There is no "car" tire presently made that will fit a stock front end of a GL1800. The best tire presently used on the front end of a GL1800 trike are the Michelin MPA and Bridgestone BT45 both of which are "rear" bias motorcycle tires.
One could add a car tire to the front end of a GL1800 with the springer front end supplied by Euro trike which at last check cost over $4K. You'd still need to have the front fender modified/widened. $4K would buy about 40 "rear" mc tires giving you a range of at least 800K miles (20K miles per tire).