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                                  Transmission and Speedometer Gears


When using 3.55 gears and greater (numerically) it's common to use the 23 tooth speedometer gear.  
The gear is not a Ford part, but it does fit.  Most of the time it will work for awhile and then you'll find
your speedometer bouncing slightly.  The teeth on the gear will break, causing the gear to skip around
on the transmission drive gear.  The only real solution is to change the transmission drive gear which will
allow you to use a lower speedometer gear (an actual Ford part that fits).  Listed below are the some of
the possible combinations to get your speedometer to read accurately once again.  Keep in mind that
changing tire heights will also effect your speedometer reading.  The following charts are based on
maintaining the stock height of the original equipment tire.  Some of Fords Charts are based on a
P225/60R15 size tire which spins at 815 revolutions per mile and is 25.7" tall.  (At 60 mph, that would
mean 815 revolutions per minute).

Transmission Drive Gear:  Located in the tail shaft of the transmission.  Sizes available are 6, 7, and 8.  
Size 6 was original equipment in some 4 cyl T-5 Transmissions.  

Speedometer Driven Gear:  Located at the end of the speedometer cable where it plugs into the
transmission.  Sizes available are 16 - 23.

Speedometer Driven Gear Teeth =
(Drive gear teeth x rear axle ratio x tire revolutions per minute) / 1000
1983 - 1989 Manual 5-Speed and Currently Produced Ford T5-Z
1996 - 1998 Cobra Manual 5-Speed
Tremec TR-3550 & TKO
Transmission Drive Gear
Speedometer Driven
Gear
Color
Ring and Pinion
7 (Yellow)   
16
Maroon
2.73
7
18
Yellow
3.08
7
19
Pink
3.27
7
20
Black
3.55
7
21
Red
3.73
7
23
White
4.10
1990 - 1998 Manual 5-Speed
(Except 1996 - 1998 Cobra)
Transmission Drive
Gear
Speedometer Driven
Gear
Color
Ring and Pinion
8 (Green)
18
Yellow
2.73
8
20
Black
3.08
8
21
Red
3.27
8
23
White
3.55
8
23
White
3.73
8
23
White
4.10
For gear ratios 3.73 and higher, consider changing the transmission drive gear to a 6
(Black) or 7 tooth gear.  You would then recalibrate with the proper speedometer gear.
1983 - 1989 Automatic AOD
Transmission Drive Gear
Speedometer Driven
Gear
Color
Ring and Pinion
7
18
Gray
3.08
7
19
Tan
3.27
7
20
Orange
3.55
7
21
Purple
3.73
7
23
Black
4.10
1990 - 1998 Automatic AOD
Transmission Drive Gear
Speedometer Driven
Gear
Color
Ring and Pinion
8
18
Gray
2.73
8
20
Orange
3.08
8
21
Purple
3.27
8
23
Black
3.55
8
23
Black
3.73
8
23
Black
4.10