Have a 1991 lasabre.the speedometer is reading about 20mph off.How do i fix this???

Y can't reset the speedometer. Your problem is the electric motor that turns the speedometer needle. I believe yours just pops in and out. But some are soldered. You can get both kinds on line.

IT CAN BE CAUSED BY THE WRONG SIZE TIRES, TO THE WRONG GEAR IN THE TRANSMISSION! IF YOU'VE CHANGED THE RIM SIZE, IT CAN CAUSE A PROBLEM! LIKE GOING FROM A 15" RIM, TO A 17, OR 19" RIM!

A reliable fix would be to send the cluster out for repair, if in fact the problem lies in the speedo head. There is a bulletin for this car that states that at 20mph the speedo can be + or - 4 mph inaccurate, and its normal operation.
I suggest first having the alternator tested. The VSS on this car produces an AC voltage signal. The alternator also produces AC voltage, which is converted to DC before it is output to the cars electrical system. If any AC voltage is leaking from the alternator, it can interfere with the VSS signal, and cause the speedo to read higher, or even show that the car is moving when sitting still.

this is in the speedometer unit itself. either purchase a known good part from an auto recycler or people still rebuild these. on these, they are individual little motors inside and they will probably just replace the speedo motor and reseal it. unfortunately you can not purchase these parts yourself.

It's a cable. If the car doesn't have the same size tires that it came with from the factory, the speedometer will be off. The only way to calibrate it would be to change the the cable drive gears in the transmission, but I have no idea what the gear ratio would be or where you would get the gears. That car came with either P215/75R14 or P205/75R15 tires.

The "needle at 20MPH" is a sure-fire indicator -- it's the speedometer head. This is a common problem in this year.

I haven't found a fix other than replacing the speedometer head, sorry.

speed sensor on trani is bad
replace it

i have a 2004 buick lasabre gauges are not working can anyone give me an answer of how to fix this

Typically on a Yukon instrument cluster this is caused by failing stepper motors on the cluster's circuit board, it can be fixed though. Check out this page for more about Yukon speedometer problems and how to get it fixed - Yukon speedometer fix

Can you fix this my headlight blink off and on and i must hold high beam light switch on blinker handle to drive at night