Fastest I've ever been on a kart was about 65 mph on a high bite 1/4 track. A comment on speeds: unless you've got a gps or speed sensor it's hard to know how fast you're going. Track sizes tend to be inaccurate (and always advertised as longer than they actually are in the groove).
So many combinations of karts and engines and tracks.
On a regular paved race track, (like the Daytona road course) in a sit up sprint kart, no body work and a KT100 pipe engine, a top speed of about 83 MPH is common.
If you put a body on the kart, built for that type of racing, quite a bit faster.
In a laydown kart with the same engine and pipe, about 103 MPH or so.
On our 1/2 mile, 800' straight sprint track a good running box stock clone on a weenie pipe will do 56-57 mph according to my MyChron gps and race studio software. In a draft of two or more karts you may see 58-59. My stock appearing clone does 65 mph. My PTK KZ 125 shifter does 80 mph.
1/5 mile dirt, my 13 yr old and his class run about 65-> down the chutes, they don't lift in the corners much, scrubbing 1500-ish rpm drop in the corners is a pretty good run (mph calculated from rpm, gear ratio, and tire roll out .. math based on lap speed (11.2x is a real nice lap) and track size backs it up) ...
7 or 8 years ago Carl Anderson had a radar gun at Lasoski speedway in Mo(high banked 1/6-1/5) The gun showed us running 52 in superhvy entering turn 1. Karts and tires are better now. I remember we were on a Nemesis so it has been a while ago.