new gears, wrong speedo reading

[mfinney]

Hello, on my 72 K20 the previous owner rebuilt the differentials and put 3:55's in instead of like 4:10's. My guess is that then my speedo reads 52-53 i'm going around 65 or so. I'm also running 255/85/R16 tires with new tread. Where is the best place to get the little gear for the speedo cable that goes into the tranny and what one might I need? Thanks.