Here's a tip, don't use either one of the free GPS speedometer apps downloaded onto my cellphone. Both of them read out only 80% of what the speedometer in my van reads. Different apps, read the same.
No idea what to trust now, and hooking up some kind of real speedo, mechanical or otherwise, is out of the question for me right now.
Going to assume you mean your expensive Juggernaut CLUTCH, not a "torque converter" (CVT or Constant Variable Transmission). The belt drive CVT's can't be figured in to a calculator for a mathematical answer to output, as the ratio is constantly changing.
If a clutch (high stall Juggernaut) can be assumed to not be slipping once it's engaged, the speed calculator should be accurate if figuring in your engine rpm's. Any decent little cheap induction tachometer can get you the rpm's.
Just figuring that in to your 5.4:1 gear ratio, and the rear tires diameter (18" tall diameter x pi, or 18 x 3.14 = 56.52" circumference) will give you the speed. 5,890 rpm's div. by 5.4 ratio gives you about 1,090 rpm's of rear axle, Translates to 61,607 inches per minute. Div. by 12" = 5,134 feet per minute. That's 308,040 feet per hour. Div. by 5,250' in a mile, that's 58.67 mph.
At least that's my desk quarterbacking on paper. I know that's not real world numbers, and there's a huge unknown about the loss from tire compression at speed, just rough calculating. You know, even if 36 mph. were only 80% real speed like my 2 apps would read, you'd still only be getting 45 mph.
I will say this, everybody agrees that right out of the box a Tillotson 212 will blow away a Predator 212, but of course both can be hotrodded up to similar results. I'd be willing to accept that your stage 1 Tillotson at that ratio could top 50 mph.
About getting more speed, I'd advise against increasing your gear ratio for less low end torque but more top end. People do that all the time, and burn up clutches all the time too.
I heartily recommend replacing the clutch with a CVT. Bought 2 from China, average about $60 each, never had a problem with them. Bolts right to the case w/4 bolts, where the clutch is now. Wheelie popping low end, scary top end.
You could probably sell the used Juggernaut online for enough to buy a CVT.