You need a special tool. It's not too expensive. Summit carries one for under $10.
http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM%2DW83125&N=700+115&autoview=sku
A piece of string or rope?! You are probably asking about the 'knots' used at sea as a measure of speed. One knot = one nautical mile per hour. Don't ever say 'knots per hour', the word 'knot'...