David,
You could have saved yourself some $$ by sending your broken speedo out to one of the various speedo shops like North Hollywood Speedometer in the San Fernado Valley. A broken odmeter gear is not uncommon, and does not require you to replace the entire speedometer. The gear commonly breaks when you rese the odmeter when you are moving. In either case, in order for you to correct the milage in your new unit, it needs to be opened up just as you would have been required in your old unit to replace the gear. I know many here have opened up their gauges, but it requires some special tools to avoid ruining the outer ring. I'd look up NHS
http://www.nhspeedometer.com/ and give them a call.