Does your cruise control work? If cruise control works and you have no Service Engine Soon/Check Engine Light illuminated when you are driving it would mean the Vehicle Speed Sensor on the transmission is OK. The problem may be ath dash instruments or wiring. On the LeSabre the tachometer and vehicle speed input to the dash come from the Powertrain Control module. Instrument panel fuse fuse 30 (10 AMP) protects all the instruments so if the gas and temperature gauge works the fuse must be OK.