if the 21 toother reads 17% too high (double check that) then you need one with 17% more teeth so 21 X 1.17 is 18.27 teeth. so 18 teeth EDIT I did my math wrong, need 24.57 teeth. MORE EDIT if those older TF's only went up to 21 then one of those mini gearboxes (I forget what they are called) that go inline on the speedo cable to alter the ratio would be needed. I have one & if I can find a pic of it I will post it. EVEN MORE EDIT it is a "speedometer gear ratio adapter", just found em on eBay & they (on the one I saw) will select the overdriven or underdriven ratio you need. GPS would be the most accurate confirmation on what you need. MORE EDIT you want the right tooth for the odometer to read correct & most all times that will then make the SPEEDO read right but if the teeth are correct for a correct ODO reading & the speedo is off then the speedo head would need to be sent to a speedometer specialty calibration shop. For the speedo, If no GPS, on the interstate with mile marker signs (which are very accurate as far as land based signs go)) at a steady 60 MPH it should take you exactly 1 minute to go 1 mile