"Darkwing" theducksmail"AT"yahoo.com writes:

Is there a certain speed an aircraft must be going for contrails to form?

No. They depend mostly on atmospheric conditions.

I have never seen a 172 with contrails but as a kid I used to go to Road
America with my dad and watch the (then CART Indycars) and in the morning
you could see the contrails off the rear wings, so that tells me it has
something to do with speed/humidity nothing to do with altitude.

It has to do with all of these. Any combination of conditions that encourages
water to condense out of the air (including water in engine exhaust).

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