Aftermarket sending unit not the same

I just put a new one in my 64 dart and it seems to be working fine.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=310274579753

Around 10 gals gives me a little over 1/2 tank
18 gals puts it past full and there is a gallon or two left at empty