How fast will a 6.5 hp go cart go?

You should be able to get between 60 and 70mph with no problem at all. Go to YouTube and look up box stock preperation. I did it to an engine I am using in a motorcycle (6.5hp Greyound from Harbor Tool and Freight) with just a clutch it manages about 55mph and with a Comet TAV transmission I get 78mph
A 6.5 go cart motor can go approximately 45 miles per hour with the right running gear. Most of these engines go in smaller carts which go between 20 and 30 miles per hour.
I have a 6.5 go-cart and it dos 70kph but it all depends on your gears and tire size
Horse power is not an indication of speed.

The speed of any vehicle depends on a variety of factors.

What medium through which it is moving. (Air, water, land, space)

The power of the engine.

The weight of the vehicle.

The streamlining.

The gear ratiOS. (If any)

The efficiency of the system.
80 kmph