The most accurate way to determine your max heart rate is to do a maximal stress test.
Many people rely on formulas like MHR = 220 - Age. These formulas are typically an approximation for a large group of people and the standard deviation is very wide. That means that the formula is very likely to be wrong (and wildly wrong) for any individual, even though it is a fairly good approximation for the population at large.
If you can trust your heart rate monitor, then the best you are likely to know is that your maximum heart rate is equal to or higher than the maximum reading you have ever taken with the monitor.
The 185 rate you got is already higher than what the common rule of thumb would predict (which is 181). The queston about the 196 number is whether it is a spike that was not a true reading, or whether something about the conditions actually put your heart rate up that high. There is no way that someone on these forums can answer that question (at least not without more information).
It is possible that the numbers are correct. On a really tough group ride about 5 years ago (at age 43), I pushed my heart rate up to 200. The readings were taken every 5 seconds and they rose to that rate pretty gradually, stayed there for several readings, and then came down gradually. So my conclusion was that the HRM was doing its job correctly. I already knew that I could push my heart rate up to 190, so it was not _that_ surprising.
I think the 196 number may be high, but it is not out of the realm of possibility.
Duffy