How to determine an unbiased estimate of threshold in a single-stimulus forced choice experiment?
Any estimate you make is going to be pretty bad. To answer this question requires so many assumptions about the data, experiment, and intended usage, it is just not possible. You would be better off running the experiment again after consulting someone who knows more about experimental design. Beyond that, you could hire a stats consultant.

Apart from the lack of catch trials, you only have 2 speeds. Even if you get lucky and they bracket threshold (e.g., d' of 1 or 75% correct), without more theory (or data), you cannot interpolate between the points.