w/4-SPD AUTO TRANS(MW9); GEAR ADAPTER, FLEX W/CASING, 83.66 IN/2125 MM LONG; CC1131
Here are some part numbers for a 1987 El Camino:
ATP Part # Y814 {Click Info Link for Alternate/OEM Part Numbers} One of our most popular parts
W/o gear adapter; Cable Length 61.00 inches
Part Image
$9.09
ATP Part # Y815 {Click Info Link for Alternate/OEM Part Numbers} One of our most popular parts
Upper; W/gear adapter; Cable Length 80.00 inches
Trans. code THM200-4R; W/o gear adapter; Cable Length 80.00 inches
Part Image
$10.43
ATP Part # Y816 {Click Info Link for Alternate/OEM Part Numbers}
Trans. code THM700-R4/4L60/MD8; W/o gear adapter; Cable with 1 female end; Cable Length 100.00 inches
Part Image
$12.30
ATP Part # Y808 {Click Info Link for Alternate/OEM Part Numbers}
Lower; W/gear adapter; w/intercon.; Cable Length 20.00 inches
Part Image
$12.53
ATP Part # Y810 {Click Info Link for Alternate/OEM Part Numbers}
Upper; Trans. code THM700-R4/4L60/MD8; W/gear adapter; Cable Length 120.00 inches
Part Image
$15.25
ACDELCO Part # 88959478
CABLE,SPEEDO Upper w/3-SPD AUTO TRANS(MV9); GEAR ADAPTER, FLEX W/CASING, 77.80 IN/1976.12 MM LONG; CC1054
Part Image
$32.79
ACDELCO Part # 88959479
CABLE,SPEEDO Upper w/3-SPD AUTO TRANS(MV9); EXC GEAR ADAPTER; FLEX W/CASING, 83.66 IN/2125 MM LONG; CC1076
Part Image
$40.79