I wouldn't ignore them, but I would tell them politely that you have a signed contract that both parties at the table agreed to when signing. Unless they were telling you 20 and DID make a mistake typing it up as 15 and you KNEW it, but hey they'd have to PROVE that in court to force the higher amount on you. If you indeed think you'll put more than 39,000 miles in that time (I sure as heck would, more like 50,000), do the math for the various scenarios and consider your options before calling back and offering them a counter proposal.