NAC(or very very little) Towing speeds- Trailer Tire Size???

[joel nevin]

I am going to be putting a hitch on my A4Q(audi
content) to tow my motorcycle to the track. I have
lead on a trailer kit, but was wondering what the max
speed of the trailer could handle with 8" wheels, I
called the company and they were useless. Saying it
was "highway speeds" well to me highway speed is
anywhere from 45-65, and my normal driving speeds are 
75-80. I really don't want to tow a bike 200 miles at
50 mph, ugh and on the other hand tow it at 75 and
have the trailer bounce all around. 
Will the 8" wheels suffice for traveling at 70-75mph?
Or should I go with the larger wheels and what is the
main factor for the speed of a trailer is the size or
construction of the tire(4-ply), probably both?
Any BTDT's would be great. 

TIA
Joel

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