I got new sprockets for my 440LTD from z1enterprises. The link above will take you to the page for the 1980 KZ440LTD-A1 parts where you can buy sprockets. Stock are 15 tooth front and 45 tooth rear. they sell those and a 16 tooth front and 41 or 33 tooth rear. gearingcommander.com will give you an idea of how your speed to rpm ratio will change to meet your application, but keep in mind that by changing the rear sprocket you may have to shorten the chain. I bought the 33T and it is much smaller than the stock 45T. I'll let you know on the gas mileage change once I switch them out.
