It seems like most people are happiest running gearmotors at 48v. If you overvolt motors too much you make a LOT of heat, which gearmotors radiate away a bit slower than dd motors. So 48v and about 30 amps seems to be a poplular and reliable setup.
There are different windings, so 30 mph is totally possible at 48v with the speed versions of the Mac, for example. Cellman in the for sale section has em.
In any case, 80 kph is pretty unrealistic for gearmotors, and often melts the crap out of decent dd motors. Not to mention that 72v and 25 amps isn't even close to enough wattage for 80 kph. Lots of us wish it was just that simple, a gearmotor and 72v.
For what you want, you need to be looking at scooter motors rated at 3000-5000 watts, not motors of any type rated for 600-1000 watts.
50-60 kph is doable with the larger direct drive motors and 120v 40 amps. And they will go at least 8 miles without melting. So much depends on your intended use. What might work fine, like 5000 watts for a drag race, will not work for a long ride.
Read a lot of the build logs in the pics section. See what guys like Methods have done, what it took, and how long it lasted before the smoke came out the motor. You won't see much in the way of gearmotors going 70 kph there though.