I have a 94 that had the same problem. It's more than likely a broken speedometer cable. Remove your gauge hood, loosen the gauge pod and remove the cable. It slips into the back of the speedometer (may have a couple of retaining screws -- I can't remember), pull it through the firewall into the engine bay. Go under the car (on jack stands, of course) and remove the bolts holding it onto the passenger side of the transmission. Bolt on the new cable, push the end through the rubber gasket on the firewall, attach it to the speedometer housing and reattach the gauge pod and hood. It's pretty simple and straightforward. You should be able to do it in an hour or so.

Email Marshall English at MedCenter Mazda (www.mazdaparts.org) and he'll give you a good price on the cable. If you buy $100 worth of parts the shipping is free, so if there's some other stuff you need now would be the time to get it. The cable should be $50 -- $60, but that's just a guess as I replaced mine four or five years ago.

Good luck,
HRColey