SPEEDOMITTER GEAR SIZE

After restoration my B Body, my speedometer is about 10 MPH slow.. I have a 355 rear end, 15'' tires.. What I'm seeing I need a 36 tooth gear.. Does this sound right? Also all I need is a gear,,, not the gear housing,,, that works as a cam setting correct? I appreacate any help.. also where can I get this gear? Thanks..

IMO, the odometer is a better indicator of correct pinion. If the odometer is correct and the speedo is off, the problem is in the speedo head.

The actual tire diameter is needed to compute the pinion needed, lots of sources for the pinions such as Brewers, e-bay, etc.