Before you order that 21 gear, consider this:
If you are reading exactly 80mph when you are actually running at true 60mph, your gears are off by 25%. If your speedo gear ratio is 2.857 (20/7), then you need to would need to step up to a 25 driven gear... 25/7 = 1.25% X 20/7. Since a 23T is the largest gear you can get, you will need to swap the drive gear as well. If you swap to a 6T drive gear, you would need a 21 or a 22 driven gear (actual needed gear would be 21.428). See how important it is to know just how far off your speedo is off? Also consider if you are going to change your tire height. This can have a big impact on accuracy.
Someone please check the math on this one...It's not one of my strengths!