Paul Newman is a very tough autograph to find. Dealers ask a lot for one because they know it will be a long time before they find another to replace it.
Though he was generally nice to fans, he just didn't like to sign autographs. On occasion he would sign a quick one, sometimes telling the recipient, "Don't tell anybody."
Your Newman autograph is worth $200 - $300.
Mario Andretti was always good about signing autographs, so there's plenty of those in the marketplace. His signature is worth $20 - $25.
A race fan might like both autographs on one piece, so it might find a buyer at a figure higher than the sum of those totals.