There are a number of ways to go about it. There are several factors that have an effect on speedometer accuracy. Tire diameter and rear-end ratios being the most commonly adjusted items. There are two (2) gears that change the speedometer/odometer readings. One in the transmission tailshaft itself and one that the cable attaches to at the tailshaft. Sometimes you can fix the problem by changing the one on the cable alone other times you must change the internal or both. There are also custom shops that will make a conversion gear set for you that resides outside of the transmission. I have contacts for one if you determine you need to go that route. As long as you are using stock z parts and nearly original diameter tires you should be able to find a stock gear combinaton that will work.
You can drive, say 10 miles/kilometers down a road with markers and note the difference between your odometer and the markers (or use a GPS). You can then calculate the percent error. Pull the cable gear out and count the teeth. You will need to change the number of teeth to match your error percent and you hopefully you can find a gear that will closely match. let me know if you need formulas.
Check with your local garages, some have a dyno to check your speedometer. That is usefull if you get a speeding ticket and if your speedo is off, sometimes it gets reduced to a inop equipment ticket. Now to fix the problem... As dbryan said, tire size, different gears in the differential will make a difference. Check out this page: http://www.geocities.com/zgarage2001/engine.html it will list the different speedo cogs for each gear ratio. To find out what cog you have, unhook the speedo cable from the tranny and then loosen the 10mm bolt that holds the cog assembly. Pull the cog assembly out (have a pan ready to catch the tranny fluid) and look at the color of the cog. By taking your car to a dyno to see how far the speedo is off, you can buy a cog to correct the difference. I can't remember the formula of how many teeth on the cog will change the speedo... they guys at the dyno should know that.