Car Demon has a wide variety of PHP constants that can be used to modify functionality.
Over time many of these constants will be turned into settings that can be adjusted on the settings page. In the meantime you can define these in your wp-config.php file right above where it says:
/* That's all, stop editing! Happy blogging. */
CD_MILEAGE_STOP if defined the mileage drop down in the search form will stop at this number.
To do this you would add the following to your wp-config.php file:
define( 'CD_MILEAGE_STOP', '250000' );