XSRACINGGT

may just be that your speedometer is off calibration. what year is your car?

MEM 302 5.0L

my car is a 91'

7upedition

the ratio for the tailshaft on a 91 auto is 1:1, and you have a 8 tooth drive gear, 23 tooth won't be exact for 3.73 let alone 4.10's, hate to break the news to ya but you'll need to do a drive gear swap if you really want to read accurate. let me try to find the chart for ya....

ADDED:

1990-1993 automatic AOD speedo gear
2.73-19 tooth
3.08-20 tooth
3.27-21 tooth
3.55-23 tooth

Calculator:
http://tchracing.com/tech/gearcalc.php
http://www.fordmuscle.com/calculators/rpm.shtml

MEM 302 5.0L

ok so how would i fix the speedometer exactly?...
i ordered the gear from steeda, it was for a 8 tooth aod tranny 91 mustang with 4.10 gears...

7upedition

yea but if you look closely they use the same 23 tooth for 3.55, 3.73, and 4.10 gears, thats mathematically impossible to work for all 3 if they are the only variable....i'm still looking around for that chart, it might be at home...

7upedition

http://www.ddperformance.com/speedo_gear_chart.htm

not the exact chart but will work, 21 tooth speedo w/ a 6t drive gear... and if you go to the home page they'll give a bunch of info on the individual tranny gear ratios that your also looking for in your other thread