TL;DR: Around 250cc.

I’ve gotten my CRF250L Rally up to an indicated 93 with a long windup and maybe a small tail wind. It’ll maintain 75 easily, and can achieve 80 without feeling like it’s about to explode.

I’ve also taken my RV200 (198cc) for short stints on the highway before and I didn’t die. Technically my state only restricts 49cc and below for highway/freeway use, but with anything slower than the aforementioned Vanvan I’d be more worried about some smoothbrained dipshit getting angry with you for being “in their way” and trying to start shit with you rather than any real world speed deficiency with your bike. There are probably quite a few 125cc class bikes that could maintain the actual 55-ish MPH speed limit that is likely to be in force on the highways around most major urban areas around the country, but the problem is people these days apparently feel entitled to exceed the speed limit by at least 30 and get frothingly butthurt at anyone or any thing they perceive as preventing them from doing so.

Seen plenty of 125cc’s on the German autobahn, they seem to keep up with the traffic ok as long as they stay on the right side. Not sure if I’d be comfortable on anything less than a 500 myself but then I’m bigger than average.

what’s cc got to do with it? my rg125 two stroke (when it’s not blowed up) can do 100mph, and 90 all day, my dax 125 will eat a cam if I push it beyond 55mph.