http://www.myradiobase.de/fmscan/form.php?ex=1
This generates coverage maps for stations, and gives estimates of how far a station travels during either the day or the night. I used it to check on KVNA's coverage, as stu said it wasn't great lately, and it looks pitiful. All you have to do is put in the call sign under "Program" next to country and region. I haven't checked it out, but I think you can do it for MEX stations and Canadian stations too.
HEA!