I have a Sigma BC 8.12 hard wired bike computer that I just installed on my two wheel 'bent which has a 20" front tire and 26" rear tire.

According to the wheel chart that came with the computer it only has one setting for a 20" tire and the problem is that the wheel calculation is for a 20" 1.75" tire and my tire is a 20" 1.5" tire. The manual states to enter "1580" for the 20" 1.75" wheel. Since my wheel is a 1.5" how much off will it be? Are we talking about less than 1 mph at say 25mph?

Is anyone here really good at math and can do the calculation for the correct setting for the 1.5" tire?