international speedometer

ok ih guy's,i have a spedo issue on my 95 model 4900. the 95 i bought a few months back had a couple item's missing[turn signal.intrument cluster]. no prob,i bought a new turn sig [$175.00 w.t.h ?]and took the instrument cluster out of one of my 95 model school buses,installed it in the 4900. well here's my prob, everything works fine but the temp gauge[sender is good ],and the speedo is inaccurate. i'am sure the spedo prob is due to the difference between axle ratio's of the bus and the 4900. my question,how do you calibrate the speedo to match the axle ratio? i have seen the spedo adjusment on the back of the cluster ,but what's the proper procedure/code? thanks, jim ..