205/65/15 is about 3.3% out, based on stock size of 205/60/15
If thats the case, then swapping the drive gear wont fix it, each tooth is equivelent to 5 or 10km/h, so it would bne the drive in the gearbox need replacing to be 100% accurate.
haha in my VH i dont have the original gear box and i dont know what gear box it is and my speedo is out by 20kmph there is a place around my area that fixes em but if its easy i i do it my self
20km/h is alot easier to fix, check with a GPS if you can, and get a percentage figure.
Once you have the right gear, you just undo 1 bolt, pull the cable out hard, whilst tearing your hand apart on the gearbox cross member, jump in pain and smack your head on the exhaust, so swear some more and throw your ratchet, which bounces and smacks your kneecap..... once its out, put the drive gear off, fit new one, clean the blood up, and push back in and put the bolt in. About a 2 minute job, wounds may take a few weeks