Remove the instrument cluster and replace the resistors on the circuit board. You can get them from radio shack, an it's not hard to do. While you are in there, I would recommend replacing all bulbs and stepper motors to your gauge needles so you won't ever have to take the cluster out again.
What year is it? If it's a 97-04 Regal, the problem lies withing the instrument panel board and must be refurbished. DO NOT try to replace the odometer or get a whole new one. You will lose mileage information. I repair instrument panels and I'm willing to help you, contact me at ANCTechnologies at Hotmail.com
there is no light bulb for the odometer. its a digital display. you have to replace the dash. used usally a 100. or so. had it happen to me. 2000 buick regal gs
The EPA mileage estimate for the 2001 Buick Regal is 17 mpg in the city, 28 mpg on the highway.
The EPA mileage estimate for the 2013 Buick Regal is 25 mpg in the city, 36 mpg on the highway.
The EPA mileage estimate for the 2014 Buick Regal is 21 mpg in the city, 30 mpg on the highway.
The EPA mileage estimate for the 2003 Buick Regal is 17 mpg in the city, 27 mpg on the highway.
The EPA mileage estimate for the 2002 Buick Regal is 17 mpg in the city, 27 mpg on the highway.
The EPA mileage estimate for the 2011 Buick Regal is 19 mpg in the city, 30 mpg on the highway.
The EPA mileage estimate for the 2004 Buick Regal is 18 mpg in the city, 28 mpg on the highway.
The EPA mileage estimate for the 2012 Buick Regal is 19 mpg in the city, 31 mpg on the highway.
you get less gas mileage
Buick Regals don't have timing belts. They have a timing chain.