What I want to do is compare an old ride to a new ride. I would like the cycle counter to pull data from the SD card and compare the old time vs the new time. eg start riding, after 5 wheel rotations look up time for the previous recorded 5th cycle of the wheel and compare. Then do this for 10,15,20 etc. Is this possible?
In its simplest form, it's easy. But to produce anything meaningful I think would be quite hard. I suppose you want to do some sort of comparison of speed versus distance so you can tell whether you went up that hill any faster today than yesterday. The problem is that the start points and distance traveled each time are unlikely to match exactly in the real world, so getting anything more than a rough comparison would be tricky. Your best bet I think would be to save the log of each journey in a CSV file and then load that up to a PC where you can compare and graph the results in a spreadsheet. Make sure you make your sample interval long enough so that you can hold a useful number of logs of your desired length on an SD card.