93 Grand Marquis speedometer reads too fast

[vittorio]

have a 93 grand marquis the trany has been changed and now speedometer read too fast at leaste 20 fast then car is moving what could it be causing this is was fine before other trany went bad

[fredjacksonsan]

Depending on the setup for your trans/speedo, it's likely one of the following--

If the electronic transmission, the computer needs to be reset to show the correct speed; the trans could be sending the wrong impulses.

but...

If it's mechanical, then there is a small gear in the transmission that drives the speedometer cable; if this gear has the wrong number of teeth, that would cause your speedo to be off.

In either case, it depends on the rear axle ratio; if the trans has the gear for the previous vehicle and that axle ratio wasn't the same as yours, there's your error, as the gear or electronics are set up for (to simplify) a 4:1 ratio rather than a 3:1.

I'm hedging a little, I don't know what year they switched to all electronics, so put both systems in, hopefully what I said was clear. I guess I drove my 86 MGM too far this weekend?

[fredjacksonsan]

....so, you'll need to do a little diagnostic, find out what your rear end axle ration is (crude backyard mechanic method: tape a string on the driveshaft, have somebody move the car slowly for one rotation of the rear wheels, and then see how many times the string is wrapped around your driveshaft. Gives an approximation). And also see if your tranny drives the speedo mechanically (Heavy duty plastic/metallic cable) or electronically (just thin wires). If mech, then you need the correct drive gear. If elec, you'll need to go to some shop that can reprogram the 'puter.

Good luck, hope these helped.