im not sure what gear im running but when i find out i need to change my speedometer sensor gear. how do i know which gear i need. there are so many different teeth sizes, how do i choose?
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89 hatch lx four inch cowl hood.
trick flow intake, bbk 80mm tb, bbk cai, grannatteli maf, b cam, electric water pump, aluminum radiator, electric fan, pulleys, a/c and p/s delete, bbk long tubes, H pipe, 40 series flows, msd 6al, ford racing 9mm wires, msd cap and rotor.
t-5 tranny, alum. drive shaft, tubular control arms, air ride launch kit, bbk lowering springs, weld drag lites, subframes
florida 5.0 guage cluster w/ autometer carbon guages, race buckets, pro 5.0 shorty
You have an 8 tooth drive gear in your tranny, so this is the setup you would want. I don't know what gear you have. But I have also heard reports that the 23 tooth gear has such thin teeth that they don't last long.
Rear Ration / Tooth Count
2.73---------18 teeth or 19 teeth (getting mixed messages on this one)
3.08---------20 teeth
3.27---------21 teeth
3.55---------23 teeth
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1986 Crown Victoria, New Crate 94-95 5.0L HO EFI, 2-1/2" dual exhaust, 2-1/2" Dynomax mufflers, T5 manual conversion, King Cobra clutch, billet flywheel, Hurst shifter, 3.55 gears, Detroit TruTrac posi, B&M linelock, TruCoil racing springs, QA1 shocks, Sway bars 1-1/8" front & 1" rear, 17X9 Custom Wheels with Sumitomo HTR-Z 275/40 tires, MRC leather racing seats, JVC KD-LX111 CD receiver, 2 Cadence 6"X9" 3-way rear, 2 Cadence 6.5" component system front.
Mid '89 is when the transmission-mounted speedometer drive gear changed from 7 teeth to 8 teeth. If you have an '89 Mustang then nobody can tell you with any certainty whether you have an 'early' or a 'late' transmission. There are only two ways to know:
1. The part number on the transmission tag will either be E6ZR-7003-FB (late '86 through mid '89), which had a 7-tooth drive gear originally, or E9ZR-7003-A (mid '89 through late '89), which had an 8-tooth drive gear originally.
2. The color of the driven gear on the transmission's output shaft with either be yellow (7T) or green (8T).
Now to figure out which driven gear is required, you must know three variables:
1. rear gear ratio
2. drive gear tooth count
3. tire revolutions per mile
Rear Gear x Speedometer Drive Gear x Tire RPM / 1000 = Speedometer Driven Gear
Tom Moss
88 GT 5spd Vert 3:73s, Flowmaster catbacks, stock cam +4°, GT40P heads & 1.7 rockers, Jet-Hot coated MAC P headers, 97 Explorer intake, 65mm TB and 19# Explorer injectors. 277RWHP/330RWTQ
awsome info, thanks alot. the gear i pulled out was white and torn up bad.
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89 hatch lx four inch cowl hood.
trick flow intake, bbk 80mm tb, bbk cai, grannatteli maf, b cam, electric water pump, aluminum radiator, electric fan, pulleys, a/c and p/s delete, bbk long tubes, H pipe, 40 series flows, msd 6al, ford racing 9mm wires, msd cap and rotor.
t-5 tranny, alum. drive shaft, tubular control arms, air ride launch kit, bbk lowering springs, weld drag lites, subframes
florida 5.0 guage cluster w/ autometer carbon guages, race buckets, pro 5.0 shorty
speedometer gear in relation to rear end gear 
